There are actually more than 30 tectonic plates. One of the smallest is the Galapagos microplate, smaller than the U.S. state of Connecticut.
Earth's crust is only 40 kilometers (25 miles) thick on average, making it the thinnest of Earth's three layers of crust, mantle, and core. This thin band contains all known life in the universe.
The crust is the outermost layer of the Earth.
Earth has three layers. Beneath the crust is the mantle, which is made of semi-solid magma and solid rocks and minerals. At the center of the Earth is the extremely hot, metal core. The lack of air, water, and moderate temperature prevents organisms from living in the mantle or core.
Earth's crust is divided into 15 major tectonic plates: the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca plates. Tectonic plates actually slide around on the mantle, causing earthquakes, mountain formation, continental drift, volcanoes, and other geologic activity on the crust.
Billions of years ago, the Earth started out as a hot, gooey ball of rock. The heaviest material, mostly iron and nickel, sank to the center of the Earth and became the core. The surface of the Earth slowly cooled off and hardened. These surface rocks became the crust.
The crust is divided into two types: oceanic crust and continental crust. Oceanic crust, found under the ocean floor, is made of dense rocks such as basalt. It is about 7 kilometers (4 miles) thick. Continental crust, found under land masses, is made of less dense rocks such as granite. Its thickness varies between 10 and 75 kilometers (6 to 47 miles).
Continental crust is almost always much older than oceanic crust. Some of the oldest rocks in the world can be found in the Nuvvuagittuq greenstone belt in Quebec, Canada. This continental crust formation has rocks that are about 4 billion years old. Unlike continental crust, oceanic crust is still being formed in places called mid-ocean ridges. Here, magma from the mantle erupts through cracks in the ocean floor, creating crust as it cools.
Oceanic crust is heavier than continental crust. The heavy oceanic crust is constantly sinking, very slowly, underneath the lighter continental crust. This important process is called subduction. A chain of volcanoes formed at a subduction zone is called a volcanic arc. One such volcanic arc exists where the oceanic crust of the Australian plate subducts under the continental crust of the Eurasian plate. The Indonesian Island Arc, which includes the islands of Sumatra and Java in Indonesia, has some of the most powerful volcanoes in the world.
Eventually, oceanic crust sinks low enough to enter the mantle. Once this happens, the crust melts, then rises up again as magma in the mid-ocean ridges. In this way, the Earth enjoys a brand-new oceanic crust once every 200 million years or so.
Term Part of Speech Definition Encyclopedic Entry air Noun
the layer of gases surrounding Earth.
Encyclopedic Entry: air basalt Noun
type of dark volcanic rock.
continental crust Noun
thick layer of Earth that sits beneath continents.
continental drift Noun
the movement of continents resulting from the motion of tectonic plates.
Encyclopedic Entry: continental drift core Noun
the extremely hot center of Earth, another planet, or a star.
Encyclopedic Entry: core crust Noun
rocky outermost layer of Earth or other planet.
Encyclopedic Entry: crust dense Adjective
having parts or molecules that are packed closely together.
our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.
Encyclopedic Entry: Earth earthquake Noun
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
at some point in the future.
type of hard, igneous rock.
chemical element with the symbol Fe.
molten, or partially melted, rock beneath the Earth's surface.
Encyclopedic Entry: magma mantle Noun
middle layer of the Earth, made of mostly solid rock.
Encyclopedic Entry: mantle metal Noun
category of elements that are usually solid and shiny at room temperature.
mid-ocean ridge Noun
underwater mountain range.
inorganic material that has a characteristic chemical composition and specific crystal structure.
landmass that forms as tectonic plates interact with each other.
chemical element with the symbol Ni.
oceanic crust Noun
thin layer of the Earth that sits beneath ocean basins.
living or once-living thing.
natural substance composed of solid mineral matter.
process of one tectonic plate melting or going beneath another.
subduction zone Noun
area where one tectonic plate slides under another.
tectonic activity Noun
movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.
tectonic plate Noun
large, moveable segment of the Earth's crust.
all known matter, energy, and space.
volcanic arc Noun
chain of volcanoes formed at a subduction zone.