How do we study the GEOSPHERE?
Until recently, the geosphere was studied primarily by mapping rocks, fossils and soils as they vary across the landscape. Based on the principles of superposition (younger rocks overlie older rocks), original horizontality (sedimentary rocks are deposited as horizontal layers) and uniformitarianism (past processes can be understood by observing similar processes today), incredibly-complex patterns on the surface can be unraveled to reveal the arrangement of rocks beneath the surface, their relative age of formation, and the processes that created them. Gaps, repeated sequences of rocks, or areas where older rocks are found above younger rocks were tools used to map faults (fractures in the surface in which movement has occurred), folds (rocks that have been bent or deformed) and unconformities (portions of the geological record that have been lost due to erosion). Based on the fossil record, geologists of the 18th and 19th century assembled a relative geologic time scale, which is the basis of the modern time scale.
During the 20th century, traditional mapping methods remained one of the most powerful tools for understanding the geosphere. Fortunately, these tools have been greatly enhanced by recent technological advances in many areas. For example, one of the earliest controversies during the turn of the 20th century involved the age of the Earth. Based on the Earth's thermal history, British physicist Lord Kelvin had calculated that the Earth could be no more than 40 million years old. This calculation conflicted with the fossil record, which suggested a much greater age. The discovery of radioactive decay by Henri Becquerel, and subsequent discovery that radioactive decay generates heat and can be used to determine the age of rocks, resolved this conflict, and the age of the Earth is now thought to be billions of years old! Since that time, many other age dating techniques have been developed, which have enabled scientists to place absolute ages on the relative geological time scale.
The discovery of radioactivity, and the fact that the number of neutrons in an element can vary forming what are called isotopes, helped establish two powerful tools used to understand the geosphere. The first, radiometric age dating, is based on the fact that certain isotopes are unstable with predictable rates of decay. If we know the amount of a particular isotope that is present when a rock formed, and the rate at which the unstable isotope is transformed into a different element, we can determine the age of the rock based on its modern-day isotopic composition.
One example of an age dating technique is carbon-14, in which this form of radioactive carbon (which has 8 neutrons, instead of 6) decays to stable nitrogen and is used to age-date materials containing carbon that are as old as 70,000 years. Another powerful tool is based on stable isotopes, in which the relative proportion of isotopes in a rock or organic compound can be used to trace its origin.
Isotopic geology forms only one of many new technologies used to study the geosphere. Other laboratory-based technological advances include the use of new instruments to determine rock and mineral composition, and advances in our understanding of magnetism and our ability to measure magnetic properties of materials. New analytical tools have revolutionized the way geologists describe rocks, which until recently were limited to what a human eye could see through a hand lens or microscope. Paleomagnetism has allowed geologists to determine not only when a rock formed, but the latitude where it formed and orientation of the rock as it cooled down and settled. Through paleomagnetism, geologists have been able to reconstruct past locations of continents and determine how far rocks have traveled from their origin.
Advances in laboratory analysis tools have been augmented by many advances in field techniques. Many of these techniques can be considered some form of remote sensing. Remotely-sensed tools include: stereo photography, sonar, seismology, and radar. Furthermore, it has become possible to take the same tools used in the laboratory and put them on an airborne platform. For example, methods for mapping certain emissions (such as gamma rays and beta particles) have been used by geologists to map radioactive materials from aircraft. Sensitive scientific equipment used to map magnetic properties can be mounted on a plane to map regional variations of the Earth's magnetic field, due to the presence of ore bodies or some other large concentration of heavy elements below the surface. The presence of many satellites in orbit (and our ability to accurately locate them in space) has lead to the development of two new tools for studying the Earth; geodesy and the Global Positioning System (GPS). Where feet, sweat, and surveying tools were once our only way to map the planet's surface, now we can map it from planes, satellites and ships across the globe, giving Earth scientists the potential to better study and understand the geosphere.
For more info on remote sensing, explore the SPHERE TOPICS sections of GEOSPHERE.
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