Geological Timeline from The Geological Society of America

Bishop, Chapter ??

Rocks

Igneous Rocks

The so-called crust of the Earth is about 20 mi thick under the continents but averages only some 4 mi beneath the oceans.  If is formed mainly of rocks of relatively low density.  Beneath the crust there is a layer of denser rock called the mantle which extends down to a depth of nearly 2000 mi.  Much of the molten rock material which goes to make up the igneous rocks is generated within the upper parts of the mantle.  This material, which is called magma, migrates upward into the Earth’s crust and forms rock masses which are known as igneous intrusions.  If magma reaches the Earth’s surface and flows out over it, it is called lava.

Within some lavas, fragments of dense, green-colored rocks are sometimes found which consist principally of olivine and pyroxene.  The fragments (xenoliths) are thought to represent pieces of the mantle, carried upward by the migrating magma.

The great majority of lavas consist of the black, rather dense rock called basalt, and most petrologists consider that the primary molten rock material which comes from the mantle has a composition which is near to that of basalt.  Although basalt is the most abundant of the lavas, granite is by far the commonest of the intrusive igneous rocks.  Granite is mineralogically and chemically different from basalt and for many years geologists have wrestled with the problem of how the two rock types are related.  If basalt is assumed to derive from the mantle, is it likely that granite, which is of quite different composition, could also come from the mantle? In recent years, ideas as to how basalt, granite and, indeed, the whole spectrum of igneous rocks described in the following pages have been generated, has been much influenced by the theory of plate tectonics.  In brief, this all-embracing idea is that the uppermost part of the Earth consists of a layer, up to about 100 mi thick, and comprising the crust and the uppermost part of the mantle, that moves over the underlying mantle.  This uppermost layer, which is known as the lithosphere, comprises a number of rigid ‘plates’ which jostle against each other.  In some places new lithosphere is being generated while elsewhere an equivalent amount is being destroyed. New lithosphere is mostly being formed along the midocean ridges (spreading ridges), which are lines, notably running along the center of the Atlantic and within the Pacific Ocean, where magma rises from deep in the mantle, erupts onto the ocean floor and is injected close to the surface, so forming new lithosphere and hence making the plates bigger.  Although this activity is generally concealed beneath the waters of the oceans, it can be seen, for instance, in Iceland, which is located on the MidAtlantic Ridge.  Along some plate margins one plate is forced downward beneath the adjacent one. At these boundaries, known as subduction zones, the lower plate may be driven hundreds of kilometers down into the mantle, where the high temperatures will cause melting. This new magma will eventually rise toward the surface where it forms a line of volcanoes.  The majority of volcanoes in the world are formed in this way and the many thousands of volcanoes that rim the Pacific Ocean are the result of the edges of the ‘Pacific Plate’ being melted as it is forced down beneath eastern Asia and North and South America.

So, the plate tectonic theory explains where and, in general terms, how igneous rocks are formed.  Returning to the problem of basalt and granite, it should be noted that the rocks found along midocean ridges are mainly basalt, while along subduction zones, although basalt is also present, the bulk of the volcanic rocks comprise a spectrum from andesite through to rhyolite, which are chemically equivalent to granite and its associated rocks.  Numerous, large bodies of granite also occur in these zones, and represent the intrusive equivalent of the rhyolite.  The variety of the igneous rocks occurring above subduction zones reflects the heterogeneity of the melted plates, which are built from most of the rock types found at the surface of the Earth.

Other important places where igneous rocks are produced are ‘hot spots’, which are areas within the plates where heat escaping from the deep Earth is concentrated.  In such areas melting may be sufficient to produce volcanoes.  Such hot spots form many oceanic islands, for example Hawaii, or volcanoes within the continents, such as Kilimanjaro.

Although the melting of heterogeneous plates explains some of the diversity of igneous rocks found along subduction zones, there is another important process at work.  When basalt magma starts to crystallize in the upper mantle, or the lower part of the crust, the overall composition of the crystals is not the same as the overall composition of the magma. This means that the liquid part will have a composition different from that of the original magma, and the further the crystallization process goes the greater will be the difference in composition between the liquid and the crystals.  If the crystals and the liquid should now be separated by some mechanism, then rocks of two types will result, each with a composition different from the original basalt.  This process, called differentiation, is capable of producing a great range of rock types, one of which is granite.

The recognition and naming of igneous rocks involves an assessment of grain size and the recognition and estimation of the relative amounts of the constituent minerals.  Additional information is obtained from color index, texture, structure, and sometimes from field relationships.

Grain Size

Grain size refers to the size (average) diameter of the mineral grains comprising the rock.  Some rocks have large crystals set in a ground mass of smaller grains (see below); in these rocks only the groundmass minerals are taken into account; the large crystals, no matter how obvious, are ignored.  Excluding the glassy rocks, three broad grain size categories are recognized:  fine-grained, in which the grains are generally below the limit of resolution of the naked eye (less than about 0.004 of an inch); medium-grained, in which the grains are recognizable with the naked eye, but minerals hard to identify (0.004 to 0.08 of an inch); and coarse-grained, in which the mineral grains can be identified by the naked eye (coarser than 0.08 of an inch).  The coarsest rocks, in which the mineral grains have diameters of several inches or more, are referred to as pegmatites.  Glassy (or vitric) rocks consist essentially of glass.  If magma, or lava, is chilled very rapidly the potential minerals are unable to crystallize and grow, and glass results.  The best known such glassy rock is obsidian.

Mineralogy

This is the most important single feature to be considered when naming igneous rocks.  Although magma is a complex silicate melt, most igneous rocks are composed of a few essential minerals belonging to a few mineral groups, namely quartz,  the feldspars and feldspathoids (the light colored or felsic minerals), and the pyroxenes, amphiboles, micas and olivine (the dark colored of mafic minerals). Minor constituents are grouped as accessory minerals. All the groups listed are silicates and except for quartz are, within limits, variable in chemical composition.  Once the grain size has been decided, rock names are assigned according to the kind and proportions of the constituent minerals.  It may be necessary sometimes to know the approximate chemical composition of one particular mineral, but this usually requires at least a microscope and so cannot be determined in the field.  The table above summarizes the mineral content and the interrelationships of most of the igneous rocks described in the following pages.

Color Index

The color index of a rock is the proportion of dark minerals iti contains on the scale 0 to 100.

Texture

Texture refers to the shape, arrangement and distribution of the minerals of the rock.  The following descriptive terms are often used.

In a granular texture, (equigranular) (page 158) all grains are of about the same size and roughly of equant shape.  Poikilitic texture (page 166) refers to large grains of one mineral enclosing smaller grains of other minerals. If pyroxene encloses plagioclase, as in many gabbros and diabases, the texture is called ophitic.  In a porphyritic texture (page 172) some large grains (phenocrysts) (or insets) are set in a finer grained or glassy matrix (groundmass). ‘Porphyritic’ is a common adjectival prefix; for example porphyritic granite, porphyritic basalt.

Flow or fluidal texture (page 170) refers to tabular or elongate crystals aligned by flow in the magma, in much the same way as logs in a river.  In glassy rocks flow is marked by swirling lines, and often by trains of bubbles.

Structure

The structure of rocks refers to the broader features of rock masses rather than those which depend on the interrelationships of the grains.

In a layered or banded structure (page 164) the rock comprises layers of contrasting mineral composition that appear on a surface as bands differing in color or texture.  A rock with a vesicular structure (page 177) contains cavities (vesicles) produced by the expansion and escape of gases. Vesicles, which frequently occur in lavas, may be spherical, elliptical or tubular.  When the vesicles are filled with secondary minerals they are referred to as amygdales, and the structure as amygdaloidal.  Xenoliths, or ‘inclusions’, (page 163) are fragments of other rocks included in igneous rocks.  They may vary greatly in shape and size.  Joints are cracks or fissures in rocks along which there has been no displacement. Lava flows sometimes show columnar jointing (page 174) in which the rock has broken on cooling into parallel hexagonal columns roughly perpendicular to the cooling surface.

Field relationships

Igneous rocks can be divided conveniently into three major groups: volcanic (extrusive) rocks are largely glassy and fine-grained and form lava flows, tuffs and agglomerates; hypabyssal rocks are largely medium-grained and occur as minor intrusions (sills, dykes); and plutonic rocks are largely coarse-grained and form major intrusions.

Igneous intrusions are described according to their shapes and their relationships with the rocks they intrude (the country rocks).

Minor intrusions

Dykes are sheetlike intrusions which are vertical or nearly so and which cut sharply across bedding (see sedimentary rocks).  Dykes range from a few inches to hundreds of feet in width.  Sills are sheetlike intrusions which are essentially horizontal and usually follow bedding or foliation. Like dykes they range from a few inches to hundreds of feet in thickness.  Veins are irregular intrusions which sometimes form a complex network.

Major intrusions

Batholiths are large, cross-cutting intrusions, usually of granitic rocks, having steeply dipping contacts and no apparent floor. Exposed batholiths may cover hundreds of thousands of square miles.  Stocks are smaller than batholiths but otherwise similar.  They occupy areas of a few square miles to tens of square miles.

Volcanic rocks

Volcanic cones of volcanoes form when lava and accompanying pyroclasts (lava fragments) are ejected from a vertical pipeilke vent.  Lava may, however, flow from a fissure from which it may travel for considerable distances forming a lava plateau.  Lava which flows into water chills rapidly and may give rise to distinctive pillow lavas (plate 174).

Schoenherr, A Natural History of California, Chapter 3

Schoenherr, Allan A., A Natural History of California. Berkeley, CA: University of California Press, 1992.

Chapter 3

Basic Geology

Geology is the study of the earth – more specifically, it is the study of rocks, the nonliving components of the earth. Scientists have divided matter into 102 pure inorganic substances known as elements.  Living organisms consist primarily of four elements: hydrogen (H), oxygen (O), nitrogen (N), and carbon (C). Ninety-nine percent of all rocks are composed of some combination of eight elements: silicon (Si), oxygen (O), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg). All the earth is composed of these elements, either alone or in a myriad of combinations called compounds.

A mineral is composed of a single element or compound.  By definition, a mineral is a naturally occurring inorganic substance with a definite chemical composition and ordered atomic structure.  Table salt, for example, is a mineral called sodium chloride (NaCl).  Its ordered structure is apparent because it occurs in crystals shaped like small cubes. Another common mineral is quartz, or silicon dioxide (SiO2). Its crystals have a specific hexagonal shape.  Gold (Au) and silver (Ag) are minerals composed of a single element.  Coal is a mineral composed entirely of carbon, originally trapped by living organisms through the process of photosynthesis.  The carbon in coal is therefore of organic origin, which leads some authorities to object to the definition of a mineral as an inorganic substance.  The controversy, over the true definition of a mineral, however, is beyond the scope of this book.

Rocks are usually composed of several minerals.  Granite is a rock with a speckled appearance caused by different minerals in crystal form, such as quartz, mica, and feldspar.  Limestone is a rock composed of a single mineral, calcium carbonate (CaCO3). On the basis of their origin on earth, rocks may be divided into three primary categories: Igneous, sedimentary, and metamorphic.

Igneous Rocks

Igneous rocks are formed from cooling and solidification of molten rock.  The term igneous refers to fire; it comes from the same root as ignite.  The high internal core temperature of the earth causes convection of heat energy, which melts rock to produce magma.  Upon cooling, magma becomes igneous rock.  The length of time magma takes to cool determines the size of the crystals of each mineral.  The texture of the rock is an indication of the length of time it took to cool.  If it remains deep in the earth, it will cool slowly, and the crystals will have a long time to form.  These rocks will be coarse-grained; the individual mineral crystals will be visible to the naked eye.  Granite is the most common of these coarse-grained rocks.  If magma comes to the surface and cools rapidly, as in a volcano, it will be fine-grained because there has been too little time for large crystals to form.  The most common fine-grained igneous rock is basalt, a heavy, black volcanic rock.  Rocks that have been cooled slowly, deep beneath the earth, are called intrusive or plutonic rocks.  Rocks that have been formed by molten material that flowed out upon the surface are called extrusive or volcanic rocks.

Igneous rocks also differ in color and composition.  Some minerals, such as quartz and feldspar, are light-colored.  Minerals that include iron and magnesium are dark-colored and are called ferromagnesian materials.  The relative proportions of quartz and feldspar; and of ferromagnesian minerals are responsible for the principal color of the rock.  Table 3.1 is a list of minerals found in igneous rocks.  On the basis of relative proportions of orthoclase feldspar, plagioclase feldspar, and quartz, geologist have named 15 different kinds of plutonic rocks alone, which is beyond the scope of this book.  For purposes of simplicity, six main types of igneous rocks based on chemical composition (color) and texture (table 3.2) will be considered here.

In California, granite and diorite are the common rocks that make up much of the Sierra Nevada.  A large intrusive block such as that of the Sierra Nevada is known as a batholith (deep rock) in reference to its origin deep beneath the surface.  Granite, associated with the Southern California batholith, is found in the Transverse Ranges, the Peninsular Ranges, and some of the ranges in the Mojave Desert.  Gabbro is a dark-colored plutonic (intrusive) rock.  It is found in the Peninsular Ranges in San Diego County and the foothills of the Sierra Nevada in El Dorado County, where it has degraded into a dark, iron-rich soil upon which many specialized (endemic) plants live.

Table 3.1 Common Minerals Found in Igneous Rocks

Mineral, Chemical Composition, Appearance

Quartz: Silicon dioxide; glassy, clear, cloudy, white, gray, pink.
 

Feldspar Plagioclase: Calcium or Sodium aluminum silicate; blocky, dark gray to white.

Feldspar Orthoclase: Potassium aluminum silicate; blocky, pink.

Mica Biotite: Complex iron silicates; thin, shiny, clear sheets.

Mica Muscovite: Complex potassium silicates; thin, shiny, clear sheets.

Ferromagnesian minerals, Pyroxenes: complex iron, magnesian silicates; short, stubby crystals, green or black.

Ferromagnesian minerals, Amphiboles: complex iron, magnesian silicates; grains or long crystals, light green to black.

Ferromagnesian minerals, Olivine: complex iron, magnesian silicates; glassy to grainy, light green.

Table 3.2 Types of Igneous Rocks

Texture, Color

Fine-grained (volcanic): Rhyolite: light; Andesite: medium; Basalt: dark.

Coarse-grained (plutonic): Granite: light; Diorite: medium; Gabbro: dark.

Note: Darker rocks indicate increased amounts of ferromagnesian materials and decreased amounts of quartz.

Of the volcanic (intrusive) rocks, basalt is highest in ferromagnesian minerals.  It is the dark black rock that forms large flows on the eastern side of the Sierra Nevada, for example, in Devil’s Postpile, near Mammoth Mountain.  Basalt is most common, however, as the main component of oceanic island such as the Hawaiian Islands.

Andesite, lighter in color than basalt, is the primary volcanic rock found on the borders of continents. The name comes from the Andes, the large mountain range on the western border of South America.  In North America, the Cascades are composed primarily of andesite.

Rhyolite, the extrusive rock with the lightest color, has approximately the same chemical composition as granite, but differs because it cooled quickly as it flowed out upon the surface.  In California, rhyolite is found in the Mojave Desert, where it forms layered mesas, flat-topped buttes that project hundreds of feet above the desert floor. Rhyolite is viscous or sticky when it flows: therefore, it often has many stones embedded in it, and when it cools it can trap large bubbles of gas.  One popular campground in the Mojave, called Hole-in-the-Wall, gets its name from large holes formed by trapped gas in the rhyolite.

Of the extrusive rocks, obsidian has the smoothest surface.  It is noncrystalline, similar to glass.  It lacks ordered atomic structure: therefore, by definition, there are no minerals in obsidian.  It is an amorphous mixture of the same elements found in granite or rhyolite, but, due to rapid cooling, atoms did not have time to become arranged in an ordered structure.  Black obsidian cooled in the absence of oxygen. Its color is due to nonoxidized (reduced) iron.

Large deposits of obsidian are found east of the Sierra Nevada north of the town of Bishop. A large mountain, prominent on the horizon there, is known as Glass Mountain, and a large butte, known as Obsidian Dome, is found north of Mammoth Lakes.  Obsidian is also found on two buttes south of the Salton Sea.  Obsidian was of great importance to early California Indians, who carried it great distances in order to make their tools.  It was used to make knives, arrowheads, and spear points.  It is not uncommon to find large numbers of obsidian flakes high in the Sierra Nevada many miles from the nearest source.

Mahogany obsidian is brown because its iron is oxidized.  Large amounts of high-quality mahogany obsidian is found in the Warner Mountains, on the eastern edge of the Modoc Plateau.  Although mahogany obsidian is found in limited amounts at other localities, Modoc Indians must have traded obsidian with people farther south because flakes of high-quality mahogany obsidian are found in chipping sites as far south as the Kern Plateau of the southern Sierra Nevada.

Obsidian commonly is found with pumice, a light weight volcanic “froth.”  The gray-colored soil that extends for miles in the Mammoth Lakes area is composed of pumice.  The rock is so light that the wind is able to pick up pea-sized pieces of gravel.  If the gravel lands upon a lake, it floats because of all the air trapped in it.

Bishop, A Guide to Minerals, Rocks, and Fossils, Chapter ?

Bishop, A. C., A. R. Woolley, and W. R. Hamilton. Guide to Minerals, Rocks, and Fossils. Buffalo, NY: Firefly Books Ltd., 2005.

Chemistry of Minerals

It is possible to write a chemical formula to express the composition of a mineral, and such formulae are used as a short way of expressing mineral chemistry. Atoms can conveniently be regarded as electrically neutral because the positive charge on the nucleus is balanced by the negative charges of the surrounding electrons. Atoms can, however, gain or lose one or more electrons and so become either negatively or positively charged, when they are called ions.  Negatively charged ions are called anions and positive ions are called cations.  A chemical compound can be regarded as being made up of two parts, a positively charged cationic part and a negatively charged or anionic part.  The resulting compound is electrically neutral because the two sets of charges are in balance.  The positive part is usually a metal and is always the first part of a written chemical formula.  The negative or anionic part of the formula can be either a nonmetallic ion such as oxygen or sulfur or else a combination of several elements to form a negatively charged group, such as carbonate (CO3) or sulfate (SO4).  The table below lists the chemical symbols of the elements referred to in this book.

Ag                               Silver

Al                                Aluminum

As                               Arsenic

Au                               Gold

B                                  Boron

Ba                               Barium             

Be                               Beryllium
Bi                                 Bismuth

C                                  Carbon

Ca                               Calcium                         

Cd                               Cadmium

Ce                               Cerium

Cl                                 Chlorine

Co                               Cobalt

Cr                                Chromium      

Cu                               Copper
F                                  Fluorine

Fe2+, Fe3+                  Iron

H                                 Hydrogen

Hg                               Mercury

K                                  Potassium

La                                Lanthanum

Li                                 Lithium
Mg                              Magnesium

Mn2+, MN3+, Mn4+   Manganese

Mo                              Molybdenum

N                                 Nitrogen

Na                               Sodium

Nb                               Niobium

Ni                                Nickel            

O                                 Oxygen

P                                  Phosphorus

Pb                               Lead

S                                  Sulfur                         

Sb                               Antimony

Si                                 Silicon

Sn                               Tin

Sr                                Strontium

Ta                                Tantalum     

Th                               Thorium

Ti                                 Titanium

U                                 Uranium

V                                 Vanadium

W                                Tungsten

Y                                  Yttrium

Zn                               Zinc

Zp                               Zircenium

Some common anionic groups and their names are given below.

                        Al2O4 etc                                         Aluminate

                        As, As2 etc                                       Arsenide

                        AsO4 etc                                           Arsenate

                        BO3, B3O4 etc                                Borate

                        Cl, Cl2 etc                                          Chloride

                        CO3                                                    Carbonate

                        CrO4 etc                                           Cromate

                        F, F2 etc                                            Fluoride

                        MoO4 etc                                         Molybdate

                        N, N2 etc                                           Nitrate

                        NO3                                                   Nitrate

                        NbO3 etc                                          Niobate

                        O, O2                                                 Oxide

                        OH, (OH)2 etc                                 Hydroxide

                        PO4 etc                                             Phosphate

                        S, S2 etc                                            Sulfide

                        SiO4, Si2O7 etc                               Silicate

                        SO4                                                    Sulfate

                        TaO3 etc                                           Tantalate

                        TiO3 etc                                            Titanate

                        UO2 etc                                             Uranate

                        VO4 etc                                             Vanadate

                        WO4 etc                                           Tungstate

In chemical formulae the subscript numerals denote the numbers of atoms of the preceding element that are present in the formula unit.  When referring to a chemical compound by name it is simply necessary to state, in turn, the cationic and then the anionic part that follows; for example, CaCO3 is calcium carbonate, FeS2 is iron sulfide, CaF2 is calcium fluoride, (Mg,Fe)SiO4 is magnesium (or) iron silicate, and so on. By contrast KAlSi3O8 is potassium aluminum silicate, or better, potassium aluminosilicate; here there are two parts of the cationic group and they are emphasized in the way shown.  Another example is K2(UO2)2(VO4)23H2O which is called hydrated potassium uranylvanadate.  Notice that water of crystallization (H2O) is referred to by the adjective ‘hydrated’. Atoms which can substitute one for the other in a mineral are written so (Mg,Fe).

Question:  What minerals, on loan from the San Bernardino County Museum, are on display at the Needles Regional Museum?  Prepare a list of them, get there chemical formulae, and group them by anionic group.  Note especially silicates.  Have C. More and others contribute a display of minerals collected in the Tri-State.

Harden, California Geology, Chapter 2

Harden, Chapter 2

California’s Rocks

Because of its long history as an active plate margin, California has an unusual assortment of rocks. Rocks that are common in many parts of the world – such as limestone – are uncommon in much of California, and rocks that are very rare in most places – such as serpentinite – are fairly abundant. Although it is beyond the scope of this book to discuss the formation and classification of rocks in detail, this chapter presents a brief overview of California’s rocks. Later chapters include more detailed discussions of some rock types, and many excellent sources are available for more comprehensive reading.

Even rocks that appear to be completely homogeneous are, in most cases, composed of a multitude of individual grains. These grains can be mineral crystals, pieces of rocks or mineral, fragments of animal or plant remains, or a combination of two or more of these. The types of grains and their proportionate amounts define a rock’s composition. The size and shape, of the grains and the way the grains fit together define its texture. Both the composition and the texture provide important clues about the origin of each rock, which is the reason that identification and classification of rocks form an important part of any geologist’s work. Field study of rocks as they appear in the context of surrounding rocks and the landscape is an important part of the identification process. Closer examination of hand samples with a magnifying lens can aid in the identification of minerals or fossils. Rocks composed of grains too small to be seen with a hand lens require examination with a microscope.

The minerals and textures of a given rock reflect its source, providing clues about either the preexisting rocks or the fluids from which it formed. For a particular mineral to form, the correct chemical “building blocks” must be present. For example, iron oxide cannot form in an environment where oxygen is absent, and quartz cannot crystallize from pure water. Even if the correct ingredients are present, some minerals only form if certain temperatures and pressure exist.  After decades of detailed laboratory studies, geologists also know the temperature and pressure conditions favorable for the formation of most minerals. The presence of certain minerals in a rock reveals whether the rock formed at or near Earth’s surface or thousands of meters beneath it.

Minerals: The Building Blocks of Rocks 

When confronted with the variety and complexity of California’s rocks, a beginning student of geology has a difficult time believing that rocks are made of well-ordered building blocks that are themselves composed of orderly materials.  Rocks are composed of minerals, minerals are composed of one or more chemical elements, and the chemical elements are made up of atoms, the basic building blocks of all matter. It is the variety of possible combinations of elements and minerals and the variety of physical and chemical conditions of formation that create the variety of rock composition. On a larger scale, the hierarchy of Earth’s building blocks continues: bodies of similar rock are grouped together in geologic formations, as discussed in Chapter 4. Rock formations can in turn constitute a distinctive geologic terrane. On a still grander scale, lithospheric plates – some of them aggregates of terranes – are the building blocks of the Earth’s crust.

A mineral is a naturally occurring, inorganic, crystalline solid. A crystalline solid is a material with atoms that are arranged in an orderly, regularly repeating arrangement. Minerals can be composed of atoms of only one element, as in the case with gold nuggets. However, most of the nearly 3,000 known minerals are made up of more than one element combined in fixed proportions in a regular crystalline arrangement. Even minerals that appear to be dirty, bumpy specks are in fact constructed with beautifully ordered atoms of their component atoms.  If minerals grow under ideal circumstance, with adequate space, time, and chemical conditions – the internal order may be reflected in the formation of crystals.

Many people are familiar with the term mineral as it is sometimes used for nutritional supplements, such as iron, calcium, or magnesium. Strictly speaking, these are chemical elements rather than minerals. Only 10 of the 105 known chemical elements make up 99.9 percent of the Earth’s crust, and two – oxygen and silicon – account for almost 75 percent. It is therefore not surprising that the most common minerals contain oxygen and silicon. These minerals are known as the silicate minerals, and they are the building blocks of most rocks. Although more than 600 minerals are found in California, and about 50 of these are found nowhere else, most of California’s rocks are made up predominantly of the common rock-forming minerals.

Some of the rare elements in the Earth’s crust are extremely valuable to society, and rare minerals and rocks that contain these elements have long been sought after by miners. Geologic events have enriched California rocks with pockets of valuable commodities such as gold, mercury, lithium, chromium, boron, and other elements. As we will see in subsequent chapters, mineral resources have always played an important role in California’s history.

The texture of a rock also reflects its conditions of formation. In general, all rocks display one of two general textures. Rocks with a crystalline texture display a network of interlocking mineral grains, indicating that crystals grew together. In contrast, rocks with clastic texture are composed of mineral or rock fragments held together by a cementing mineral. Rocks with a clastic texture are formed at least in part from pieces of older, preexisting rocks or from pieces of once-living organisms.

Igneous, Sedimentary, and Metamorphic Rocks

Igneous Rocks 

Rocks are classified into three major groups, based on the way in which they formed. Igneous rocks form by the solidification of magma, molten silicate material formed by melting or partially melting rocks. Most magma is created at active plate boundaries or hot spots. Igneous rocks are further broken down into two major subgroups: volcanic rocks that crystallize rapidly from lava, which is magma that reaches the surface; and plutonic rocks that crystallize slowly beneath the surface. Beneath the surface, molten magma rises into solid rocks and intrudes into them: for this reason, plutonic rocks are also commonly termed intrusive rocks. As we will see in Chapter 8, not all of the mechanisms of plutonic rock formation are completely understood.

Plutonic rocks crystallize slowly underground because the overlying rocks are good heat insulators. A body of magma might require many thousands of years to completely solidify. In contrast, magma that reaches the Earth’s surface as lava cools very rapidly because heat dissipates rapidly into the atmosphere or ocean.  Because plutonic rocks have so much more time to cool, minerals grow to larger sizes, producing a rock with characteristic coarse-grained texture. Volcanic rocks are characteristically finer-grained, and mineral grains are commonly so small that they can be seen only by using a microscope. Igneous rocks displaying both visible and microscopic minerals – a porphyritic texture – have undergone slow cooling followed by rapid crystallization. In all three of these cases, the igneous rocks display crystalline texture, indicating that crystals grew together. Some volcanic rocks produced during violent eruptions display clastic texture, produced by broken fragments. Lava might also cool so rapidly that the atoms in the magma have no time to form orderly crystals. When this occurs, volcanic glass forms.  Obsidian is the most common type of volcanic glass. Volcanic eruptions and the rocks they produce are discussed more completely in Chapter 5, which includes a classification table for volcanic rocks.

Plutonic and volcanic rocks are further classified according to their mineral composition, which in turn reflects the composition of the magmas that generate them. The relative abundance of silicon and oxygen (silica) and iron and magnesium are the key elements of igneous rock classification. Although there are dozens of volcanic and plutonic rock types, some of which are discussed in subsequent chapters, they can be grouped into general categories. Mafic (sometimes called basaltic) rocks are relatively low in silicon and oxygen (silica) and high in iron and magnesium. It should be noted that mafic magma still contains about 50 percent silica. The term mafic denotes the high content of magnesium and iron, whose chemical symbol is Fe. About 90 percent of all erupted lava is mafic lava. Volcanic rock formed from mafic lava is called basalt, and its plutonic equivalent is gabbro. Both are dark colored because of the high content of dark, mafic silicate minerals. Ultramafic rocks contain only about 40 percent silica. Silicic or felsic magma has a high silica content (about 70 percent) and is low in iron and magnesium. Light-colored minerals dominate in granite, the plutonic rock formed from silicic magma, and rhyolite, its volcanic equivalent.

Sedimentary Rocks 

Sedimentary rocks form at or near the Earth’s surface as accumulation of mineral, rock, and (or) plant and animal fragments.  These sedimentary rock types display clastic (from the Greek word meaning broken) texture.  Other sedimentary rocks from by the crystallization of minerals from groundwater, or from evaporating sea or lake water.  These rocks display crystalline texture, but, in contrast to igneous rocks with crystalline texture, they contain minerals that form at near-surface temperatures and pressures.  Other sedimentary rocks are organic, formed from the remains of living organisms.  It is easier to conceptualize the formation of sedimentary rocks than other rock types, because the processes take place in surface environments, where they can be observed.  In the daily lives people deal with many of the agents that transport sediment, including rivers, waves, and wind.  The processes that form marine sedimentary rocks (those formed in the ocean) are less familiar, but they can be directly observed by oceanographers.

All rocks and minerals at the Earth’s surface are affected by weathering.  Chemical reactions between minerals and water can decompose rocks or cause new minerals to form.  Minerals can also be chemically changed by interactions with the atmosphere and water.  One example of chemical weathering is the formation of iron oxide on the surface of rocks.  The outside of a freshly broken piece of rock almost always appear more red or brown than the inside, because iron oxide has formed on the exposed surface.  Acting together with chemical weathering, physical processes cause rocks to break into smaller pieces.  Freezing and thawing, crystallization of salt, and the growth of roots are examples of physical weathering processes that disintegrate rocks.  Over geologic time, weathering weakens even the most solid rocks.  Massive rock disintegrates into smaller pieces available for transport by water, wind, or glaciers.  After decomposition and disintegration by weathering, rock materials are removed by erosion.

Sedimentary rocks are usually layered.  The layering, or bedding, results from the successive accumulation of sediments over time.  Sedimentary beds, each of which generally represents a single depositional event such as a windstorm or a flood, can be millimeters or meters thick.  The bedding is visible because of differences between layers.  An interval of no deposition or weathering between successive deposits also produces layering.  Volcanic rocks can also be layered, but these are easily distinguished from sedimentary layer because they are composed of volcanic materials.
 

When classifying sedimentary rocks, geologist are most interested in determining the environment where the sediments were originally deposited.  In the case of clastic rocks, the nature of the sediment reflects the energy conditions in the environment where it accumulated.  One important diagnostic characteristic of sediment and sedimentary rocks is the grain size of the particles in the rock.  For example, in high-energy environments like steep mountains streams, huge boulders might be part of the river channel.  In contrast, in the almost flat channels of a delta, only the finest mud is found.  Other important features of sediments and clastic sedimentary rocks are their sorting and rounding.  Sediments that are will sorted contain particles of about the same size and density, whereas those that are poorly sorted contain particles of many different sizes.  The rounding of grains is another measure of how well-traveled and tumbled the grains are.  Sand dunes and beaches are the two environments that produce highly rounded and well-sorted sediments.

Using all of these characteristics, together with the composition of the grains and the type of fossils present, geologist are able to determine with some certainty the environments where sedimentary rocks originally formed.  For, example, a shale might have been deposited in a coastal lagoon.  Even though the lagoon and its surrounding landscape have long since vanished, the rock preserves key evidence about the size, location, water chemistry, and surrounding of the environment.

Other distinctive features could indicate a sedimentary rock’s environment of deposition very precisely.  For example, mud cracks are evidence that the surface of the sediment experienced episodic wetting and drying, indicating an environment such as a tidal flat or intermittently flooded lake bed.  Other sedimentary features that aid interpretation of depositional environments include ripple marks, channels, cross-beds, mud lumps, and, of course, fossils.  California fossils will be discussed further in Chapter 3.

Sedimentary rocks can also provide important information about the tectonic setting of a region.  The sudden appearance of coarse-grained sediments in a section of rocks might indicate that uplift has occurred in nearby areas, causing an influx of sediment to be shed into nearby lowlands.  If the sediments contain distinctive rock or mineral types, it might be possible to identify their geologic source with some precision, even if the sediments have been displaced from their sources by faulting.  Indicators of current directions in the sediments might identify the direction of sediment transport.  Through careful study of sedimentary rocks, geologist are thus able to identify the precise location of newly uplifted mountain ranges long after those mountains have been eroded away.  Reconstruction of the ancient geography or paleogeography of an area can, in turn, provide clues about past tectonic events.

Metamorphic Rocks

Metamorphic rocks have been subjected to enough heat and pressure to cause grains to grow together, but not enough to melt them.  During metamorphism, hot fluid containing reactive chemicals can also circulate through rocks and substantially change their composition.  In classifying metamorphic rocks, geologist are particularly interested in interpreting the degree of metamorphism and the evidence of stresses during metamorphism.  By reconstructing the temperature and pressure conditions that existed during metamorphism, geologist can estimate how deep beneath the Earth’s surface the rocks were when they were metamorphosed.

One type of metamorphism occurs when magma intrudes into a body of rock.  The unmelted rocks surrounding the magma are heated.  This type of metamorphism is known as contact metamorphism because it affects rocks that are in contact with igneous intrusions.  Hot fluids can also be released into rocks, reacting with them to form new minerals; this process is known as hydrothermal alteration.  One important result of contact metamorphism is the concentration of valuable mineral deposits during hydrothermal alteration.
 

Most metamorphic rocks form during regional metamorphism.  Large areas are subjected to high temperature and pressure when rocks are intensely deformed.  The most obvious setting for regional metamorphism is along a convergent plate boundary, where rocks are buried to great depths in a subduction zone.  An important characteristic of regional metamorphism is that rocks are subjected to stresses that are not equal in all directions.  Rocks that are compressed by directed stress during metamorphism develop a foliated texture.  Foliation is a distinctive layered texture that develops during metamorphism.  If rocks are subjected to stress that is stronger in one direction, then new metamorphic minerals will crystallized in a preferred orientation.  It is the alignment of metamorphic minerals that produces foliation.

As the degree of metamorphism increases, rocks are subjected to higher temperatures and directed stresses.  As a result, more highly metamorphosed rocks show more obvious foliation.  The obvious layering of schist results from the growth of platy silicate minerals like mica, chlorite, or talc during metamorphism.  Less metamorphosed slate is also foliated, but the new metamorphic minerals are very small.  Foliated metamorphic rocks can also contain other silicate minerals formed only by metamorphism, such as garnet, kyanite, or staurolite.  Gneiss is a banded metamorphic rocks composed mostly of mica, feldspar, and quartz, indicating a very high degree of metamorphism.

Non-foliated metamorphic rocks commonly display a grainy texture caused by the intergrowth of minerals with no preferred alignment.  Metamorphism of limestone and dolomite produces larger crystals of calcite and dolomite with a characteristically sugary texture – the metamorphic rock marble.  Metamorphism of shale without directed stresses produces a hard, dark-colored, non-foliated rock called hornfels.  California’s metamorphic rocks are discussed in more detail in Chapter 8 and Chapter 12.

The Rock Cycle

The rocks that we find at any given location have often experienced a complex history of transformations as they have been subjected to the forces that shape the Earth. Throughout all but its earliest history, when major extraterrestrial impacts were frequent, the mass of the planet has changed very little. Only a tiny amount of new material has been added to the Earth by collision with meteors.  The endless creation of new rocks and the destruction of old rocks result from continuous recycling of rock material because of plate tectonics. The transformation from one rock type to another is referred to as the rock cycle.  The concept of the rock cycle conveys the idea that a rock can be changed over geologic time. Although it is useful to use a diagram to visualize the rock cycle, the diagram fails to convey the almost endless possibilities created as rocks are subjected to geologic forces over million, or even billions of years.
 

Rocks are melted when they are subducted to depths where temperatures are sufficiently high. Also because of plate tectonics or at isolated hot spots, magma rises and cools to form new igneous rocks. Rocks that are buried to great depths without melting can be changed to metamorphic rocks. As mountains are uplifted, they are worn down as rocks are weathered, eroded, transported, and deposited to form sediments. These sediments can then be gradually buried by younger sediments, eventually becoming sufficiently compacted and cemented to form sedimentary rocks ( the process of lithification).

By identifying the sediments that make up a sedimentary rock such as a sandstone, geologist might be able to decipher the sandstone’s ancestor rock.  For example, 140 million-year-old sandstone found today in the Great Valley contains abundant fragments of volcanic particles, suggesting that the source of the sediments were volcanoes. The type of volcanic particles in the sandstone might even enable the geologist to infer what type of rocks were melted to generate the magma that in turn created the volcanoes. Using the principles of the rock cycle to trace the history of a rock back through geologic time is not unlike tracing a family’s genealogy. In many cases, the “parents” of rocks can be identified, but their lineage becomes very sketchy beyond that – not surprising considering the time spans involved!

It is important to realize that rocks can be transformed by many different pathways. For example, a volcanic rock can be transformed directly into a metamorphic rock – specifically a metavolcanic rock in this case – by subjecting it to sufficient heat and pressure. Belts of slate and schist in the western foothills of the Sierra Nevada and the Klamath Mountains were originally layers of volcanic rock formed from lava erupted from submarine volcanic chains. Any type of rock can be melted to form magma. As we will see in Chapter 5, lavas formed by melting oceanic rocks are recognizably different from lavas formed by melting continental rocks.

All of the rocks in California’s landscape are subjected to erosion and weathering.  In Death Valley, sand dunes contain quartz grains that were eroded from sedimentary rocks that formed hundreds of millions of year ago. Some of those ancestral rocks were themselves deposited in ancient sand dunes several hundred million years ago. The sediments carried by modern rivers from the high Sierra to San Francisco Bay might someday become sandstone. That sandstone will carry compositional clues that it was eroded from the Sierra Nevada – mountains of mostly granitic rock – even if the mountains themselves have been completely eroded away. Any rock has almost endless possibilities of becoming a very different type of rock in its geologic future. The only certainty is that, if a rock melts to become magma, the magma can only be transformed to an igneous rock.

Common Rocks in California

Igneous Rocks

Igneous rock types common in California include the following:

Granitic plutonic rocks. Granitic rocks make up much of the Sierra Nevada Province. Smaller granitic bodies are found in the Klamath Mountains, in the Coast Ranges west of the San Andreas fault, in Mojave Desert, and in the Peninsular Ranges of southern California.  Types of granitic rocks and their origins are discussed in Chapter 8.
 

Mafic and ultramafic plutonic rocks. Serpentinite, which is actually a metamorphosed ultramafic rock, is included in this unit. Geologist believe that these rocks formed in the lower crust and upper mantle beneath oceanic plates. These rocks are discussed further in Chapter 9.

Intermediate and silicic volcanic rocks. Young volcanic rocks cover most of the Cascades Province, are abundant east of the Sierra Nevada, and are also found in the northern Coast Ranges. In these three areas, young volcanic rocks include large deposits of rocks with intermediate and high silica contents. California’s volcanoes and volcanic rocks are discussed further Chapter 5.

Mafic volcanic rocks. Recently erupted mafic volcanic rocks, mostly basalt, are most common in California in the Modoc Plateau, the Mojave Desert, and in the Basin and Ranges of southeastern California.

Basalt covers more of the Earth’s surface than any other type of rock.

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Even these metavolcanic rocks can show well-developed rounded forms called pillows, formed when lava entered the ocean and congealed into blobs. Studies have confirmed that many of California’s metavolcanic rocks formed in ancient midocean ridges, seamounts, and island arcs and were then accreted to the North American continent.  Mafic volcanic rocks are discussed further in Chapter 5 and Chapter 12.

Sedimentary Rocks

On the California geologic map, sedimentary rocks are grouped according to their age.  From youngest to oldest, the sedimentary rocks are colored tan, green, and blue.  California’s metamorphic rocks are grouped with the metasedimentary and metavolcanic rocks of Mesozoic (green) and Paleozoic (blue) age.  Serpentinite is included with ultramafic rocks (purple), and highly metamorphosed rocks of unknown or pre-Paleoazoic age are shown in dark brown.

Sedimentary rocks common in California include the following:

Sandstone.  Sandstone is found in all provinces of California.  The majority of California’s sandstone was originally deposited in marine environments.  Much of the sandstone in California is gray or brown and contains chips of shale and other rocks.  Because of its location along a steep continental margin, California has produced these types of “dirty” sandstones rather then quartz-rich sandstones.  The deposition of these sandstones, called greywacke, is discussed further in Chapter 12.

Shale.  Dark gray or black marine shale is also very common in California and usually is found together with sandstone (see Chapter 12).  Metamorphosed sandstone and shale, examples of metasedimentary rocks, are common in the Sierra Nevada, Klamath Mountains, and other provinces.
 

Chert.  Chert is abundant in the Coast Ranges and can be black, red, green, brown, or white.  Much of California’s chart formed by the slow settling of microorganisms called radiolaria onto the floors of the deep ocean basins.  It is commonly found together with submarine volcanic rocks.  Chert is also found in the Klamath Mountains, the Transverse Ranges, and the metamorphic rocks of the Sierra Nevada.  Chert is discussed further in Chapter 3 and Chapter 12.

Limestone and dolomite (carbonate rocks).  Carbonate rocks are common only in the mountain ranges of the Basin and Ranges and Mojave Desert provinces.  Metamorphosed carbonate rocks (marble) are also common in the eastern Klamath Mountains and parts of the Sierra Nevada.  Limestone also occurs as small blocks within the Coast Ranges and in the mountains of southern California.  The formation of these rocks is discussed further in Chapter 7.

Metamorphic Rocks

Metamorphic rocks common in California include the following:

Serpentinite.  This is California’s state rock, commonly found along faults in the Coast Ranges, Klamath Mountains, and Sierra Nevada.  Serpentinite forms when rocks originally formed as ultramafic rocks in the mantle are hydrated (see Chapter 9).

Slate.  Slate is abundant in the western Sierra Nevada foothills.  It was originally mafic volcanic rock, shale, or sandstone that changed during metamorphism (see Chapter 8).

Schist.  Schist occurs in the Santa Monica, San Bernardino, and San Gabriel Mountains in southern California, in the Sierra Foothills, and in the Klamath Mountains.  Like slate, many of the schists in California were originally sedimentary or volcanic rocks.  A distinctive type of schist, blueschist, is found in the Coast Ranges.  Metamorphic rocks and their significance are further discussed in Chapter 8 and Chapter 12.

Marble.  Marble, metamorphosed limestone or dolomite, is relatively rare in California and occurs in blocks within the high Sierra Nevada, in the northwestern Sierra Foothills, and in the Klamath Mountains.

Unconsolidated sediments are recent deposits that have not been buried, compacted, and cemented to form sedimentary rocks.  These sediments cover virtually all of the low-lying areas of California and are shown in yellow on the California geologic map.  Most of these sediments are alluvium, sediments deposited by rivers.  The processes that deposit nonmarine sediments are discussed further in Chapter 6, 10, and 11.  The processes of marine and beach deposition along the California coast are discussed in Chapter 15 and Chapter 16.
 

Glossary for Chapter 2

serpentine  1.  A green, platy, hydrated silicate mineral containing magnesium.  2.  Internal name for metamorphic rocks composed of serpentine; syn. serpentinite,

composition  The type and proportion of constituent elements or minerals in a rock..

texture  The shape, size, and orientation of particles in a rock, and the way they fit together.

minerals  A naturally occurring, inorganic, crystalline solid composed of one or more chemical elements.

formations   A recognizable geologic unit that can be identified and mapped.

crystalline  The state of matter in which atoms are arranged in a regular, repeating pattern.

silicate   A mineral belonging to the group that contains the elements silicon and oxygen bonded together in a tetrahedron.

clastic   Rock texture in which fragments are held together by a cementing mineral.

igneous  Fire-formed; rocks crystallized from cooling magma.

magma Molten silicate material.

volcanic  Rocks crystallized from magma at the earth’s surface.

lava  Molten silicate material (magma) at the Earth’s surface.

plutonic Rocks crystallized from magma beneath the earth’s surface; syn. intrusive

intrusive  Igneous rock that solidified from magma below the earth’s surface; also called plutonic rock.

porphyritic  Texture describing igneous rocks with some large, early formed crystals in a mass of finer grained crystals or glass.

mafic Term describing igneous rocks or minerals with proportions of iron and magnesium and relatively low silica; syn. basaltic.

basalt  Mafic volcanic rock.

  

gabbro  Mafic plutonic rock.

ultramafic  Describes igneous rocks with very high iron and magnesium and low (about 40%) silica.

silicic  felsic  silica  A term describing igneous rocks or minerals with high proportions of silica and relatively low iron and magnesium; syn. falic.

granite  Silicic plutonic rock.

rhyolite  Silicic volcanic rock.

weathering  The physical disintegration and chemical alteration of rocks at the earth’s surface.

chemical weathering  The chemical decomposition of minerals and rocks at the earth’s surface.

physical weathering  Disintegration of rocks into smaller pieces through physical processes such as freezing and thawing, crystallization of salt, or the growth of roots.

erosion  The removal of solid particles and irons in solution from an area at the surface.

bedding  The layering in sedimentary rocks

grain size  Refers to the size of individual mineral crystals or particles within a rock or sediment deposit.

sorting  A term used to describe how similar in size the particles in sediment or sedimentary rock are.

rounding  One measure of how well-traveled and tumbled grains of sediments are; refers to a lack of sharp edges.

paleogeography  The ancient environments and landscapes reconstructed, from the types and location of rocks found in an area.

 contact metamorphism  Changes in rocks adjacent to intruding magma.

hydrothermal alteration  The changing of minerals and crystallization of new minerals caused by the chemical action of groundwater, particularly at hot springs and geysers.

regional metamorphism  Changes in rocks over a large scale in response to major magmatic and/or tectonic events.
 

foliated, foliation  Sheet-like layering in metamorphic rocks that results from the growth of minerals in a preferred orientation.

schist  Strongly foliated metamorphic rock with visible platy minerals.

chlorite  Green silicate mineral with sheet structure, formed during low-grade metamorphism.

talc  Silicate of magnesium common among metamorphic minerals, greasy, and extremely soft due to sheet structure.  Sometimes known as soapstone.

slate  Fine-grained, foliated metamorphic rock that breaks into thin, regular layers.

gneiss  High-grade metamorphic rock with characteristic light and dark banding.

marble  Non-foliated metamorphic rock formed from carbonate sedimentary rocks.

hornfels  A high-temperature, low-pressure metamorphic rock of uniform grain size showing no foliation.  Usually formed by contact metamorphism.

rock cycle  A conceptual model used to illustrate the origins of and the relationships between earth materials.

lithification  The process by which sediments become sedimentary rocks.

metasedimentary  Sedimentary rocks transformed by heat and pressure.

radiolarian   A single-celled marine organism whose tests are composed of silica; pl. radiolaria.

unconsolidated sediments  (unconsolidated material).  Nonlithified sediment that has no mineral content or matrix binding its grains.

alluvium  sediment deposited by a river.