Geology

Geology is the natural science that studies the Earth and other astronomical bodies.

It focuses on the rocks that compose these bodies. Geologists also study the various processes that change these rocks over time. The name comes from the Ancient Greek words for "earth" and "study of." This field is essential for understanding the structure of our planet. It helps us explore the history of life and past climates.

Modern geology overlaps with many other Earth sciences, including hydrology. It is also integrated into planetary science and Earth system science.

To understand the Earth, geologists study minerals and rocks. Minerals are naturally occurring elements or compounds. They have a definite chemical composition and an ordered atomic arrangement. Some substances that look like minerals are called mineraloids. Geologists identify minerals using several specific physical tests. They might check the color or the luster, which is how light reflects off the surface. They can test the hardness by seeing how much resistance a mineral has to being scratched. Some tests involve chemistry, such as dripping hydrochloric acid on a mineral to check for effervescence, or fizzing. They may also test for magnetism or even taste, like the mineral halite, which tastes like salt.

Rocks are solid masses made of minerals or mineraloids. Most geological research focuses on rocks because they record Earth's history.

There are three main types of rock: igneous, sedimentary, and metamorphic. Igneous rocks form when melt, called magma or lava, solidifies and crystallizes. Sedimentary rocks form when existing rocks are weathered and eroded. These pieces are then redeposited and undergo lithification to become solid rock. Metamorphic rocks are created when igneous or sedimentary rocks are changed by intense heat and pressure. This process changes the mineral content and creates a characteristic fabric. These rocks can eventually melt to become new magma, continuing the rock cycle.

In the 1960s, scientists discovered the theory of plate tectonics.

This theory explains how the Earth's lithosphere moves. The lithosphere includes the crust and the rigid uppermost part of the mantle. It is divided into several tectonic plates. These plates move across the asthenosphere, which is a solid but plastic layer of the upper mantle. This movement is driven by mantle convection, which is heat transfer caused by the slow movement of ductile rock. This discovery provided a "grand unifying theory" for geology. It explained how continents move and why certain geological features appear in long, linear regions.

Plate boundaries are categorized by how the plates interact. Divergent boundaries occur at mid-ocean ridges where plates move apart. This movement often creates hydrothermal vents and volcanoes. Convergent boundaries happen when one plate subducts, or moves under another plate. This process often creates arcs of volcanoes and causes earthquakes. Transform boundaries occur where plates slide horizontally past each other. A famous example of this is the San San Andreas Fault system.

These movements are responsible for much of the Earth's surface deformation.

We can also map the deep interior of the Earth using seismology.

Seismologists study the arrival times of seismic waves to image the Earth's layers. These studies revealed that the Earth has a liquid outer core and a dense solid inner core. The Earth is organized into a layered model: the lithosphere on top, the mantle below, and the core at the center. The mantle is divided by seismic discontinuities at 410 and 660 kilometers. In the 1970s, new techniques like seismic full-waveform inversion allowed for more detailed imaging. This is similar to how a doctor uses a CT scan to see inside a human body.

The history of the Earth is recorded on a massive geological time scale.

This scale begins with the formation of the Solar System about 4.567 billion years ago. The Earth itself formed approximately 4.54 billion years ago. This marked the start of the Hadean eon. The scale tracks major milestones, such as the start of photosynthesis around 3.5 billion years ago. It also tracks the rise and fall of supercontinents like Pangaea. Geology helps us place human history, such as the appearance of modern Homo sapiens 200,000 years ago, into the much larger context of the planet's life.