Historical geology is the study of the earth’s history and the changes it has undergone over millions of years. It involves the study of fossils, rocks, and other geological features to understand the evolution of the earth’s surface and the life that has inhabited it. This guide aims to provide a comprehensive overview of the key topics and concepts in historical geology, specifically tailored to students at Old Dominion University.

The Earth’s History

The earth is estimated to be around 4.5 billion years old, and its history is divided into different eons, eras, periods, and epochs, based on significant geological events and changes in the earth’s climate and environment.

The oldest eon is the Hadean eon, which lasted from the formation of the earth to around 4 billion years ago. This eon is characterized by intense volcanic activity, the formation of the earth’s crust, and the emergence of the first oceans.

The Archean eon followed the Hadean eon and lasted from 4 billion to 2.5 billion years ago. During this eon, the earth’s atmosphere began to form, and life is thought to have emerged in the form of simple bacteria.


The Proterozoic eon followed the Archean eon and lasted from 2.5 billion to 541 million years ago. This eon saw the emergence of more complex life forms, including multi-cellular organisms, and the formation of the first continents.

The Phanerozoic eon is the most recent eon and began around 541 million years ago. It is divided into three eras: the Paleozoic, the Mesozoic, and the Cenozoic. Each era is characterized by significant geological events and the emergence of new life forms.

Paleozoic Era

The Paleozoic era lasted from 541 million to 252 million years ago and is divided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

During the Cambrian period, life exploded in diversity, with the emergence of many new species, including trilobites, brachiopods, and mollusks. The Ordovician period saw the emergence of jawless fish and the diversification of marine life.

The Silurian period is known for the emergence of the first land plants and the diversification of invertebrates. The Devonian period saw the emergence of amphibians and the formation of the first forests.

The Carboniferous period is known for the formation of coal deposits and the emergence of reptiles. The Permian period saw the emergence of new reptile groups and the formation of the supercontinent Pangaea.



Mesozoic Era

The Mesozoic era lasted from 252 million to 66 million years ago and is divided into three periods: the Triassic, Jurassic, and Cretaceous.

During the Triassic period, the first dinosaurs emerged, along with the first mammals and the first turtles. The Jurassic period saw the emergence of many iconic dinosaurs, including the stegosaurus and the tyrannosaurus rex.

The Cretaceous period saw the emergence of new dinosaur groups, including the triceratops and the velociraptor. It also saw the emergence of flowering plants and the extinction of the dinosaurs at the end of the period, likely caused by an asteroid impact.

Cenozoic Era

The Cenozoic era began around 66 million years ago and is divided into two periods: the Paleogene and the Neogene. The Paleogene period saw the emergence of the first primates and the diversification of mammals.

The Neogene period saw the emergence of many modern mammal groups, including cats, dogs, and horses. It also saw the emergence of humans, with the first hominids appearing around 6 million years ago.


Geological Processes

Geological processes are the forces and mechanisms that shape the earth’s surface and create the different geological features we see today. These processes can be divided into two categories: endogenic processes and exogenic processes.

Endogenic processes are those that originate within the earth itself, such as volcanic activity, tectonic plate movement, and mountain building. Volcanic activity occurs when molten rock, or magma, rises to the earth’s surface and erupts, creating new landforms such as islands and mountains.

Tectonic plate movement occurs when the earth’s crust is divided into large plates that move and interact with each other, creating features such as mountains, volcanoes, and earthquakes. Mountain building occurs when two plates collide, causing the crust to buckle and fold, creating mountains such as the Himalayas.

Exogenic processes are those that originate from external forces, such as erosion, weathering, and deposition. Erosion is the process by which soil, rock, and other materials are worn away by wind, water, and other natural forces.

Weathering is the breakdown of rocks and other materials through exposure to the elements, including wind, rain, and temperature

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