REVIEW OF PHYSICAL GEOLOGY

Sedimentary and Metamorphic Rocks

Weathering and Sedimentation

When rocks are exposed on the surface, they are subject to mechanical and chemical weathering. Mechanical weathering involves such things as freeze-thaw action and abrasion of rocks and minerals by materials carried in streams. Chemical weathering is the result of chemical reactions between the minerals in the rocks and air, water, and the chemicals dissolved in water (notably carbonic acid, which forms when carbon dioxide dissolves in water).

Some minerals are more resistant to weathering than others, and these minerals are more likely to be found in clastic sediments. (Clastic sediments are those composed of particles ranging from clay-sized to boulders. Each "particle" is called a clast.) With a longer period of weathering, the less-resistant minerals tend to disappear. Only the most resistant minerals are left in sediment that has been extensively weathered. Here is a list of some important minerals and how they weather.

• Calcite. Easily weathered in moist conditions. This property is the reason caves and sinkholes are common in limestone, which is largely composed of calcite. Dissolves to calcium and bicarbonate ions.

• Quartz: Very resistant to weathering, partly due to the fact it has no cleavage planes (planes along which you can split the crystal) and partly due to its chemically inert structure. The grains often are the main constituent of sandy beaches and dunes, because the other minerals have largely been eliminated by weathering.

• Feldspars: Weather more easily than quartz, partly because feldspars have two cleavage planes. They weather to form clay minerals, which are highly resistant to further weathering.

• Ferromagnesian silicates: Are sources of of metal oxides when weathered (essentially the same thing as when iron rusts).

Sedimentary Rocks

There are two (or three, depending on how you look at it) types of sedimentary rocks:

• Clastic sedimentary rocks: These rocks consist of particles of minerals and rock fragments that have been solidified (lithified) into rock. Lithification requires compaction (by burial) and cementation (the gluing together of the particles). The most important cements are silica (silicon and oxygen), calcite, and iron oxides (such as hematite).

• Chemical/Biological sedimentary rocks: These are rocks that have been precipitated from water by either chemical or biological means. As a result they are often rock hard as soon as they form.

Clastic Sedimentary Rocks

These rocks are classified on the basis of the size of the clasts that comprise them. The following list names these sizes and a gives a brief description.

• Clay - microscopic in size. Rocks made entirely of clay are very smooth to the touch.

• Silt - mostly microscopic, but gives rocks a gritty texture you can feel when you rub them on a fingernail or tooth.

• Sand - is exactly what you think it is (although it does have an official size range, which isn't important here).

• Gravel - also essentially what you think it is.

• Cobble - a size you can hold in your hand (or maybe two for the larger ones) but larger than gravel. (You wouldn't want to drive on a loose cobble road.)

• Boulder - again, mostly a self-evident definition.

The following lists some major clastic sedimentary rocks, briefly describes them, and tells what their likely depositional environment was. The main factor that determines what size of clast is deposited by water is the speed or degree of agitation of the water. Geologists refer to this as low-energy vs high-energy environments.

• Claystone - a sedimentary rock made up almost entirely of clay. Deposited in quiet water (very low energy) such as in lakes, floodplains, tidal flats, or in seas far from shore.

• Shale - also composed of clay but can be split along planes of weakness. Also deposited in very low-energy environments.

• Siltstone - a rock composed mostly of silt-sized particles. Deposited in rather low energy environments such as lakes, floodplains, tidal flats, and seas far from shore.

• Mudstone - made of clay and silt (mud). Tidal flats, lakes, etc.

• Sandstone - predominately made up of sand-sized particles and comprise a wide variety of rocks laid down by water and/or wind. Deposited in fairly energetic environments (beaches, alluvial fans, submarine fans, dunes, streams, etc.).

• Conglomerate - composed of gravel-sized clasts and larger. The clasts range from very rounded to somewhat rounded (due to weathering) but do not have sharp angles. They were laid down by streams or along beaches with strong wave action.

• Breccia - like a conglomerate but with angular to very angular clasts. A breccia indicates a rather violent event, such as a landslide, either on land or beneath the sea.

What Clastic Sedimentary Rocks Tell the Historical Geologist

Three properties of clastic sedimentary rocks are particularly revealing.

Clast size: The first products of weathering are often large clasts, and these tend to be deposited near their place of origin while smaller clasts are washed away. Hence, for example, the course of an ancient river might be retraced by following its clasts to see in which direction they get progressively smaller. Also, the size of the clast indicates the energy of the depositional environment.

• Sorting: A rock with its clasts all close to the same size is termed well-sorted. Well-sorted rocks indicate a long period of working (such as by waves) and/or transport by wind or water. A rock with a wide range of clast sizes is termed poorly sorted. It indicates just the opposite; that is, that the sediment was not transported far and not extensively worked. Generally speaking, the smaller the particle size the better sorted the sediment. One exception is quartz sandstone, which is usually very well-sorted despite the size of its clast.

• Rounding: The more a clast is subject to weathering and/or working, the more rounded it gets. Hence a well-rounded clast (besides being well-versed in art, music, science, and athletics) is an indicator of extensive weathering.

Chemical/Biological Sedimentary Rocks

These rocks have varied characteristics. Here are two most important types with a brief résumé.

• Limestone – largely made up of the mineral calcite. It can be deposited by chemical precipitation from water or (more often) by the accumulation of calcareous shells or tests of organisms that live in water or by a combination of both. It is the most abundant chemical/biological rock on earth. It often indicates deposition in a warm, shallow sea, where conditions for deposition are most favorable. There is a huge variety of different types of limestones, depending on the exact depositional environment. To make things more complicated, there are actually clastic limestones composed of water-worked hard parts of marine animals.

• Evaporites – rocks formed by chemical deposition due to evaporation of water in a closed basin or a basin with restricted water flow. The two most common evaporites are rock salt (consisting of halite) and rock gypsum (consisting of the mineral gypsum).

Depositional Environments

There are three main categories of depositional environments, described below.

• Continental: These include deposits in lakes and by rivers and streams. Alluvial fans are fan-shaped deposits along the sides of desert mountain ranges mostly due to flash floods. Also important are windblown deposits such as dunes.

• Shore: These include relatively high-energy environments such as beaches to low-energy ones such as tidal flats, and lagoons. Intermediate environments include river deltas, where sediment is deposited as the river enters the sea.

• Marine: Here we have reefs (limestone) and back-reef environments (less energetic than fore-reef environments), submarine fans (formed by flows of water and sediment mixed called turbidity currents), and the quiet deep marine environment.

Metamorphic Rocks

Metamorphic rocks are produced when heat and/or pressure alter the mineralogy and/or crystalline character of a "parent" rock. Because both heat and pressure increase with depth in the earth, many metamorphic rocks are formed by deep burial. Others are formed when rising magma comes into contact with preexisting rock (country rock). The term grade is used to indicated the degree of heat/pressure that has altered the rock. Low-grade metamorphic rocks were subjected to relatively low levels of heat and/or pressure, whereas high-grade rocks were subjected to high levels. Two major types of metamorphism are regional metamorphism and contact metamorphism. The two types of metamorphic rock are foliated and unfoliated.

Regional Metamorphism

This occurs when a large region of the earth's crust is subjected to elevated temperatures and/or pressures. It typically accompanies a mountain-building episode.

Contact Metamorphism

This occurs when country rock is invaded by magma. The rock near the magma body may be heated sufficiently to induce metamorphism.

Foliated Metamorphic Rocks

Foliated is a term related to the word foliage, meaning leaf-like. In foliated metamorphic rocks platy minerals such as mica and chlorite are aligned parallel to each other. Foliated rocks are categorized according to grade. The following table lists the major categories of foliated rocks.

Nonfoliated Metamorphic Rocks

Of primary importance for the sake of the historical geology is to know the major types. A list follows.

• Marble - metamorphosed limestone or dolostone. The calcite or dolomite has been recrystallized to a more compact form, but the minerology hasn't been changed. (Marble made of calcite will fizz with dilute hydrochloric acid.) The grade may be low to high.

• Quartzite - metamorphosed quartz sandstone. The quartz sand grains have been recrystallized to interlocking quartz crystals.

• Metaconglomerate - metamorphosed conglomerate. The gravel and cobbles are still apparent.

• Hornfels - a dark, fine-grained metamorphic rock typically formed from fine-grained sedimentary rocks, such as shale, by contact metamorphism.