09 The History of Earth
Trace how geologists read Earth’s record, how the planet and its life changed over time, and how geological processes continue to reshape the world.
Reading Earth’s Record
Earth is about billion years old. Its history is reconstructed from evidence preserved in minerals, rocks, fossils, and landscape shapes. Geologists arrange this immense span into nested units—eons, eras, periods, and smaller intervals—whose boundaries often reflect important changes in rocks, climate, or life.
The includes the Hadean, Archean, and Proterozoic eons. The Eon contains the Paleozoic, Mesozoic, and Cenozoic eras. The earliest part of the geological record is incomplete because many ancient rocks have been changed or destroyed.
Geologists use more than one kind of evidence to establish timing. In undisturbed sedimentary rock, lower layers are generally older than the layers above them. Fossils help geologists compare layers from different locations. estimates numerical ages by measuring radioactive isotope decay in minerals. Used together, these methods help place events in sequence and estimate when they occurred.
Takeaway: Geological time provides an organized framework, while rocks and fossils supply evidence for the events placed within it.
Earth Takes Shape
Earth formed as rocky material gathered in the young Solar System. Frequent impacts and heat from within left the early planet extremely hot, with widespread molten rock. Dense, iron-rich material sank inward to form the core, while lighter materials rose to form the mantle and an early crust. A giant impact with another planetary body is the leading explanation for the Moon’s formation.
As Earth cooled, minerals crystallized and solid rock became more widespread. Volcanoes released gases, including water vapor, carbon dioxide, and nitrogen, helping to create an early atmosphere. As the surface cooled, water vapor condensed and liquid water accumulated to form oceans. Impacts may also have delivered water and other materials. The details and timing of these early changes remain active areas of research.
Takeaway: Earth’s early development involved heating, separation into internal layers, cooling, and the emergence of a crust, atmosphere, and oceans.
Minerals and the
A is a naturally occurring solid with a characteristic chemical composition and crystal structure. Minerals make up rocks, which are classified by how they form:
crystallize as magma or lava cools.
form when particles or dissolved minerals are deposited and then compacted or cemented. They can preserve fossils and evidence of past environments.
form when existing rocks are changed by heat, pressure, or fluids without fully melting.
These categories are connected through the . For example, weathering can break granite into grains. carries the grains away, and they may be deposited and eventually form sandstone. Burial and heat can change sandstone into metamorphic rock. If rock melts and the resulting magma later cools, new igneous rock can form.
Takeaway: A rock’s type describes how it formed, and rocks can change from one type to another over time.
Moving Plates and Changing Landscapes
Heat inside Earth drives the slow motion of . Where plates spread apart, new ocean crust forms. Where they converge, they collide, and one plate may sink beneath another. At transform boundaries, plates slide past each other. Plate motion rearranges continents and ocean basins, builds mountains, and fuels much volcanism.
Stress can build along faults until rocks suddenly slip, producing an . At the same time, weathering breaks down rock, while transports it through the action of water, wind, ice, and gravity. Sediment can be carried toward basins and become part of new rock layers. These processes both create and destroy parts of the geological record.
Takeaway: Earth’s surface is continually reshaped by processes within the planet and by processes acting at its surface.
Life Transforms and Diversifies
The oldest widely accepted evidence of life is at least billion years old. Early life consisted of single-celled organisms in an environment with very little free oxygen. Some microbes evolved photosynthesis, releasing oxygen. At first, oxygen reacted with materials in the oceans and rocks; later, it accumulated in the atmosphere during the , around billion years ago. This environmental change reshaped life and made oxygen-based metabolism possible for many organisms.
Cells with nuclei evolved later, followed by multicellular organisms. In the late , larger and more varied life appeared in the oceans. Around million years ago, the Cambrian Period began, with a major diversification of animal life preserved in fossils. Plants later colonized land, followed by diverse land animals.
Over time, changing sea levels and climate, plate movement, volcanism, and altered habitats and influenced evolution. The history of life is therefore connected to changes in Earth’s atmosphere, oceans, climate, and surface.
Takeaway: Life changed Earth’s environment, and environmental change in turn influenced the development and diversification of life.
Life and Continents Through the
The lasted from about to million years ago. Marine life diversified, plants and animals established themselves on land, and forests spread. Continents gradually joined to form the supercontinent Pangaea. The era ended with the largest known mass extinction, at the Permian–Triassic boundary.
The , from about to million years ago, began as Pangaea broke apart. Dinosaurs dominated many land ecosystems, while mammals and birds also evolved. The era ended with a mass extinction associated with a large asteroid impact alongside other environmental stresses. Non-avian dinosaurs disappeared, while groups including birds and mammals survived.
The began million years ago and continues today. Mammals and flowering plants diversified, continents moved toward their present positions, and mountain ranges rose. Repeated ice ages reshaped landscapes. carved valleys and deposited sediments, while continuing plate movement caused earthquakes and volcanism. Humans arrived very late in Earth’s history: modern humans have existed for only a tiny fraction of the planet’s age.
Takeaway: The eras of the mark major changes in life and geography, but Earth’s physical and biological history is a continuous process.