What is Geology? - Definition, History, Facts & Topics

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  • 0:02 Definition
  • 0:35 Two Main Types
  • 1:18 History
  • 5:35 Topics in Geology
  • 6:53 Lesson Summary
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Lesson Transcript
Instructor: Kimberly Schulte

Kimberly has taught at the university level for over 17 years.

Did you know Earth is over 4.5 billion years old? Did you also know that the continents move and have joined together to create supercontinents? Learn about geology, what it is, its history and some interesting facts.


Geology is the study of the earth (geo means earth, and ology means study of). This is a very simple definition for something so complex. Geology involves studying the materials that make up the earth, the features and structures found on Earth as well as the processes that act upon them. Geology also deals with the study of the history of all life that's ever lived on or is living on the earth now. Studying how life and our planet have changed over time is an important part of geology.

Two Main Types of Geology

Typically, geology is divided into two categories: physical geology and historical geology. Physical geology deals with the study of the physical features of the earth and the processes acting on them. This includes volcanoes, earthquakes, rocks, mountains and the oceans; just about any feature of the earth.

Historical geology is the study of the history of the earth. Historical geologists focus on what's happened to Earth since its formation. They also study the changes in life throughout time. In historical geology, you essentially get to travel back in time to the formation of the earth and move forward through time, witnessing the changes in Earth itself and the life on it.

Geologists study the earth, its subsurface and all the processes acting on the earth.
The Earth


Geology has been of interest to humans as far back as ancient Greece in the 4th century. Aristotle was one of the first people to make observations about the earth. This was also the first time that scientists and philosophers noted a difference between rocks and minerals. The Romans became very adept at mining certain rock for use in building their empire, especially marble.

In the 17th century, fossils were being used as a way to understand what happened to the earth over time. These fossils played a key role in the debate about the age of Earth. For a while and even in some cases today, theologians and scientists have been at odds about the age of Earth. Theologians believed Earth was only about 6,000 years old while the scientists believed it to be much older.

In the 18th century, scientists started focusing on minerals and mineral ores since mining was an important part of global economies. During this century, two main theories came forward explaining some of the physical features of the earth. One theory believed that all rocks were deposited by the oceans during flooding events. The second theory believed that some rocks were formed through heat or fire.

This debate continued into the 19th century until James Hutton proved that some rocks are formed by volcanic (heat & fire) processes and others are formed by sedimentation. Hutton also explained that all the processes we see going on today, are the same processes that happened in the geologic past and that they occurred very slowly.

In other words, the erosion that is occurring to our mountains today is the same process that eroded mountains in the past. This theory became known as Uniformitarianism, which simply stated says 'the present is the key to the past.' James Hutton is known as the Father of Modern Geology.

James Hutton, the Father of Modern Geology.
James Hutton

Once Uniformitarianism was accepted by the scientific community, all the geologic pieces started to fall into place. Geologists began to understand how fossils could help them date the earth and different rock layers called strata. The fossils acted as markers that allowed geologists to place them in order of occurrence, allowed them to correlate rock strata found over great distances, and helped them understand the changes in life over time and the changes in Earth's environment through time.

The next big leap for geology happened in the early 1900s. A scientist, Alfred Wegener proposed a theory called Continental Drift. Wegener suggested that the continents moved around on the surface of the earth and came together to form a supercontinent known as Pangaea.

Alfred Wegener.
Alfred Wegener

He cited several pieces of evidence to prove his theory—the continents all fit together like puzzle pieces, the same rock unit or fossil could be found on both sides of an ocean and similar features such as mountains could be found on continents when they were all together.

The supercontinent, Pangaea. You can see how the continents fit together like puzzle pieces.

He suggested that continents 'floated' or 'drifted' to their positions. However, he could not explain how this happened. The scientific community rejected his theory until the 1940s. The technology boom associated with WWII brought advances in sonar and radar. In 1947, two geologists mapped the ocean floor, which revealed evidence that oceanic crust is created at mid-ocean ridges.

A close up look at a mid-ocean ridge where magma from the subsurface pushes through cracks in the ocean floor.
Mid Ocean Ridge

This became known as seafloor spreading. These mid-ocean ridges, are found on the bottom of the oceans and are major cracks or vents in oceanic crust. Magma from the mantle pushes its way up through the cracks (think of squeezing toothpaste from its tube). As it does this, it pushes the existing crust causing continents to move around. This led to the Theory of Plate Tectonics, which is based on the idea that Earth is broken into tectonic plates and these plates move in response to seafloor spreading.

Imagine taking a hard-boiled egg and dropping it on the floor. The egg cracks all around. The areas between the cracks are called plates and the cracks are called boundaries. The same principle applies to the earth. If we could shake off all the water on the planet so we could see the ocean floor, we would be able to see these cracks and boundaries.

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