Earth is our home. It is a rocky planet, and the only planet in our Solar System that is known to contain life. Earth shares similarities with the the neighboring planets that formed in the inner solar system.
The planets closest to the Sun—Mars, Earth, Venus, and Mercury—are made mostly of rock. The rocky planets all formed in our inner solar system. Their geological history is preserved on their surfaces. Their landscapes reveal the processes that shaped them: impacts, crustal movements, volcanic activity, and erosion. Gravity, temperature, air, and water all play leading roles in their geological stories.
Rocky worlds like Earth can reshape themselves from internal forces that push and pull at their crustal materials, a process called tectonics. Contractional forces shove crustal material together to create cliff-like fault scarps, ridges, and mountains. Extensional forces stretch and pull the crust apart to form trough sand rift valleys. Planets and other celestial bodies across our solar system also bear the scars of collisions. Impact craters form on their surfaces when a dust particle, rock, asteroid, or comet smashes into them. While impacts are sudden, tectonic forces operate over long periods of time. The landforms they create can take millions of years to form.
Volcanism also plays an important role on Earth. The Earth has a hot interior core surrounded by hot mantles. One way rocky worlds release interior heat is through volcanic activity. This can involve molten rock, or magma, being forced into the crust. As the interior cools, it shrinks, causing the crust to wrinkle like the skin of an apple as the core dries and shrivels over time. Volcanism creates a variety of landforms, not just volcanoes, depending on the properties of the lava (such as viscosity and composition) and on the planetary environment (like gravity and presence of an atmosphere). The last featured characteristic of Earth is erosion. Erosion wears down landforms, but it also creates new ones. On Earth, water and wind are the main agents of erosion.
Earth’s “outer shell”— its crust and upper mantle, called the lithosphere — is broken into a mosaic of about 12 plates. They move around and forcibly interact in a process called plate tectonics. Earth appears unique in this regard.
As the Earth slowly cools, heat loss creates currents in the mantle that move the crustal plates. On Earth, most tectonic landforms are created by forces that push, pull, and slide tectonic plates. Mountain ranges, rift valleys, and long faults like the San Andreas in California mark the borders of plates.
Impact craters come in all sizes and shapes, depending on the impacting object size, impact angle, and surface into which the object crashes. Earth has impact craters all over its surface. Vredefort basin in South Africa is the largest known impact crater on Earth. It is more than 185 miles (300 kilometers) across.
Scientists have found evidence of volcanic activity on all the rocky planets. Volcanic activity can may cause magma to erupt onto the surface to become lava. A pyroclastic flow is a mix of hot ash, gases, and lava fragments that moves rapidly downslope. This pyroclastic flow spilled down the flank of Popocatépetl in Mexico.
Shiprock in New Mexico is a preserved core of a volcanic vent exposed by erosion. The vent was active 30 million years ago. Several sheet-like lava intrusions called dikes, in the foreground, radiate from the core on the left. Scientists have found evidence of volcanic activity on all the rocky planets.
Wind-blown sand has eroded parallel ridges into the bedrock in many places on Earth. Scientists call these ridges yardangs. The yardang seen here is in the Lea-Yoakum Dunes near Meadow, Texas.
