Earth was formed about 4.65 billion years ago. Scientists use the geologic time scale to describe events that have happened throughout Earth’s history.
If the entirety of Earth’s history was represented on a clock, humans would appear at 11:59:40 PM.
- 4.65 billion years ago: Earth is formed
- 4 billion years ago: The first life on Earth was simple, prokaryotic bacteria.
- 600 million years ago: Aquatic plants and animals evolved. It is likely that life originated in water because it offered early organisms more temperature stability compared to land, currents provided early movement, and they didn’t have to fight gravity.
- 500 million years ago: Huge period of evolution and diversification of life known as the Cambrian Period.
- 400 million years ago: Terrestrial plants and animals evolved.
- 65-250 million years ago: Dinosaurs lived during the Mesozoic Era.
- 200,00-300,000 years ago: Homo sapiens, early ancestors of humans, evolved.
At least three times in Earth’s history, the planet was engulfed almost entirely in ice–an event called . These events happened between 580 and 750 million years ago. Evidence for Snowball Earth comes from sedimentary rocks. In a normal ice age, the types of rock deposited by glaciers would be found mostly near Earth’s poles. However, geologists found glacial rocks of similar ages around the world, at both the poles and equatorial regions. This led to the Snowball Earth theory.
Snowball Earth was caused by a chain of events called a . Ice has a high and is very insulating, so it is not heated up efficiently by sunlight. As ice accumulated on the planet, it increasingly reflected more sunlight and cooled Earth even further. Thus, this created a cycle of ice formation, increased albedo, and cooling of Earth which continued until the entire planet was covered in ice.
Why is Earth not covered in ice now if the positive feedback loop continues forever? The leading theory is that continued erupting during Snowball Earth, emitting high levels of carbon dioxide, which was trapped under the ice. Eventually, the carbon dioxide built up enough to start melting the ice. Ice turned to water, which has a lower and thus absorbed more of the Sun’s energy. This led to a new positive feedback loop that resulted in warming of the planet and the end of Snowball Earth. Additionally, it is believed that bacterial life on Earth survived the harsh conditions during Snowball Earth events by living near the heat sources from volcanoes.
2-ESS2-3. Obtain information to identify where water is found on Earth and that it can be solid
Due to , Earth’s land is constantly shifting and changing over time. 250 million years ago, all of Earth’s landmass was united in a single supercontinent called . About 200 million years ago, this supercontinent began to break up into pieces as the plates moved away from each other. Smaller landmasses were formed until, eventually, the continents we recognize today were shaped. It is easy to see how certain continents, like South America and Africa, once fit together. Even today, the location of the continents continues to fluctuate. Scientists theorize that a new supercontinent, called Pangaea Proxima, could form 250 million years in the future.
Proposed by scientist Alfred Wegener, the theory of was one of the earliest ways to explain why continents moved over time. Wegener noticed that the shape of Earth’s continents, such as South America and Africa, could fit together like a jigsaw puzzle. He also studied fossils from different continents that showed the remains of identical plants and animals spread throughout the world. For example, dinosaurs lived during the Triassic, Jurassic, and Cretaceous periods. As Pangaea broke into different pieces, fossils show that dinosaurs spread to different continents and evolved into different species over time. Based on this evidence, Wegener theorized that all of Earth’s continents were originally united in the supercontinent Pangaea. Over time, they drifted apart to their current positions.
While parts of this theory were correct, Wegener’s ideas had some flaws. For example, Wegener was unable to explain how the continents had separated over time. Consequently, this theory was replaced by the theory which is discussed in the next chapter, “Earth’s Structure: At the Surface and Underground”.
Timeline of Earth’s History:
|2-ESS1-1.||Use information from several sources to provide evidence that Earth events can occur quickly or slowly. [Clarification Statement: Examples of events and timescales could include volcanic explosions and earthquakes, which happen quickly and erosion of rocks, which occurs slowly.] [Assessment Boundary: Assessment does not include quantitative measurements of timescales.]|
|2-ESS2-2.||Develop a model to represent the shapes and kinds of land and bodies of water in an area. [Assessment Boundary: Assessment does not include quantitative scaling in models.]|
|2-ESS2-3.||Obtain information to identify where water is found on Earth and that it can be solid or liquid.|
|4-ESS2-2.||Analyze and interpret data from maps to describe patterns of Earth’s features. [Clarification Statement: Maps can include topographic maps of Earth’s land and ocean floor, as well as maps of the locations of mountains, continental boundaries, volcanoes, and earthquakes.]|
|5-ESS2-1.||Develop a model using an example to describe ways the geosphere, biosphere, hydrosphere, and/or atmosphere interact. [Clarification Statement: Examples could include the influence of the ocean on ecosystems, landform shape, and climate; the influence of the atmosphere on landforms and ecosystems through weather and climate; and the influence of mountain ranges on winds and clouds in the atmosphere. The geosphere, hydrosphere, atmosphere, and biosphere are each a system.] [Assessment Boundary: Assessment is limited to the interactions of two systems at a time.]|
|5-ESS2-2.||Describe and graph the amounts of salt water and fresh water in various reservoirs to provide evidence about the distribution of water on Earth. [Assessment Boundary: Assessment is limited to oceans, lakes, rivers, glaciers, ground water, and polar ice caps, and does not include the atmosphere.]|
|MS-ESS2-2.||Construct an explanation based on evidence for how geoscience processes have changed Earth’s surface at varying time and spatial scales. [Clarification Statement: Emphasis is on how processes change Earth’s surface at time and spatial scales that can be large (such as slow plate motions or the uplift of large mountain ranges) or small (such as rapid landslides or microscopic geochemical reactions), and how many geoscience processes (such as earthquakes, volcanoes, and meteor impacts) usually behave gradually but are punctuated by catastrophic events. Examples of geoscience processes include surface weathering and deposition by the movements of water, ice, and wind. Emphasis is on geoscience processes that shape local geographic features, where appropriate.]|
|MS-ESS2-3.||Analyze and interpret data on the distribution of fossils and rocks, continental shapes, and seafloor structures to provide evidence of the past plate motions. [Clarification Statement: Examples of data include similarities of rock and fossil types on different continents, the shapes of the continents (including continental shelves), and the locations of ocean structures (such as ridges, fracture zones, and trenches).] [Assessment Boundary: Paleomagnetic anomalies in oceanic and continental crust are not assessed.]|
- Some events happen very quickly; others occur very slowly, over a time period much longer than one can observe. (2-ESS1- 1)
- Maps show where things are located. One can map the shapes and kinds of land and water in any area. (2-ESS2-2)
- Water is found in the ocean, rivers, lakes, and ponds. Water exists as solid ice and in liquid form. (2-ESS2-3)
- Earth’s major systems are the geosphere (solid and molten rock, soil, and sediments), the hydrosphere (water and ice), the atmosphere (air), and the biosphere (living things, including humans). These systems interact in multiple ways to affect Earth’s surface materials and processes. The ocean supports a variety of ecosystems and organisms, shapes landforms, and influences climate. Winds and clouds in the atmosphere interact with the landforms to determine patterns of weather. (5-ESS2-1)
- Nearly all of Earth’s available water is in the ocean. Most fresh water is in glaciers or underground; only a tiny fraction is in streams, lakes, wetlands, and the atmosphere. (5-ESS2-2)
- The planet’s systems interact over scales that range from microscopic to global in size, and they operate over fractions of a second to billions of years. These interactions have shaped Earth’s history and will determine its future. (MS-ESS2-2)
- Maps of ancient land and water patterns, based on investigations of rocks and fossils, make clear how Earth’s plates have moved great distances, collided, and spread apart. (MS-ESS2-3)
- Water’s movements—both on the land and underground—cause weathering and erosion, which change the land’s surface features and create underground formations. (MS-ESS2-2)
- Standard units are used to measure and describe physical quantities such as weight and volume. (5-ESS2-2)
- Patterns in rates of change and other numerical relationships can provide information about natural systems. (MS-ESS2-3)
- Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small. (MS-ESS1-4),(MS-ESS2-2)
A period of extreme glaciation where a positive feedback loop led to Earth being covered almost entirely by ice.
A process where one change triggers the next in a continuous cycle that encourages the initial change.
The amount of energy reflected by a surface. Light surfaces tend to have a high albedo because they reflect more energy. Dark surfaces tend to have a low albedo because they absorb more energy.
A vent that allows magma, rock fragments, ash, and gases to escape to the surface of a planet or moon.
Theory that Earth's crust is divided into many pieces, called tectonic plates, which move over time due to convection currents in the mantle.
A supercontinent made of all of Earth's landmass that existed 200-250 million years ago.
Theory that states that all of Earth's land was originally a united supercontinent that drifted apart over time. This theory was eventually replaced by the theory of plate tectonics.