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The science of geology ranges from the study of individual rocks, to the study of the composition of the entire Earth. In between, geologists work to explain why the surface of the land looks the way it does, where valuable mineral resources might be found, how the land used to look based on the minerals deposited there, and how the continents have moved over time, creating new mountain ranges and oceans. Much to the dismay of Fundamentalist Christians, all of this provides evidence for the fact that this is a very old planet. Indeed it would be very difficult to conduct any sort of mineral exploration using a young-Earth hypothesis.
Branches of geology
- Geochemistry — Using chemistry to solve problems in geology
- Geophysics — Using physics to solve problems in geology
- Bio-geology — Using geology to solve problems in biology
- Volcanology — Geological study of volcanoes
- Mineralogy — Geological study of minerals
- Engineering Geology — Interdisciplinary study of geology and engineering
- Geomorphology — Study of processes that shape the Earth and its landscapes. It is one of the fields studied in geology. Geomorphology analyses the dynamics of change in the Earth's landscape and serves as a guide to what the surface of the land looked like in the past and to extrapolate how landscapes might be subject to change in the future.
Rocks are made of minerals and mineraloids. Minerals have a crystalline structure (i.e. crystals of silica = quartz), whereas mineraloids (like silica glass) do not. Three types of rock are recognized: igneous, sedimentary, and metamorphic.
Igneous (from ignis, Latin for fire, compare English ignite) rock is produced by the welling up of molten magma from inside the Earth, which then solidifies as it cools. Igneous rocks are divided into "intrusive" and "extrusive"; extrusive rock cools quickly and near the surface via volcanoes and rifts, and tends to have small crystals (barely visible with the naked eye), as can be seen in basalt. Rock is said to be intrusive when magma cools deep under the surface of the Earth. Here the crystals form very slowly and are thus much larger, as for example, the large crystals of quartz, biotite, and feldspar that are visible in plutonic rocks like granite.
Sedimentary rocks are aggregates of smaller bits of rocks, minerals, and other solids that have been compacted and fused together by various processes. They may form from remnants of any rock type, including other sedimentary rocks.
Over time the action of weathering and erosion via temperature changes, rain, wind, chemical reactions, and the moving action of streams and rivers, breaks rock down into smaller and smaller pieces. These are washed downstream and eventually deposited in an ocean, river, lake, etc. as sediment. Lithification (turning to stone) of sediments occurs over thousands of years as layers of deposited sediments accumulate, and at depth are compacted by the weight of overlying sediments. The grains are fused together by the dissolution of minerals (just like salt sticking to your body after leaving the ocean) in a process called cementation. As a sedimentary example: quartz sandstones form where quartz sand is the primary sediment, and is typically cemented by silica, though in oceanic settings, calcium carbonate is also quite common.
Sedimentary rocks can also be biological in origin. Limestone is a common sedimentary rock consisting of calcium carbonate, which is formed by some shell-bearing organisms like corals, bivalves (clams, oysters, etc.), and foraminifera (or forams: a micro-organism). Indeed, the White Cliffs of Dover are composed almost entirely of the skeletal remains of forams.
These are rocks which used to be either sedimentary or igneous, but have undergone chemical changes thanks to being subjected to enormous heat and/or pressure. Through the action of the movement of the Earth's crust, particularly continental collision, or when the Earth's crust is pulled downwards towards the interior in a process called subduction (more about this later), rocks of any kind can be subjected to enormous pressure and heat and may be transformed, or metamorphosed into metamorphic rock. Metamorphic rocks are divided by how strongly they have been metamorphosed from the original rock. Low-grade metamorphic rocks look a lot like the rocks they came from (e.g. slate is the metamorphosed form of shale, and they look almost identical, but are different physically), while high-grade metamorphic rocks look like igneous rocks, but can be distinguished by looking for various unusual textural features such as strong veining or lineation.
Extensive exploration and mapping of the seafloor after World War II led to the discovery of a deep rift running down the center of the Mid-Atlantic Ridge. In the early 1960s Harry Hess of Princeton University and Robert Dietz of the University of California suggested that the seafloor separates along the rifts in mid-oceanic ridges and that new seafloor forms by upwelling of hot mantle materials in these cracks, followed by lateral spreading. By 1967, separate lithospheric plates had been identified, which explained phenomena such as high levels of volcanic and earthquake activity that take place between the plates. By the end of the 1960s the theory of plate tectonics proved to be a unifying concept that pulled together diverse theories and explained a the large body of observations in the field.
About twelve large plates, and several smaller plates, slide over a weak asthenosphere, carrying with them the continental and oceanic crust. Where plates collide, tectonic forces cause the continental crust to buckle upward, producing mountains such as the Himalayas or the Rocky Mountains. Oceanic plates are denser than continental plates, and at boundaries called subduction zones, the extra density causes one oceanic plate to sink beneath another, re-entering the mantle and recycling the oceanic crust. Where plates move apart, at mid-oceanic ridges and at continental rifts like East Africa, hot material from the mantle rises to fill the gap, eventually melting to create new sea floor.
Composition of the Earth
Volcanism and deformation bring rocks to the surface of the Earth from depths as great as 50 to 100 km. Scientists can make inferences about some of the properties of the Earth at these depths by studying these rocks. But far more information has been provided through the use of seismic waves created by natural earthquakes, and by controlled explosions designed to learn more about the composition of the Earth, including underground nuclear explosions. These data have revealed that the Earth is composed of three main layers, the crust, the mantle and the core. The crust, or outermost layer, varies in thickness from about 5 km under oceans to about 40 km under continents. The mantle consists of an outermost zone of about 100 km thick named the lithosphere, which is relatively strong and makes up the bottom part of tectonic plates. The layer below is a weak, partially fluid solid called the asthenosphere, which ends at a depth of about 300 km. As seismic waves penetrate deeper into the mantle they indicate that between 400 km and about 2900 km atoms are packed closer and closer together by extreme pressures, creating a crystalline structure with larger grains, and different minerals, than the layers above. The Earth's core extends from a depth of 2900 km to the center. Seismic wave propagation through the core indicates that the upper two-thirds of the core are liquid, while below a depth of 5100 km pressures are so great that the core becomes solid again.
Age of the Earth
- Press, Frank and Siever, Raymond (1998). Understanding Earth, 2nd ed. W. H. Freeman and Company.