1.5: The Study of Geology


Geologists apply the scientific method to learn about Earth’s materials and processes. Geology plays an important role in society; its principles are essential to locating, extracting, and managing natural resources; evaluating environmental impacts of using or extracting these resources; as well as understanding and mitigating the effects of natural hazards.

Geology often applies information from physics and chemistry to the natural world, like understanding the physical forces in a landslide or the chemical interaction between water and rocks. The term comes from the Greek word geo, meaning Earth, and logos, meaning to think or reckon with.

Why Study Geology?

Geology plays a key role in how we use natural resources—any naturally occurring material that can be extracted from the Earth for economic gain. Our developed modern society, like all societies before it, is dependent on geologic resources. Geologists are involved in extracting fossil fuels, such as coal and petroleum; metals such as copper, aluminum, and iron; and water resources in streams and underground reservoirs inside soil and rocks. They can help conserve our planet’s finite supply of nonrenewable resources, like petroleum, which are fixed in quantity and depleted by consumption. Geologists can also help manage renewable resources that can be replaced or regenerated, such as solar or wind energy, and timber.

Resource extraction and usage impact our environment, which can negatively affect human health. For example, burning fossil fuels releases chemicals into the air that are unhealthy for humans, especially children. Mining activities can release toxic heavy metals, such as lead and mercury, into the soil and waterways. Our choices will have an effect on Earth’s environment for the foreseeable future. Understanding the remaining quantity, extractability, and renewability of geologic resources will help us better sustainably manage those resources.

Geologists also study natural hazards created by geologic processes. Natural hazards are phenomena that are potentially dangerous to human life or property. No place on Earth is completely free of natural hazards, so one of the best ways people can protect themselves is by understanding the geology. Geology can teach people about the natural hazards in an area and how to prepare for them. Geologic hazards include landslides, earthquakes, tsunamis, floods, volcanic eruptions, and sea-level rise.

Finally, geology is where other scientific disciplines intersect in the concept known as Earth System Science. In science, a system is a group of interactive objects and processes. Earth System Science views the entire planet as a combination of systems that interact with each other via complex relationships. This geology textbook provides an introduction to science in general and will often reference other scientific disciplines.

Earth System Science includes five basic systems (or spheres), the Geosphere (the solid body of the Earth), the Atmosphere (the gas envelope surrounding the Earth), the Hydrosphere(water in all its forms at and near the surface of the Earth), the Cryosphere (frozen water part of Earth), and the Biosphere (life on Earth in all its forms and interactions, including humankind).

Rather than viewing geology as an isolated system, earth system scientists study how geologic processes shape not only the world but all the spheres it contains. They study how these multidisciplinary spheres relate, interact, and change in response to natural cycles and human-driven forces. They use elements from physics, chemistry, biology, meteorology, environmental science, zoology, hydrology, and many other sciences.

Rock Cycle

The most fundamental view of Earth materials is the rock cycle, which describes the major materials that comprise the Earth, the processes that form them, and how they relate to each other. It usually begins with hot molten liquid rock called magma or lava. Magma forms under the Earth’s surface in the crust or mantle. Lava is a molten rock that erupts onto the Earth’s surface. When magma or lava cools, it solidifies by a process called crystallization in which minerals grow within the magma or lava. The resulting rocks are igneous rocks. Ignis is Latin for fire.

Igneous rocks, as well as other types of rocks, on Earth’s surface was exposed to weathering and erosion, which produces sediments. Weathering is the physical and chemical breakdown of rocks into smaller fragments. Erosion is the removal of those fragments from their original location. The broken-down and transported fragments or grains are considered sediments, such as gravel, sand, silt, and clay. These sediments may be transported by streams and rivers, ocean currents, glaciers, and wind.

Sediments come to rest in a process known as a deposition. As the deposited sediments accumulate—often underwater, such as in a shallow marine environment—the older sediments get buried by the new deposits. The deposits are compacted by the weight of the overlying sediments and individual grains are cemented together by minerals in groundwater. These processes of compaction and cementation are called lithification. Lithified sediments are considered a sedimentary rock, such as sandstone and shale. Other sedimentary rocks are made by the direct chemical precipitation of minerals rather than eroded sediments and are known as chemical sedimentary rocks.

Pre-existing rocks may be transformed into a metamorphic rock; meta- means change and -morphos means form or shape. When rocks are subjected to extreme increases in temperature or pressure, the mineral crystals are enlarged or altered into entirely new minerals with similar chemical makeup. High temperatures and pressures occur in rocks buried deep within the Earth’s crust or that come into contact with hot magma or lava. If the temperature and pressure conditions melt the rocks to create magma and lava, the rock cycle begins anew with the creation of new rocks.

Plate Tectonics and Layers of Earth

The theory of plate tectonics is the fundamental unifying principle of geology and the rock cycle. Plate tectonics describes how Earth’s layers move relative to each other, focusing on the tectonic or lithospheric plates of the outer layer. Tectonic plates float, collide, slide past each other, and split apart on an underlying mobile layer called the asthenosphere. Major landforms are created at the plate boundaries, and rocks within the tectonic plates move through the rock cycle. Plate tectonics is discussed in more detail in Chapter 2.

Earth’s three main geological layers can be categorized by chemical composition or the chemical makeup: crust, mantle, and core. The crust is the outermost layer and composed of mostly silicon, oxygen, aluminum, iron, and magnesium [29]. There are two types: continental crust and oceanic crust. Continental crust is about 50 km (30 mi) thick, composed of low-density igneous and sedimentary rocks, Oceanic crust is approximately 10 km (6 mi) thick and made of high-density igneous basalt-type rocks. Oceanic crust makes up most of the ocean floor, covering about 70% of the planet [30]. Tectonic plates are made of crust and a portion of the upper mantle, forming a rigid physical layer called the lithosphere.

The mantle, the largest chemical layer by volume, lies below the crust and extends down to about 2,900 km (1,800 mi) below the Earth’s surface [31]. The mostly solid mantle is made of peridotite, a high-density composed of silica, iron, and magnesium [32]. The upper part of the mantel is very hot and flexible, which allows the overlying tectonic plates to float and move about on it. Under the mantle is the Earth’s core, which is 3,500 km (2,200 mi) thick and made of iron and nickel. The core consists of two parts: a liquid outer core and solid inner core [33; 34; 35]. Rotations within the solid and liquid metallic core generate Earth’s magnetic field (see figure above) [36; 37].

Geologic Time and Deep Time

“The result, therefore, of our present enquiry is, that we find no vestige of a beginning; no prospect of an end.” (James Hutton, 1788) [22]

One of the early pioneers of geology, James Hutton, wrote this about the age of the Earth after many years of geological study. Although he wasn’t exactly correct—there is a beginning and will be an end to planet Earth—Hutton was expressing the difficulty humans have in perceiving the vastness of geological time. Hutton did not assign an age to the Earth, although he was the first to suggest the planet was very old.

Today we know Earth is approximately 4.54 ± 0.05 billion years old. This age was first calculated by Caltech professor Clair Patterson in 1956, who measured the half-lives of lead isotopes to radiometrically date a meteorite recovered in Arizona [38]. Studying geologic time, also known as deep time, can help us overcome a perspective of Earth that is limited to our short lifetimes. Compared to the geologic scale, the human lifespan is very short, and we struggle to comprehend the depth of geologic time and slowness of geologic processes. For example, the study of earthquakes only goes back about 100 years; however, there is geologic evidence of large earthquakes occurring thousands of years ago. And scientific evidence indicates earthquakes will continue for many centuries into the future.

Eons are the largest divisions of time, and from oldest to youngest are named Hadean, Archean, Proterozoic, and Phanerozoic. The three oldest eons are sometimes collectively referred to as Precambrian time.

Life first appeared more than 3,800 million years ago (Ma). From 3,500 Ma to 542 Ma, or 88% of geologic time, the predominant life forms were single-celled organisms such as bacteria. More complex organisms appeared only more recently, during the current Phanerozoic Eon, which includes the last 542 million years or 12% of geologic time.

The name Phanerozoic comes from phaneros, which means visible, and zoic, meaning life. This eon marks the proliferation of multicellular animals with hard body parts, such as shells, which are preserved in the geological record as fossils. Land-dwelling animals have existed for 360 million years, or 8% of geologic time. The demise of the dinosaurs and subsequent rise of mammals occurred around 65 Ma, or 1.5% of geologic time. Our human ancestors belonging to the genus Homo have existed since approximately 2.2 Ma—0.05% of geological time or just $$\frac{1}{2,000}^{th}$$ the total age of Earth.

The Phanerozoic Eon is divided into three eras: Paleozoic, Mesozoic, and Cenozoic. Paleozoic means ancient life, and organisms of this era included invertebrate animals, fish, amphibians, and reptiles. The Mesozoic (middle life) is popularly known as the Age of Reptiles and is characterized by the abundance of dinosaurs, many of which evolved into birds. The mass extinction of the dinosaurs and other apex predator reptiles marked the end of the Mesozoic and beginning of the Cenozoic. Cenozoic means new life and is also called the Age of Mammals, during which mammals evolved to become the predominant land-dwelling animals. Fossils of early humans, or hominids, appear in the rock record only during the last few million years of the Cenozoic. The geologic time scale, geologic time, and geologic history are discussed in more detail in chapters 7 and 8.

The Geologist’s Tools

In its simplest form, a geologist’s tool may be a rock hammer used for sampling a fresh surface of a rock. A basic toolset for fieldwork might also include:

• Magnifying lens for looking at mineralogical details
• Compass for measuring the orientation of geologic features
• Map for documenting the local distribution of rocks and minerals
• Magnet for identifying magnetic minerals like magnetite
• Dilute solution of hydrochloric acid to identify carbonate-containing minerals like calcite or limestone.

In the laboratory, geologists use optical microscopes to closely examine rocks and soil for mineral composition and grain size. Laser and mass spectrometers precisely measure the chemical composition and geological age of minerals. Seismographs record and locate earthquake activity, or when used in conjunction with ground-penetrating radar, locate objects buried beneath the surface of the earth. Scientists apply computer simulations to turn their collected data into testable, theoretical models. Hydrogeologists drill wells to sample and analyze underground water quality and availability. Geochemists use scanning electron microscopes to analyze minerals at the atomic level, via x-rays. Other geologists use gas chromatography to analyze liquids and gases trapped in glacial ice or rocks.

Technology provides new tools for scientific observation, which leads to new evidence that helps scientists revise and even refute old ideas. Because the ultimate technology will never be discovered, the ultimate observation will never be made. And this is the beauty of science—it is ever-advancing and always discovering something new.

1.5: The Study of Geology is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher.