Download Earth`s Internal Heat

Earth's Internal Heat
Earth's interior is the site of great amounts of heat. Most of this heat is produced by the decay of
radioactive elements. Overall, the flow of Earth's internal heat is outward toward Earth's surface.
Many geologic processes and features, such as tectonic plate motion, volcanic activity, and
geysers, are related to the Earth's internal heat. Large convection currents in the Earth's mantle
cause heat to circulate within the Earth's interior. These convection currents are linked to tectonic
plate motion and geologic activity at plate boundaries.
Convection in Earth's Mantle
Convection is typically discussed as it relates to heat flow in liquids and gases. Warmer parts of a
fluid tend to rise, while cooler parts tend to sink. This results in convection currents that help
distribute heat more evenly throughout the fluid.
Convection currents occur when warmer parts of a fluid rise, while cooler parts sink.
Convection can also occur in some solids. For example, pressure and temperature conditions in
the Earth's mantle allow mantle rock to slowly convect. Hotter rock rises and cooler rock sinks in
mantle convection cells, or currents. This process is related to several other processes, including
the movement of tectonic plates and the outward transfer of Earth's internal heat.
Mantle Convection & Plate Tectonics
The movement of Earth's tectonic plates relates to many factors, including mantle convection
and density differences in the plates. Earlier work on plate tectonics theory suggested that mantle
convection is the driving force behind plate motion.
However, more recent studies suggest that although mantle convection was likely a key factor in
beginning plate motion long ago, plate motion may actually be the main driving force behind
present mantle convection.
Regardless of cause-and-effect relationships between convection and plate motion, the two
processes are linked. The image below is a cross section through the Earth's interior.
Relationship between mantle convection and tectonic plate motion.
Image courtesy of USGS.
Tectonic plates of the lithosphere are shown in grey at the Earth's surface. The Earth's mantle is
shown in red-orange. Black arrows in the lithosphere show the directions in which the plates
move. Red arrows in the mantle represent convection currents.
As shown in the diagram, convection currents and tectonic plates tend to move in the same
directions near Earth's surface. Also, convection currents begin to move upward beneath midocean ridges, where plates pull away from each other, and convection currents begin to move
downward beneath trenches, where one plate plunges beneath another.
Plate Tectonics
Plate tectonics is a theory that describes the movements of large pieces, or plates, of the Earth's
outermost physical layer. The movements of the plates cause geological events, such as earthquakes
and the formation of volcanoes and mountain ranges.
Earth's Tectonic Plates
Earth's lithosphere (LITH uh sfeer) is divided into a dozen or more plates. As shown in the figure
below, Earth's continents and ocean floors are part of the tectonic plates. The rigid plates move
constantly but slowly (centimeters per year) on top of a less-rigid layer of the Earth, called the
asthenosphere (as THEEN uh sfeer). As the plates move around, they push into each other,
move away from each other, or slide past each other along their boundaries. There are many
possible forces driving these motions, including push/pull forces in the plates and convection
deep within the Earth.
Image courtesy of USGS.
Plate tectonics is a relatively new scientific theory. Earlier ideas about the movement of
continents and Earth's crust, such as continental drift and sea-floor spreading, couldn't explain
how the movements occurred. Plate tectonics explains these movements as well as many other
features and events on Earth, especially those associated with plate boundaries.
Plate Boundaries
There are three major types of plate boundaries.

Convergent plate boundaries exist where two tectonic plates move toward each other. If two
continental plates push together, they buckle and fold over millions of years to form mountain
ranges.
A boundary where an oceanic plate moves toward a continental plate is called a subduction
zone. In a subduction zone, the oceanic plate plunges (subducts) underneath the continental
plate, forming a deep-ocean trench on the seafloor. As the oceanic plate continues to plunge
downward into Earth's interior, hot molten rock (magma) rises up to form a chain of volcanoes
on the continental plate. This chain of volcanoes is called a continental volcanic arc.
A subduction zone will also form where two oceanic plates move toward each other. As one
oceanic plate subducts beneath another, rising magma forms volcanic island arcs on the upper,
overriding oceanic plate.

Divergent plate boundaries exist where two tectonic plates move away from each other. Where
two oceanic plates pull apart, magma rises and erupts as lava at the surface. The lava quickly
cools and hardens to form new crust. However, the newly formed crust is still much hotter than
older crust farther away from the plate boundary. Because the new crust is hotter, it is less
dense. Because of its lower density, the crust along the plate boundary forms a mid-ocean
ridge, which is 2 to 3 km higher than the surrounding ocean floor. In many areas of the world,
mid-ocean ridges have narrow valleys along their center-line where the new crust is forming.
These valleys are called rift valleys.
Rift valleys can also form on land. When part of a continental plate is stretched and thinned, a
long valley called a continental rift forms, and pieces of the plate begin to pull away from each
other along the rift. An example of a continental rift is the East African Rift.

Transform plate boundaries occur where two plates slide alongside each other without
significant vertical movements or major volcanic activity. Like other boundaries, though,
earthquakes are associated with transform plate movements.
Earthquakes & Volcanoes at Plate Boundaries
Although earthquakes and volcanoes occur in many different settings, they are especially
common along tectonic plate boundaries. At plate boundaries, rocks grind against each other,
releasing energy as earthquake waves. As mentioned above, volcanic activity is common at
convergent plate boundaries. Volcanoes along convergent boundaries at the edge of the Pacific
Plate form a prominent pattern, called the "Ring of Fire".
Image courtesy of USGS.
The world map above shows the locations of tectonic plate boundaries (black lines) and active
volcanoes (red dots). The pattern of active volcanoes along the edge of the Pacific Plate is called
the "Ring of Fire."
Volcanic activity is also common at divergent boundaries. For example, at the Mid-Atlantic
Ridge, lava regularly erupts and hardens to form new seafloor as the North American and
African Plates pull away from each other. Depending on eruption rates, lava flows may build on
top of each other to form tall seafloor volcanoes in this setting. Some of these volcanoes build up
above sea level, while others remain completely underwater. Another example of volcanic
activity at a divergent boundary can be seen at the boundary between the African and Arabian
Plates. As shown in the image below, several active volcanoes have formed where these plates
pull away from each other.
Image courtesy of USGS.
The map above shows the locations of active volcanoes (red triangles) along the tectonic plate
boundary between the African Plate and the Arabian Plate. Also, a new divergent plate boundary
is forming along the East African Rift Zone (dotted lines). In the East African Rift Zone, active
volcanoes have formed where the Nubian and Somalian sections of the African Plate are pulling
away from each other.
Folding & Faulting
The Earth’s crust is divided into large sections called tectonic plates. These plates move very
slowly over time. This movement causes stress in some parts of the crust, especially at the
boundaries where two different plates are touching each other.
Layers of rock tend to deposit in horizontal layers, with the newest layers on top. The law of
superposition states that newer layers form on top of old layers. The top of each layer is usually
flat, rather than curved, because sediment fills in the lowest valleys first.
Bent or broken rock indicates that the layers have changed since the time period in which they
formed. Geologists can track how the Earth’s crust has changed over time by noticing how the
layers of rock are folded, or broken and shifted along faults. Folding and faulting can change the
arrangement of rock layers so that the law of superposition no longer applies to all of the rock.
Faulting
When extreme stress and pressure cause rock to fracture, or break, it is called a fault. This break
will occur along weak spots in the rock. Faults are the result of either compressional (pushing)
or tensional (pulling) stress. Faults are most common at the borders of the tectonic plates, but
faults can occur anywhere that stress builds up in the rocks.
The stress along a fault is released when the pieces of crust move. The rocks rubbing together
can cause an earthquake. For this reason, earthquakes are more common at the boundaries
between the different tectonic plates.
Faulting
When the rock layers break and move along a fault, the two sides of the fault can shift vertically,
horizontally, or both. When the land shifts upward or downward (vertically) along a fault,
valleys and mountain ridges can be formed. When it shifts parallel to the ground (horizontally),
then the moving rocks can shift terrain and change the path of streams and runoff.
Folding
Stress and pressure do not always break rocks. Sometimes the pressure in the Earth shifts the
layers of rock upward or downward through folding. Large areas of folding may form valleys or
mountains.
Folding
Hotspots & Geysers
Earth's internal heat reaches the surface in geological and hydrogeologic features such as hotspots and
geysers.
Hotspots
There are many places on Earth's surface where the outward transfer of Earth's internal heat is
concentrated. Often, these areas are hotspots, which are areas of concentrated, long-lasting
volcanic activity. Some hotspots are associated with tectonic plate boundaries, but many are not.
Rather, they are locations where an isolated body of magma rises upward toward the Earth's
surface, transferring large amounts of heat.
The Hawaiian Islands are volcanic mountains that are forming near the middle of the Pacific
Plate. This hotspot has been active for millions of years. The image below shows a simplified
model in which volcanic eruptions from the mantle form the Hawaiian island chain as the Pacific
Plate moves over the fixed hotspot. Current research is exploring whether hotspots remain fixed
in position or shift locations.
Formation of the Hawaiian island chain over a hotspot
Image courtesy of U.S. Geological Survey and found at
http://oceanexplorer.noaa.gov/explorations/05galapagos/background/hotspots/media/Hotspot_Ca
rtoon_600.jpg
Geysers
Hydrogeologic features called geysers are often found at hotspots, as well as at areas of volcanic
activity in general. Geysers are eruptions of hot water from the ground. The heat required to
form geysers comes from the Earth's mantle. As mantle heat encounters groundwater percolating
downward, the water is heated and eventually erupts from the surface as hot water and steam.
Yellowstone National Park is located at a huge hotspot in North America and is the site of many
active geysers (see image below).
Grand Geyser, which is the tallest predictable geyser in the world, is just one of Yellowstone's
many geysers. This geyser can erupt as high as 60 m.
Image from http://mms.nps.gov/yell/ofvec/exhibits/eruption/images/grand2_lg.jpg.
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