REVIEW OF PHYSICAL GEOLOGY

Introduction, Minerals, Igneous Rocks

The Nature of Geological Investigation

Geology is not primarily a laboratory science like physics, chemistry, or even biology. It is more like astronomy, where data is largely derived by observation and conclusions are made from analysis based on the laws of physics and chemistry.

The Doctrine of Uniformitarianism

In recent decades the doctrine of "uniformitarianism" has been modified from the view that all geological observations can be understood on the basis of geological processes acting today, to the inclusion of occasional catastrophic events that occur rarely but have tremendous consequences.

The major ice-breaker for this opinion change was the discovery that a huge asteroid collided with the earth 65 million years ago and, at least in part, wiped out a large percentage of the earth's species, including dinosaurs, flying reptiles, and large marine reptiles. Subsequently, it has become better appreciated that not only occasional asteroid collisions, but many other types of rare catastrophic events can be important, such as vast floods like those that occurred at the end of the last glacial cycle due to failure of ice dams, nearby supernova explosions, cataclysmic volcanic eruptions, etc.

Uniformitarianism can now be described as, most importantly, the observation that the laws of physics and chemistry do not seem to have changed over time. This allows us to apply them to past events with some confidence. Also, in spite of the rare occurrence of catastrophes, much of the geological record still reflects the operation of gradual geological processes such as occur today. For example, chemical weathering of rock into sediment is a time-consuming process that can't be accelerated by catasprophes.

The Rock Cycle

Rocks come in three major groups:

• Igneous: Rocks that cool from a magma.

• Sedimentary: Rocks that form from material that collects at the earth's surface.

• Metamorphic: Rocks that have been recrystallized by heat and/or pressure.

The rock cycle is the name for the overall process whereby igneous, sedimentary, and metamorphic rocks can be converted into each other.

Minerals

Minerals are the crystalline entities that make up rocks. To be a mineral a substance must be of a specific chemical makeup (or at least a specific chemical range) and have a specific crystal structure.

Silicates

The most abundant group of minerals at the earth's surface are the silicates, which are composed largely of silicon and oxygen. The basic chemical unit of the silicates is the silicate tetrahedron, consisting of the tetrahedral arrangement of four oxygen atoms around a central silicon atom. (A tetrahedron is a pyramid with four triangular faces.) It has an ionic charge of -4.

Silicates are categorized by their iron and magnesium content and the way in which the silicate tetrahedron is employed in the crystalline structure.

The first categorization divides the silicates into ferromagnesian and nonferromagnesian types. Ferromagnesian minerals are high in magnesium and/or iron and relatively low in silicon and oxygen. Nonferromagnesian minerals are relatively high in silicon and oxygen and low in iron and/or magnesium.

The categories corresponding to crystalline structure are as follows.

• Isolated tetrahedra: The tetrahedra are not joined in these minerals and positive ions must be present to produce charge neutrality. One of the most important of these silicates is the ferromagnesian mineral olivine, which contains iron (Fe) or magnesium (Mg) or both. This mineral is abundant in the important volcanic rock, basalt.

• Chain silicates: In these silicates the tetrahedra are joined together in either single or double chains. Two very important groups of rock-forming minerals with this structure are the pyroxenes (single chains) and the amphiboles (double chains). Perhaps the most important pyroxene and amphibole, as far as abundance in rocks is concerned, are augite and hornblende, respectively. Both are dark colored and considered ferromagnesian with augite being more so. They are the major constituents of many igneous and metamorphic rocks.

• Sheet silicates: The tetrahedra are arranged into sheets and the sheets are stacked one on top of the other. The bonds between the sheets are not as strong as within the sheets so these minerals can be split into sheet-like forms. The most abundant mineral of this structure is biotite mica, which is dark-colored due to the iron it contains and occurs in igneous and metamorphic rocks. Another important mica is muscovite, which doesn't have the iron and is light-colored to clear. Finally, many sedimentary rocks, including shales, claystones, mudstones, siltstones, and conglomerates are composed partly or largely of clay minerals, which are typically microscopic to submicroscopic crystals.

• Framework silicates: The tetrahedrons in these minerals are arranged into three-dimensional structures. The feldspars, the most abundant type of mineral at the surface of the earth, is a framework silicate. Many rock types contain abundant feldspar, especially igneous and certain metamorphic rocks and sedimentary rocks deposited in dry environments. Another common framework silicate is quartz, which is found in many of the same rocks that contain feldspars and very frequently in various kinds of sandstones. It is often the cement that holds the sand in sandstone together. Feldspar and quartz are highly nonferromagnesian minerals.

There is a relationship between the ferromagnesian-nonferromagnesian categorization and the crystalline structure of silicates. Generally speaking, the more the tetrahedra are connected together, the more nonferromagnesian the silicate is. For example, olivine, augite, and hornblende are ferromagnesian, whereas quartz and feldspar are nonferromagnesian, with biotite falling somewhere in the middle.

Other Minerals

There are lots of minerals other than those mentioned above, silicate and non-silicate, but some are more important for historical geology than others. Here is a list and description of some of them.

• Calcite: This mineral is chemically calcium carbonate and contains calcium, carbon, and oxygen. It is important because it is the mineral that makes up the different kinds of limestone, which is an extremely important sedimentary rock from the point of view of historical geology.

• Dolomite: Dolomite is calcite with the calcium replaced by magnesium due to a chemical reaction between magnesium-bearing subsurface fluids and the calcite. A limestone transformed this way is called dolostone. Dolostone is essentially important to historical geology in the same way limestone is.

• Halite: The mineral that is the source of common table salt. Vast deposits of halite were deposited in ancient evaporation basins.

• Gypsum: Another mineral that may form in evaporating water. Used to make wallboard.

• Hematite: This is an iron oxide that is important as a cement in many sedimentary rocks, giving them a reddish color.

• Limonite: Another iron oxide that serves as a cement, giving sedimentary rocks a yellowish color.

• Zircon: A single-tetrahedron silicate that contains the element zirconium. Its importance is that, since it is extremely stable chemically and contains uranium as an impurity, it can be used in radiometric dating.

• Glauconite: A sheet silicate that often forms by chemical reaction in sediments at the same time the sediment hardens into rock. It contains radioactive potassium and can be used to date sedimentary rocks.

Rock Types

The three rock types are igneous, sedimentary, and metamorphic.

Igneous Rocks

Igneous rocks come in two varieties: plutonic (intrusive) and volcanic (extrusive).

Plutonic rocks cool and harden underground from magma. The vast majority of igneous rocks are (thankfully) plutonic. They occur in a number of different types of structures, depending on how much magma there was and how the emplacement occurred. Bodies of this type of rock are called plutons, the main types of which are as follows.

• Batholith: A very large ("county-sized") pluton. When they are exposed by erosion they often form mountains (such as the Sierra Nevadas).

• Stock: A small ("town-sized") batholith or a smaller pluton connected to a batholith.

• Laccolith: A mushroom-shaped pluton with a rounded top and a flat bottom formed when magma intrudes between sedimentary beds and causes the overhead beds to arch upward.

• Dike: A sheet-shaped pluton where magma has forced its way upward through a fissure, cutting across the rocks already there.

• Sill: A sheet-shaped pluton where magma has forced its way between two sedimentary or lava beds. (If the magma pools and forces the overhead rocks upward, it becomes a laccolith.)

• Volcanic pipe: A cylindrically-shaped, vertical pluton where magma feeding a volcano finally solidified. When exposed by erosion it is called a volcanic neck.

Volcanic rocks cool and undergo final solidification on the earth's surface from material that is erupted from fissures or volcanoes. The following is a brief list of some different types of volcanism.

• Shield volcano: A volcano that produces primarily "runny" lava (usually basaltic) such that the volcano has gently sloping sides. The Hawaiian volcanoes are this type.

• Fissure eruption: Occurs around the flanks of volcanoes and is the means by which most lava is extruded. You have likely seen video of the fissure eruptions on the slopes of the Hawaiian volcano Kilauea.

• Composite volcano: A volcano that undergoes different types of eruptions, including explosive ones and more viscous types of lava. They have relatively steep slopes. Mount St. Helens is this type.

• Flood Basalt: (called basalt plateaus in the textbook) A series of fissure eruptions taking place over millions of years that produces vast quantities of runny basalts. In each eruption the basalt flows over the land somewhat like a flood of water, hence the name.

Types of Igneous Rocks

Plutonic and volcanic rocks show the same range of chemical makeup. For example, the volcanic rock rhyolite has essentially the same chemical makeup as the plutonic rock granite. The difference is that rocks that cool underground (plutonic) cool more slowly allowing the crystals to grow larger than rocks that cool rapidly on the surface (volcanic). The following is a table that shows the major plutonic and volcanic rocks. Rocks in each column are chemically equivalent. Rocks in the left column are made up of mostly nonferromagnesian minerals and are called felsic rocks (from the words feldspar and silica), since they contain a lot of feldspar and other nonferromagnesian minerals. Rocks in the right column are mafic, composed of ferromagnesian minerals. Rocks in the middle column are of intermediate composition. Note that this table highly simplifies the actual situation in nature.

In addition to the above igneous rock types, there is an important though rare plutonic rock called peridotite, which is extremely high in iron and magnesium ("ultramafic"). This rock is important because its presence indicates extreme deformation, since it is thought to come from very deep in the earth.