Earth's Surface

The History of Earth

The Earth formed around 4.6 billion years ago. During this time, and even today, the surface of the Earth is ever-changing. The period of time between the creation of our planet and when the first life arose is still full of unknowns. About 3.7 billions years ago, life appeared on Earth. Only fossil evidence gives scientists an understanding of life that existed long before us.

Fossil Evidence

A fossil is any preserved evidence of an organism. Plants, animals, and even some types of bacteria can be fossilized. Many species that are fossils are extinct today, and the tiny fraction of fossils discovered can contain information from millions of years ago. Scientists who study fossils are called paleontologists. Fossils are almost like a prized possession, because the organism cannot decompose and must be quickly buried in sediment in order to be fossilized.

Any fossil serves as evidence of an organism that has lived before us. Scientists have discovered fossils of animals similar to some animal species today, but are not exactly the same. This connection is evidence of the progression of evolution.

How deep a fossil is buried is evidence of the age of the organism that was fossilized. The Earth's crust (the outermost layer) consists of horizontal layers of sedimentary rock. The deeper the fossil is found, the older it is. This also provides evidence of more complex organisms forming after simple organisms.

Earth's Interior

The Earth has four (4) main layers. The crust, mantle, outer core, and inner core.


The Earth's crust is the outermost layer. This brittle layer extends from 5 to 70 km deep, the thinnest of the four layers. The crust is a rocky layer made up of common elements including oxygen, silicon, aluminum, and iron. There are two types of crust: continental and oceanic crust. Oceanic crust is denser, which is why land masses rise above the oceans. About 95% of the crust is igneous rock.


The Earth's mantle is just below the crust. The mantle is about 2,900 km thick, and is the majority of Earth's volume. The mantle was originally molten rock, but gradually solidified into mostly silicates. The temperature in the mantle varies from 1000 to 3700 degrees Celsius. The less viscous areas of the mantle are responsible for plate tectonics and volcanic activity. The mantle is commonly separated into the upper and lower mantle.

Outer Core

The outer core is the Earth's third layer. It is the only fluid layer and comprises mostly of molten iron and nickel (known as magma) that rises into the mantle. The churning of liquid rock is responsible for magnetic fields and convection. The outer core is about 2200 km thick, and is up to 6000 degrees Celsius - as hot as the surface of the Sun.

Inner Core

The inner core is the innermost layer of Earth. It is a solid, dense ball made of mostly iron. It is relatively thin, which a radius of 1250 km. The inner core is the hottest of the four layers. Its temperature reaches up to 9,400 degrees Celsius. Although this temperature is far above the melting point of iron, the inner core is so pressurized that metal plasma behaves as a solid.

Plate Tectonics

Plate tectonics is a theory that Earth's crust is divided into several plates, or layers, that glide over the mantle. The movement of these plates are responsible for geographical occurrences we see on Earth, like mountains, volcanoes, trenches, earthquakes, and more.

Plate tectonics occur due to churning of magma in the mantle, known as convection. Convection is when hot material in the Earth's core rises and cooler mantle rock sinks. You can connect this to when a pot of water boils on a stove.

The Earth's rocky lithosphere consists of the crust and the upper mantle. The lithosphere contains plate boundaries, which are large masses of Earth's crust. Plate boundaries rest and slide over the asthenosphere, a layer of partially molten rock in the mantle.

In plate tectonics, plate boundaries are classified in three ways: divergent, convergent, and transform boundaries.

Divergent Boundaries

Divergent boundaries are two plates that move away from each other. This produces lava spews, geysers, and earthquakes. The gap where plates diverge is called a rift. Beneath the rift, magma rises from the mantle. Magma fills the gap and hardens, forming a dense, rocky substance called basalt. This basalt is the basis of oceanic crust.

Convergent Boundaries

Convergent boundaries are two plates that come together. The two plates will either buckle together their edges and form mountain ranges, or one plate will slide under the other and form trenches. A chain of volcanoes will often form parallel to the mountain range or trenches. The impact of converging plates causes massive earthquakes on both sides of the boundary.

Transform Boundaries

Transform boundaries are two plates that slide past on another. These may form canyons or faults. There may be earthquakes as the two plates bump into each other and rocks are crushed into smaller particles. Unlike divergent and convergent boundaries, no magma is produced. Instead, a fissure is created in the crust.

Weathering, Erosion, and Deposition

There are many processes that continuously changed the Earth's surface. Some of these processes break down or build up rocks that make up the Earth's crust.


Weathering is a process that break down rocks into smaller particles called sediment. There are two types of weathering: physical and chemical weathering.

Physical weathering is a type of mechanical weathering that occurs when water freezes in cracks and expands, causing the rock to break into pieces. Physical weathering is also caused by rain and wind. Chemical weathering is when rain containing gases or chemicals wear away surfaces gradually.


Erosion is when natural forces like gravity, rain, waves, and wind move rocks, sediments, and soil from one place to another. Most erosion is caused by rainwater carrying sediments through streams and rivers downhill.


Deposition is the process of building up rocks. This occurs when wind or rain and gradually lays down sediment. In other words, previously weathered material are deposited in new locations. Deposition changes the shape of the land.

Human Impact

Human activity has had a deep and lasting impact on Earth's surface. We are the number one reason for environmental degradation. Humans deplete resources such as air, water and soil. These are also resources that ecosystems rely on. We produce pollution that affects the natural processes used to regulate Earth's surface. It is important to stay aware of our actions and protect the planet that has supported life for over three billion years.

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