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.
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