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.