Physical geology

Physical geology is the study of the earth's rocks, minerals, and soils, how they formed, and how they function and change. Geology is the study of both the earth's past and its future. Processes such as erosion and glaciation have affected the landscape over time. Sudden movements of crustal plates cause earthquakes, while the release of heat and energy is known as a volcanic eruption. Geologists study these and other continuous processes. Humans rely on geologic products, including water, metals, and energy sources, every day.

Scientists' understanding of geology has changed considerably over thousands of years, from ancient Greece to modern study. Geology emerged as a scientific discipline late in the eighteenth century, and the term first appeared in print in its modern use, describing knowledge of the earth, in 1778. Geoscientists have since determined that the earth is about 4.5 billion years old. Geologists regard the last 50 million years—about 1 percent of Earth's history—as the recent past because no major changes in plate tectonics have occurred. Plate tectonics refers to conditions in the solid part of the earth.

Geology is an integrative science—geologists must integrate elements of biology, chemistry, mathematics, and physics to figure out how the earth works. The two broad branches of geology are physical geology and historical geology. Historical geology focuses on the historical development of the earth by studying rocks and includes fields such as paleontology and stratigraphy. Physical geology also includes many branches of study, including mineralogy, petrology, geomorphology, geophysics, sedimentology, structural geology, economic geology, and engineering geology.

Background

Ancient humankind was aware of different materials, such as metals, found in the earth. They often attributed the creation of such material, as well as events such as earthquakes, to supernatural origins.

Ancient Greeks were aware of fossils, and they were confused that the fossils were often found far from the seas and oceans. Some believed that the waters had once been deeper and covered much more of the land. Eratosthenes in the third century BCE believed the level of the Mediterranean Sea had been lowered by the opening of a strait. Strabo, who was born in the first century BCE, thought the sea level was raised by earthquakes, volcanic eruptions, and other geologic events, while the sea level fell due to landslides and collapses of terrain.

Volcanic eruptions, earthquakes, and tsunamis were well-documented events, so such events seemed likely to scholars of the era. They and other early scientists had no idea how old the fossils were, but they observed changes and proposed ideas based on observation. Many commonly believed the earth to be eternal, which meant that what degraded through erosion or some other event must eventually be built back up. Mountains might fall, but others had to be created in some way to replace them.

During the sixteenth and seventeenth centuries, a number of scientists proposed theories about the formation of the earth. These were often strongly influenced by Christianity but gradually were replaced by scientific theories. This Age of Reason or Enlightenment ranged from about 1685 to 1815. By the late eighteenth century, a number of sciences emerged, including geology and biology, as the industrial age took hold and scientists and laypeople alike became keenly interested in science. Many scientific societies, including the Royal Society, thrived in Europe.

Twentieth-century efforts to map the ocean floors contributed to scientists' understanding of crustal evolution, including plate tectonics and seafloor spreading. Research into the atom and radioactivity enabled geologists to use radiocarbon dating on samples to determine the age of materials.

Overview

Many branches of geology have developed since the science emerged. Geoscientists study the composition of the earth to understand its history, evaluate risks, and profit through the extraction of useful materials, including fossil fuels and minerals. Geology has become increasingly important as human populations have expanded into new areas and face greater risks of events such as natural disasters. Humanity also has a greater need of resources that can be found using geology.

Mineralogy is the branch of geology concerned with the study of minerals. Mineralogists may study crystal structures, chemical compositions, and physical properties of minerals. They may work in many fields, including the mining industry.

Petrology is the study of rocks, including composition, structure, and texture, as well as where they are found and their origin. The three major types of rocks are igneous, metamorphic, and sedimentary. Experimental petrology involves creating rocks in laboratories to better understand how rocks form. Petrography uses a petrographic microscope to study thin sections of rock to better classify and describe them.

Geomorphology is the study of landforms, including the processes that produced them, their formation, and sediments on the surface of the earth. Climate and weather are among the influences on the landscape, but so are large events, including landslides and floods; and geologic hazards, such as tsunamis, volcanic activity, and earthquakes; and long-term actions, such as glaciation.

Geophysics uses geology, mathematics, and physics to learn about how the earth works. Study involves observation, computational and theoretical modeling, remote imaging, and laboratory experiments. Geophysicists may be involved in working to analyze hazards associated with potential earthquakes and finding and analyzing petroleum reservoirs.

Sedimentology involves the study of sediments—including sand and clay—and how they are formed, transported, deposited, and changed by forces (the process of becoming sedimentary rock). Most rocks are sedimentary, which often contain fossils and other historical markers. The presence of sedimentary rock can help petroleum geologists locate petroleum deposits.

Structural geology is the study of the three-dimensional distribution of large bodies of rock. This includes the rock surfaces, composition of their interiors, events that may have affected them, and their historical geological environments. Such information can help petroleum geologists determine the likelihood of natural resources such as natural gas or petroleum being trapped inside the rock bodies. Mining geologists can analyze the information to determine the chances of finding metal ore deposits.

Economic geology is focused on finding Earth materials of use to humankind. These may include coal and other fossil fuels, metals, minerals, including salt and other natural crystals, stone used in construction, and water.

Engineering geology involves using geology to assess geological hazards and find engineering solutions. This includes analyzing the stability of soil and rock, as well as factors such as the presence of groundwater. Engineering geologists analyze sites for their uses for construction of buildings, locating landfills, and other purposes, as well as advise clients about issues such as subsidence. They may also offer expertise on the best choices of construction materials, such as gravel, in a given situation.

Hydrogeology is concerned with groundwater. The study includes the effects and location of groundwater. Oceanography involves the study of the ocean.

Paleontology is the study of fossils to understand forms of life that existed in prehistoric times. Such information has helped scientists understand more about early life, such as bacteria found in rocks that are billions of years old and trilobites and other aquatic organisms, which emerged about 600 million years ago.

The composition of the earth and the theory of plate tectonics are central to the study of geology. The earth has four concentric zones. The inner core is at the center. It has a radius of about 730 miles and is believed to be solid iron. The outer core is about 1,362 miles thick and is thought to be molten liquid containing iron and nickel. Over the outer core is the mantle, a flowing, solid yet putty-like rock layer. It is about 1,740 miles thick. The outermost layer, the crust, varies from 3 to 30 miles thick. The oceanic crust is thinner than the crust that makes up the continents. The outer shell comprises about seven major plates and a number of smaller plates, which glide over the mantle. The theory of how these plates act is called plate tectonics.

The crust and upper mantle make up the solid lithosphere, while the more fluid zones make up the asthenosphere. The interactions between these areas greatly affect the crust. Convection currents, for example, take place throughout the earth. Deep within the earth, radioactive decay of elements generates heat, which melts the rock, creating magma. Convection currents in the fluid zones transfer heat to the surface areas. Scientists believe this convection influences the plates at mid-ocean ridges by pushing them and spreading them apart, while convection pulls plates downward at subduction zones. Many volcanoes develop along subduction zones. The Tyrrhenian Basin below the Mediterranean Sea is one area where the plates are spreading apart. Earthquakes in this region are common and have been recorded for centuries; geophysicists attribute them to this seismically active area. Other plate boundaries known as transform margins are areas where two plates grind together. One example is the San Andreas fault in California, where the North America and Pacific plates press together. Another plate line between the African and Eurasian plates pushes the ground beneath the Alps mountain upward.

Subduction zones are constantly consuming the lithosphere, while volcanic activity creates new solid land. The oldest seafloor is about 200 million years old, but chunks of continental crust in Greenland are at least 3.8 billion years old. These date to before the time when geologists believe modern plate tectonics began, about 3 billion years ago. The older crust may have been the result of plate tectonics, but it may not have operated in the same way.

Plate tectonics explains, among other things, how fossils from different continents can look the same. About 300 million years ago, a supercontinent or giant landmass called Pangaea comprised modern Africa, Europe, and North and South America. Over time, the continents moved apart, but their similar Atlantic Coast shorelines still indicate how they fit together. An even earlier supercontinent, Rodinia, existed about 1 billion years ago.

Geologists are helping to learn more about global climate change. Records of ancient climates exist in the strata of rocks, where types of marine organisms and characteristics of their shells tell scientists about the temperatures of the times when they lived. The fossil record also indicates whether the climate was wet or dry at a given time. For example, evidence of the last ice age, which ended about 20,000 years ago, can be found in the rock layers. The climate varied widely, and annual snowfall layers discovered in glaciers and marine sediment show large fluctuations over time. Scientists have also captured bubbles of air trapped in glacial ice to analyze the amount of carbon dioxide and other gases, calculated the effects of carbon dioxide on leaves that are found in the fossil record, and concluded that the cooling trend of the last 50 million years correlates with an increase in carbon dioxide levels. They believe this is related to actions on Earth and influences climate variability. At the same time, scientists have also discovered evidence of highly variable local climates.

Evidence of the Eocene Optimum about 45 million years ago, when Earth's climate was so warm that the poles were not frozen, shows that cold-blooded animals such as crocodiles lived in the Arctic region. The fossil record also shows a number of sea level changes lasting thousands of years. Researchers are using geologic evidence to try to determine future changes in climate and their effects on the earth and living organisms.

Bibliography

"Branches of Geology." Columbia Electronic Encyclopedia. 6th ed., Columbia UP, 2012.

"Convection Currents: Currents in the Earth's System." University of California Museum of Paleontology, www.ucmp.berkeley.edu/education/dynamic/session1/sess1‗earthcurrents.html. Accessed 3 Nov. 2023.

CTI Reviews. Earth, an Introduction to Physical Geology. Cram 101 Textbook Reviews, 2016.

Fletcher, Charles. Physical Geology: The Science of Earth, 2nd ed., Wiley, 2014.

Ghose, Tia. "Italy Earthquake: Complex Geology Drives Frequent Shaking." LiveScience, 24 Aug. 2016, www.livescience.com/55866-italy-earthquake-complex-geology.html. Accessed 3 Nov. 2024.

Gohau, Gabriel. A History of Geology. Translated by Albert V. Carozzi and Marguerite Carozzi, Rutgers UP, 1990, pp. 10–14.

Menke, William. "What Geology Has to Say About Global Warming." State of the Planet, Earth Institute, Columbia University, 11 July 2014, blogs.ei.columbia.edu/2014/07/11/what-geology-has-to-say-about-global-warming/. Accessed 3 Nov. 2024.

"New Study Models the Transmission of Foreshock Waves Towards Earth." Science Daily, 22 Dec. 2022, www.sciencedaily.com/releases/2022/12/221222101015.htm. Accessed 3 Nov. 2024.

Means, Tiffany. "What Is Plate Tectonics?" LiveScience, 26 May 2021, www.livescience.com/37706-what-is-plate-tectonics.html. Accessed 3 Nov. 2024.

Plummer, Charles C., Diana Carlson, and Lisa Hammersley. Physical Geology, 17th Edition, McGraw-Hill, 2021.