Subduction zone of the Juan de Fuca Plate and North America Plate, creating the Mt. St. Helens and Mt. Hood volcanoes.
Are the tectonic plates floating on magma?
Earth’s tectonic plates rest upon the asthenosphere, the upper layer of Earth’s mantle. Over short timescales, the mantle behaves like a solid. If you could reach down and grab a handful of mantle rock it would be just that; solid rock. However, over long geologic timescales the mantle can behave like a thick liquid that slowly flows at about the same rate that fingernails grow.
Magma does exist in the asthenosphere in some places along subduction zones and spreading centers. Water that is brought down by subduction allows rock to melt at lower temperatures. Decreased pressure where the plates are separating also allows mantle rock to melt, but overall, these areas of melt are comparatively small. They are represented by the red blobs against the orange asthenosphere in this cartoon.
The only place in Earth's interior that is entirely liquid is the outer core. This is well-known because of the way it effects seismic waves; it stops S-waves and the velocity of P-waves are sharply reduced.
Learn More: This Dynamic Earth – The Story of Plate Tectonics
Related Content
How fast do tectonic plates move?
A good approximation used by geologists is that the plates move roughly the same speed as your nails grow. However, tectonic plates move at different speeds and in different directions. We can measure crustal motion using Global Positioning Systems (GPS) that measure within a fraction of a millimeter per year. We can also estimate velocities over the span of geologic time using rocks in ocean...
What was Pangea?
From about 300-200 million years ago (late Paleozoic Era until the very late Triassic), the continent we now know as North America was contiguous with Africa, South America, and Europe. They all existed as a single continent called Pangea. Pangea first began to be torn apart when a three-pronged fissure grew between Africa, South America, and North America. Rifting began as magma welled up through...
Where can I find information about the geology and natural history of National Parks?
Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. A source of information from the USGS is our Geology and Ecology of National Parks website. The website has listings for regions of the country; selected parks are listed within each region...
What are igneous rocks?
Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies. Intrusive Igneous Rocks: Intrusive, or plutonic, igneous rock forms...
What are metamorphic rocks?
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates...
What are sedimentary rocks?
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks: Common sedimentary rocks include sandstone...
Subduction Zone
Subduction zone of the Juan de Fuca Plate and North America Plate, creating the Mt. St. Helens and Mt. Hood volcanoes.
Plate Tectonics Map
Plate Tectonics is the theory supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another. Slip on faults that define the plate boundaries commonly results in earthquakes.
Plate Tectonics is the theory supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another. Slip on faults that define the plate boundaries commonly results in earthquakes.
PubTalk 7/2007 — Plate Tectonics in Action
Exploring the Earth with the new edition of This Dynamic Planet map and Web site
By Robert Tilling, Volcanologist, and Stephen Kirby, Earthquake Geophysicist
Exploring the Earth with the new edition of This Dynamic Planet map and Web site
By Robert Tilling, Volcanologist, and Stephen Kirby, Earthquake Geophysicist
Subduction of the Juan de Fuca Plate beneath the North American Pla...
Block diagram showing subduction of the Juan de Fuca Plate beneath the North American Plate along the Cascadia Trench, which is the western edge of the Cascadia subduction zone. Oceanic crust forms by eruptions along the Juan de Fuca Ridge.
Block diagram showing subduction of the Juan de Fuca Plate beneath the North American Plate along the Cascadia Trench, which is the western edge of the Cascadia subduction zone. Oceanic crust forms by eruptions along the Juan de Fuca Ridge.
Related Content
How fast do tectonic plates move?
A good approximation used by geologists is that the plates move roughly the same speed as your nails grow. However, tectonic plates move at different speeds and in different directions. We can measure crustal motion using Global Positioning Systems (GPS) that measure within a fraction of a millimeter per year. We can also estimate velocities over the span of geologic time using rocks in ocean...
What was Pangea?
From about 300-200 million years ago (late Paleozoic Era until the very late Triassic), the continent we now know as North America was contiguous with Africa, South America, and Europe. They all existed as a single continent called Pangea. Pangea first began to be torn apart when a three-pronged fissure grew between Africa, South America, and North America. Rifting began as magma welled up through...
Where can I find information about the geology and natural history of National Parks?
Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. A source of information from the USGS is our Geology and Ecology of National Parks website. The website has listings for regions of the country; selected parks are listed within each region...
What are igneous rocks?
Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies. Intrusive Igneous Rocks: Intrusive, or plutonic, igneous rock forms...
What are metamorphic rocks?
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates...
What are sedimentary rocks?
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks: Common sedimentary rocks include sandstone...
Subduction Zone
Subduction zone of the Juan de Fuca Plate and North America Plate, creating the Mt. St. Helens and Mt. Hood volcanoes.
Subduction zone of the Juan de Fuca Plate and North America Plate, creating the Mt. St. Helens and Mt. Hood volcanoes.
Plate Tectonics Map
Plate Tectonics is the theory supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another. Slip on faults that define the plate boundaries commonly results in earthquakes.
Plate Tectonics is the theory supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another. Slip on faults that define the plate boundaries commonly results in earthquakes.
PubTalk 7/2007 — Plate Tectonics in Action
Exploring the Earth with the new edition of This Dynamic Planet map and Web site
By Robert Tilling, Volcanologist, and Stephen Kirby, Earthquake Geophysicist
Exploring the Earth with the new edition of This Dynamic Planet map and Web site
By Robert Tilling, Volcanologist, and Stephen Kirby, Earthquake Geophysicist
Subduction of the Juan de Fuca Plate beneath the North American Pla...
Block diagram showing subduction of the Juan de Fuca Plate beneath the North American Plate along the Cascadia Trench, which is the western edge of the Cascadia subduction zone. Oceanic crust forms by eruptions along the Juan de Fuca Ridge.
Block diagram showing subduction of the Juan de Fuca Plate beneath the North American Plate along the Cascadia Trench, which is the western edge of the Cascadia subduction zone. Oceanic crust forms by eruptions along the Juan de Fuca Ridge.