ASVAB General Science: Geology
Geology, the study of Earth's structure, composition, and the processes that shape it over time, offers a fascinating window into our planet's long and dynamic history. To understand this history, geologists have divided Earth's past into four major eras, each marked by significant changes in life forms and geological processes.
The story of our planet begins with the Precambrian Era, stretching from Earth's formation about 4.6 billion years ago to 541 million years ago. This vast span of time, covering roughly 88% of Earth's history, saw the formation of Earth's crust, the emergence of simple life forms, and the development of a stable atmosphere. Following this came the Paleozoic Era, lasting from 541 to 252 million years ago. This era was characterized by the evolution of complex life forms, including fish, amphibians, and early reptiles, and ended with the largest known mass extinction event.
The Mesozoic Era, often called the "Age of Dinosaurs," ran from 252 to 66 million years ago. During this time, reptiles dominated the planet, and we saw the emergence of early mammals and birds. Finally, we arrive at the Cenozoic Era, which began 66 million years ago and continues to the present day. This current era has been marked by the diversification of mammals and the evolution of humans.
Central to our understanding of Earth's geological processes is the theory of plate tectonics. This fundamental concept explains that Earth's outer layer is divided into several large tectonic plates that move relative to each other, driven by convection currents in the mantle. The interactions between these plates cause various geological phenomena, including earthquakes, volcanic activity, and mountain formation.
Closely related to plate tectonics are geologic faults, which are fractures in Earth's crust where blocks of rock move relative to one another. There are three main types of faults. Normal faults occur when rocks pull apart, reverse faults result from compressional forces, and strike-slip faults involve horizontal movement along the fault line.
As we delve deeper into geology, we encounter the building blocks of Earth's crust: rocks. Geologists classify rocks into three main categories based on their formation. Igneous rocks form from cooled and solidified magma or lava. These can be further divided into intrusive (or plutonic) rocks like granite, which cool slowly beneath the surface, and extrusive (or volcanic) rocks like basalt, which cool quickly on the surface.
Sedimentary rocks, on the other hand, form from the deposition and consolidation of sediments. These can be clastic rocks like sandstone, formed from fragments of other rocks, or chemical rocks like limestone, formed from the precipitation of minerals from solution.
The third category, metamorphic rocks, forms when existing rocks are subjected to high heat and pressure. These can be foliated rocks like slate, which have a layered appearance, or non-foliated rocks like marble, which do not show layering.