GENERAL GEOLOGY (GEOL 1113)

STUDY GUIDE FOR EXAM III

The following study guide is provided as an aid to help you identify the major concepts you should have learned concerning WEATHEIRNG (Ch. 5), SEDIMENTARY ROCKS (Ch. 6), METAMORPHIC ROCKS (Ch. 7). Key vocabulary with which you should be familiar is highlighted in GREEN UPPER CASE throughout this document and on future documents of this type for the course. Additionally, some key words are highlighted in blue and underlined which means they contain hyperlinks to additional information of interest. Clicking on the blue words will transport you to various Internet locations or on-line images to enhance your studying. You should, at the very least, be able to define the highlighted terms in order to complete the exam. Ideally, however, I hope you will be able to do more than simply respond to definitions. I would like you to learn to be able to apply the definitions and the concepts they represent to a fuller understanding of the Principles of Geology and Earth as a planet by visiting the added hyperlinks.

GENERAL BACKGROUND

We have learned some very basic facts about the different types of rocks which are found on Earth. We now are aware that there are 3 major types of rocks: IGNEOUS ROCKS, SEDIMENTARY ROCKS, and METAMORPHIC ROCKS. For each of these rock types, the name given to a specific rock depends on the minerals that compose the rock. However, the processes which give rise to those mineral associations are very different.

Each of these major rock types forms through processes which dominate in different geological environments on Earth. IGNEOUS PROCESSES are those processes related to the MELTING of rocks to form magma and re-CRYSTALLIZATION or SOLIDIFICATION of magmas to form igneous rocks.

SEDIMENTARY PROCESSES are those processes related to the WEATHERING, transport, deposition and cementation (or LITHIFICATION) of pre-existing rocks exposed at Earth’s surface. In some instances, weathering is a mechanical process which simply breaks larger rock masses into smaller rock masses. In other instances, weathering is a chemical process which alters the basic minerals of a pre-existing rock to produce a new suite of minerals (and thus a new rock because rocks names are defined by the aggregate of minerals which compose them).

Finally, METAMORPHIC PROCESSES are those which act on pre-existing rocks to change their mineral composition thereby resulting in a new rock composed of new minerals. It is important to note that METAMORPHISM is a process where an existing rock type (igneous, sedimentary or metamorphic) is transformed in the SOLID STATE to a new rock type.

WEATHERING AND WEATHERING PROCESSES: Chapter 5

Rocks exposed at the Earth’s surface are constantly being altered by wind, rain, blowing dust, changing temperatures and other environmental factors. Taken as a whole, those processes which act to degrade rocks exposed at the Earth’s surface are known as WEATHERING.

Weathering processes are subdivided into two categories: MECHANICAL WEATHERING and CHEMICAL WEATHERING.

Mechanical weathering refers to any process which acts to break large rocks into smaller rock fragments. The most important types of mechanical weathering are FROST WEDGING, ROOT WEDGING, SALT WEDGING, and ABRASION (or MECHANICAL BREAKAGE)

FROST WEDGING is most prevalent in temperate climates where daily freeze-thaw cycles are most pronounced. The freezing of water in small fractures within a rock mass will exert tremendous force as the ice expands. Over time, repeated freezing and thawing will widen fractures and may lead to disintegration of the rock mass into large angular blocks called TALUS.

ROOT WEDGING is a process similar to frost wedging in that plant seeds may come to rest in small cracks in large rock masses. As the plant grows, its roots will progressively widen the crack and can actually exert enough force to split the rock mass apart, again forming piles of angular blocks called talus.

SALT WEDGING occurs in arid regions where alternating wetting and evaporation might be common. Water seeps into cracks in rocks and as it evaporates, precipitates dissolved salts of various kinds. As these salt crystals grow, they also exert tremendous force which can split rocks.

CHEMICAL WEATHERING processes also act to degrade rocks exposed on the Earth;s surface. However, rather than simply breaking large rocks into smaller fragments (as observed with mechanical weathering processes), chemical weathering will alter the composition of exposed rocks to form new minerals or rocks.

Among the important agents of chemical weathering are DISSOLUTION, OXIDATION, and HYDROLYSIS.

DISSOLUTION is the process whereby rock material is dissolved in water. Water is a very reactive substance on Earth and will dissolve virtually everything to some degree. Weathering by dissolution is most pronounced in areas underlain by a sedimentary rock known as limestone. Limestone is composed primarily of the mineral, calcite (CaCO₃), and is readily dissolved by water which contains naturally occurring acids (such as carbonic acid or various organic acids produced by the decay of organic matter).

Extensive dissolution of limestone leads to formation of CAVES.

OXIDATION is the reaction of rocks exposed at the Earth's surface with oxygen in the atmosphere. This reaction alters the chemistry of rocks, producing new minerals (commonly of iron oxide).

HYDROLYSIS is a complicated series of chemical reactions involving water and exposed rocks. Through several steps, water will react with exposed rocks to liberate a variety of CATIONS (such as potassium, sodium, calcium, magnesium), altering rock-forming minerals like feldspar to CLAY and SILICA (silicon dioxide, or quartz).

Most chemical reactions proceed more rapidly at higher temperatures and may require the presence of water. Thus, effects of chemical weathering processes are most pronounced in the tropics, where average temperatures are high and there is an abundance of water.

SEDIMENTS AND SEDIMENTARY ROCKS: Chapter 6

The products of both mechanical and chemical weathering are SEDIMENTS. Through the processes of TRANSPORT, DEPOSITION, COMPACTION and LITHIFICATION these particles of degraded rock can become SEDIMENTARY ROCKS.

Sediments (the weathered residuum of other rocks) accumulate at the Earth’s surface. As such, these accumulations of unconsolidated debris may be TRANSPORTED by moving water and wind. Depending on the size of the particles and the transporting medium, some sediments can be transported great distances from their source.

Sediment particles are found in a broad spectrum of sizes ranging from BOULDERS to CLAY. Geologists have developed a classification system for sediment sizes similar to that used to classify igneous rocks.

This classification scheme depends on the particle diameter, or GRAIN SIZE of the individual sediment particles.

CLASTIC SEDIMENTARY ROCKS
GRAIN SIZE (mm)	PARTICLE NAME	SEDIMENT NAME	ROCK NAME
larger than 256	BOULDER	GRAVEL	CONGLOMERATE ( if rounded) BRECCIA (if angular)
256 - 64	COBBLE	GRAVEL	CONGLOMERATE ( if rounded) BRECCIA (if angular)
64 - 2	PEBBLE	GRAVEL	CONGLOMERATE ( if rounded) BRECCIA (if angular)
2 - 0.062	SAND	SAND	SANDSTONE
0.062 - 0.004	SILT	SILT	SILTSTONE
less than 0.004	CLAY	CLAY	SHALE

Sediments composed of fragments of weathered rock listed above are referred to as CLASTIC SEDIMENTS and the sedimentary rocks they form are CLASTIC SEDIMENTARY ROCKS.

Another class of sedimentary rocks are those which form through PRECIPITATION of various minerals from solutions. Sedimentary rocks formed from CHEMICAL PRECIPITATES are known as CHEMICAL SEDIMENTARY ROCKS.

Among the more common chemical sedimentary rocks are LIMESTONE, CHERT and EVAPORITES (such as ROCK SALT and ROCK GYPSUM).

Because sedimentary rocks form near the Earth’s surface by surface processes, they typically preserve clues to both their ENVIRONMENT OF DEPOSITION and EARTH SURFACE CONDITIONS during the time they formed.

Among the important clues geologists use to interpret sedimentary rocks are SEDIMENTARY STRUCTURES. These are features found in sedimentary rocks which form during the deposition of the sediment and reflect the nature of the environment into which the sediments are deposited.

The most obvious sedimentary structure in sedimentary rocks is BEDDING - the occurrence of sedimentary rocks in successive layers stacked one on top of the other. Because most sediments are deposited by fluids, they are usually deposited in HORIZONTAL LAYERS.

CROSS-BEDDING is a special form of bedding where sediments may be deposited in a non-horizontal fashion. Cross-beds in sedimentary rock can be used to determine the direction the current which deposited the sediment was flowing.

GRADED BEDDING is a layer in which the grain size of particles changes vertically through the layer. Often, one will find a thin layer where the base of the layer contains abundant gravel, but this gravel gradually gives way to sand, then silt, then, possibly, clay-size particles. Such deposits are indicative of relatively rapid deposition in a fluid under the influence of gravity.

RIPPLE MARKS often form on the top of sediment layers as the material is being deposited. The style and orientation of these ripple marks may also be used as an indicator of which direction the fluid was flowing at the time the sediment was deposited.

In some arid environments characterized by brief, episodic rain storms, MUD CRACKS will form in sediments as the water evaporates following torrential rain. These mud cracks can be preserved and provide a useful indicator of Earth environment in rocks that can be 100’s of millions of years old.

Sedimentary rocks may also be host to FOSSILS, preserved remains of once living organisms. Fossils are particularly useful as indicators of environment.

METAMORPHIC ROCKS: Chapter 7

As we have already seen, temperature rises as one descends into the Earth's interior. This rate at which temperature rises with depth in the Earth is called the GEOTHERMAL GRADIENT. While this gradient varies somewhat from place to place across the Earth, it averages about 30^oC PER KILOMETER of depth down to about 100 KM DEPTH.

This is a diagram of the GEOTHERM (i.e. the increase in temperature with depth in the Earth). The slope of the GEOTHERM is called the GEOTHERMAL GRADIENT (which is the CHANGE IN TEMPERATURE PER KILOMETER). Note that the geothermal gradient is very large between the surface and 100 km depth. Below 100 km, the geothermal gradient is only about 1oC per kilometer.

Note that the PRESSURE surrounding rocks in the Earth’s interior also increases very rapidly with depth.

It is the combined effects of both HEAT and PRESSURE that causes pre-existing rocks to be METAMORPHOSED. Any type of rock is subject to metamorphism under the appropriate conditions - the parent or precursor rock type might be an igneous rock, a sedimentary rock or even another type of metamorphic rock.

Generally, geologists consider the temperature range of metamorphism to be from 200^oC up to the point where a rock begins to melt (about 800^oC).

Point to ponder: Why is rock melting the cut-off point for metamorphic processes?

Metamorphism is a CONTINUUM PROCESS, meaning that TEMPERATURES & PRESSURES ASSOCIATED WITH METAMORPHISM occur over a broad range from the lowest temperatures and pressures of metamorphism to the highest (near the melting point of rocks).

We can create a "map" of metamorphic environments by plotting a diagram of TEMPERATURE versus PRESSURE.

In the diagram above, TEMPERATURE is plotted against PRESSURE (which can be translated to DEPTH in the Earth). The field of the diagram is further subdivided into 9 regions showing the relative temperature and pressure conditions in each field. Note the temperature increases to the right and pressure increases downward.

Point to ponder: In the illustration above, are the boundaries between the different metamorphic regions represented better by the white lines or the gradational color pattern? Why?

In addition to heat, the other major agent of metamorphism is PRESSURE. Pressure increases with increasing depth as a consequence of the mass of overlying rocks. For a rock at a given depth beneath Earth’s surface, the pressure is distributed uniformly in all directions around the rock and is referred to as CONFINING PRESSURE.

Pressure is measured in units called BARS. At sea-level, the pressure created by the Earth's atmosphere is about 1 bar. Note in the diagram above that pressure within the Earth is measured in KILOBARS. 1 kilobar is 1000 times the pressure we experience at sea-level.

In some geological settings, local or regional conditions may lead to pressure in a preferred direction over and above the confining pressure being exerted on the rocks. Such a pressure field is referred to as DIRECTED STRESS. Directed stress is an important agent of metamorphism, and many metamorphic rocks retain features which allow geologists to interpret the stress field which surrounded the rock during it’s formation.

Directed stress applied to rocks will result in FOLIATION - an apparent "layering" observable within the rock that is caused by PREFERENTIAL ALIGNMENT OF MINERAL CRYSTALS in the rock. Metamorphic rocks which display foliation are referred to as FOLIATED METAMORPHIC ROCKS whereas rocks which do not display foliation are referred to as NON-FOLIATED METAMORPHIC ROCKS.

Using the Temperature-Pressure diagram, we can examine likely pathways (or TRAJECTORIES) of metamorphism on Earth.

The arrows on this diagram illustrate known pathways or TRAJECTORIES of metamorphism on Earth. The magenta trajectory is that associated with CONTACT METAMORPHISM. The green trajectory is associated with BURIAL METAMORPHISM. The red trajectory is that which occurs during REGIONAL METAMORPHISM.

CONTACT METAMORPHIC rocks form from the HEAT associated with igneous intrusions. Rocks which are in contact with the margins of the magma body will experience relatively high temperatures causing metamorphism.

Though magmas are hot they do not ordinarily induce directed pressure fields, so contact metamorphic rocks are NON-FOLIATED, being affected primarily by the heat of the nearby magma. Note in the diagram above that contact metamorphism occurs at relatively low pressure.

BURIAL METAMORPHISM is responsible for generating the largest quantity of metamorphic rocks. It occurs as rocks are naturally buried within the Earth. As rocks are buried, both temperature and pressure will increase, following the indicated trajectory on the diagram.

Since pressure (that is, DIRECTED STRESS) is an important component of BURIAL METAMORPHISM, these rocks are typically FOLIATED.

REGIONAL METAMORPHISM is associated with PLATE TECTONICS and large-scale MOUNTAIN BUILDING events which have occurred episodically through Earth’s history (such as those responsible for the Appalachians, Urals, Alps, Himalayas, and Ouachitas; see section below on PLATE TECTONICS for details).

REGIONAL METAMORPHISM almost always involves generation of extreme directed stress fields and, therefore, the metamorphic rocks generated by tectonic metamorphism are always FOLIATED.

The increasing heat and pressure of metamorphism yields a variety of metamorphic rocks. So called LOW-GRADE METAMORPHIC ROCKS are only slightly altered from their parent rock, and retain features which allow the parent rock to be identified. SLATE is a typical low-grade metamorphic rock formed when the sedimentary rock, SHALE, is subjected to low grade metamorphism.

PHYLLITE and SCHIST are common types of INTERMEDIATE GRADE to HIGH GRADE METAMORPHIC ROCKS.

HIGH-GRADE METAMORPHIC ROCKS are those which have been subjected to extreme metamorphism, rendering them unrecognizable from their parent rock. GNEISS is an example of a high-grade metamorphic rock.

A typical sequence of metamorphism from LOW GRADE to HIGH GRADE might be as follows:

TEMPERATURE	LOW				HIGH
PRESSURE	LOW				HIGH
ROCK TYPE	SLATE	PHYLLITE	SCHIST	GNEISS	MIGMATITE

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View an animation illustrating the transformation of shale during progressive metamorphism by clicking this button.

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