Geology, study of the planet earth, its rocky exterior, its history, and the processes that act upon it. Geology is also referred to as earth science and geoscience. The word geology comes from the Greek geo, “earth,” and logia, “the study of.” Geologists seek to understand how the earth formed and evolved into what it is today, as well as what made the earth capable of supporting life. Geologists study the changes that the earth has undergone as its physical, chemical, and biological systems have interacted during its 4.5 billion year history.
Geology is an important way of understanding the world around us, and it enables scientists to predict how our planet will behave. Scientists and others use geology to understand how geological events and earth’s geological history affect people, for example, in terms of living with natural disasters and using the earth’s natural resources. As the human population grows, more and more people live in areas exposed to natural geologic hazards, such as floods, earthquakes, tsunamis, volcanoes, and landslides. Some geologists use their knowledge to try to understand these natural hazards and forecast potential geologic events, such as volcanic eruptions or earthquakes. They study the history of these events as recorded in rocks and try to determine when the next eruption or earthquake will occur. They also study the geologic record of climate change in order to help predict future changes. As human population grows, geologists’ ability to locate fossil and mineral resources, such as oil, coal, iron, and aluminum, becomes more important. Finding and maintaining a clean water supply, and disposing safely of waste products, requires understanding the earth’s systems through which they cycle.
The field of geology includes subfields that examine all of the earth's systems, from the deep interior core to the outer atmosphere.
The field of geology includes subfields that examine all of the earth's systems, from the deep interior core to the outer atmosphere, including the hydrosphere (the waters of the earth) and the biosphere (the living component of earth). Generally, these subfields are divided into the two major categories of physical and historical geology. Geologists also examine events such as asteroid impacts, mass extinctions, and ice ages. Geologic history shows that the processes that shaped the earth are still acting on it and that change is normal.
Many other scientific fields overlap extensively with geology, including oceanography, atmospheric sciences, physics, chemistry, botany, zoology, and microbiology. Geology is also used to study other planets and moons in our solar system. Specialized fields of extraterrestrial geology include lunar geology, the study of earth’s moon, and astrogeology, the study of other rocky bodies in the solar system and beyond. Scientific teams currently studying Mars and the moons of Jupiter include geologists.
II GUIDING PRINCIPLES OF GEOLOGY
Geologists use three main principles, or concepts, to study earth and its history. The first concept, called plate tectonics, is the theory that the earth’s surface is made up of separate, rigid plates moving and floating over another, less rigid layer of rock. These plates are made up of the continents and the ocean floor as well as the rigid rock beneath them. The second guiding concept is that many processes that occur on the earth may be described in terms of recycling: the reuse of the same materials in cycles, or repeating series of events. The third principle is called uniformitarianism. Uniformitarianism states that the physical and chemical processes that have acted throughout geologic time are the same processes that are observable today. Because of this, geologists can use their knowledge of what is happening on the earth right now to help explain what happened in the past.
Plate tectonics is the unifying theory of geology. It was established in the 1960s, making it one of the most recent revolutions in all of science. The theory describes the lithosphere (the outer rocky layer of the earth) as a collection of rigid plates that move sideways above a less rigid layer called the asthenosphere. The asthenosphere is made up of rock that is under tremendous pressure, which softens it and allows it to move and circulate slowly. Plate tectonics is useful in the field of geology because it can be used to explain a variety of geologic processes, including volcanic activity, earthquakes, and mountain building. See also Earth.
B Geologic Cycles
A second guiding principle of geology is the principle of recycling materials, or using materials many times. All processes in geology can be viewed as a series of mostly closed cycles, meaning the materials of the cycles are found on earth, and very few materials from outside our world are introduced into these cycles. The energy that drives geologic recycling comes from two sources: the sun and the earth's interior. Two examples of geologic cycles are the rock cycle and the water cycle.
The rock cycle begins as rocks are uplifted, or pushed up by tectonic forces. The exposed rocks erode as a result of surface processes, such as rain and wind. The eroded particles, or sediment, travel by wind or moving water until they are deposited, and the deposited material settles into layers. Additional sediment may bury these layers until heat and pressure metamorphose, or change, the underlying sediment to metamorphic rock. Additional sediment may compact the layers into sedimentary rocks. Rocks can also be subducted (sunk down into the lower layers of the earth) by plate tectonic processes. Buried and subducted rocks may also melt and recrystallize into igneous rocks (see Magma). Metamorphic, sedimentary, and igneous rocks may then be uplifted, starting the rock cycle again.
The water cycle is also known as the hydrologic cycle. Phases of the water cycle are storage, evaporation, precipitation, and runoff. Water is stored in glaciers, polar ice caps, lakes, rivers, oceans, and in the ground. Heat from the sun evaporates water from the earth’s surface and the water then condenses to form clouds. It falls back to the earth as precipitation, either as rain or snow, then runs into the oceans through rivers or underground and begins the cycle again.
C Uniformitarianism
Geologists use their knowledge of modern processes and events to reconstruct the past.
Uniformitarianism, or actualism, helps geologists use their knowledge of modern processes and events to reconstruct the past. The principle of uniformitarianism depends on the "uniformity of laws," which assumes that the laws of physics and chemistry have remained constant. To test uniformity of laws, geologists can examine preserved one-billion-year-old ripples that look very much like ripples on the beach today. If gravity had changed, water and sand would have interacted differently in the past, and the ripple evidence would be different. Also, minerals in three-billion-year-old rocks are the same as minerals forming in rocks today, confirming the uniformity of chemical laws. Uniformitarianism contrasts with, for example, the idea that past events such as floods or earthquakes were caused by divine intervention or supernatural causes. Catastrophism, which calls on major catastrophes to explain earth’s history, is also sometimes contrasted with uniformitariism. However, uniformitarianism can include past catastrophes.
III THE GEOLOGIC TIME SCALE
Geologists have created a geologic time scale to provide a common vocabulary for talking about past events. The practice of determining when past geologic events occurred is called geochronology. This practice began in the 1700s and has sometimes involved some personal and international disputes that led to differences in terminology. Today the geologic time scale is generally agreed upon and used by scientists around the world, dividing time into eons, eras, periods, and epochs. Every few years, the numerical time scale is refined based on new evidence, and geologists publish an update.
Geologists use several methods to determine geologic time. These methods include physical stratigraphy, or the placement of events in the order of their occurrence, and biostratigraphy, which uses fossils to determine geologic time. Another method geologists use is correlation, which allows geologists to determine whether rocks in different geographic locations are the same age. In radiometric dating, geologists use the rate of decay of certain radioactive elements in minerals to assign numerical ages to the rocks.
The process of determining geologic time includes several steps. Geologists first determine the relative age of rocks—which rocks are older and which are younger. They then may correlate rocks to determine which rocks are the same age. Next, they construct a geologic time scale. Finally, they determine the specific numerical ages of rocks by various dating methods and assign numbers to the time scale.
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