Tectonic Plates:
Earth's outer shell is broken up into irregularly shaped pieces called tectonic plates that fit together like a jigsaw. The plates have taken on many different shapes and sizes since the Earth began cooling 4 billion years ago. Currently, these plates are divided into seven major plates and several dozen smaller ones. Each plate consists of both continental and oceanic crust. The solid, brittle lithosphere, in which the plates move with is made up of both the Earth's crust and the upper-most mantle. The semi-solid asthenosphere, which lies just below the lithosphere is composed of the lower portions of the mantle. The highly viscous asthenosphere allows the lithosphere to move over it much like a conveyor belt.
Tectonic Plates in Motion:
The Earth's tectonic plates are in constant motion with one another drifting a few inches per year. Scientists believe that convection currents deep within the mantle are the driving forces behind plate tectonic movement. The theory of plate tectonics explains geological processes, such as earthquakes, volcanic activity, mountain building, deep-sea trenches, and many other phenomena. Plate tectonics is a development of the earlier theory of continental drift proposed by a German meteorologist named Alfred Wegener. Be sure to click on image for better quality viewing of this diagram.
Plate Boundaries:
Plate boundaries are zones around the edges of tectonic plates where a high degree of tectonic activity, such as earthquakes and volcanic eruptions occur. Valuable ore and mineral deposits are often found along these plate boundaries. There are three major types of boundaries. They include divergent, convergent, and transform. Although not a boundary, a fourth type of tectonic feature called "hot spot" will be covered with plate tectonics.
Divergent Plate Boundary:
Divergent plate boundaries occur where plates slowly move apart through time. Such boundaries include two types: continental rifts and mid-ocean ridges. The image to the left represents a mid-ocean ridge. Beneath the center of the mid-ocean ridge, upwelling hot magma fills in the gap and new oceanic floor is formed. Note the convection currents rising upward and then sinking back down.
These currents recycle themselves along with the upper portion of the Earth's mantle. The oceanic plates above the mantle follow the same direction as do the convection currents. Therefore, at divergent plate boundaries, the plates move away from each other (diverge). Note, however, that the plates do not continue to move on forever. If they did, our Earth would continue to grow larger throughout time. Once the oceanic plates move far enough away from the hot upwelling magma at the mid-ocean ridge, they cool off and sink beneath the weight of a continental plate or another oceanic plate. The "rule of thumb" is, the heavier plate will sink (subduct) beneath the lighter plate. A common name given to this geological process is "subduction zone".
Continental Rifts:
A new divergent boundary forms where an upwelling plume of magma rises under a section of a continental crust. When this occurs, it causes the crust to soften, weaken, stretch, and become thin. Long linear faults (fissures) appear in the crust, which leads to the formation of volcanoes. A good example of a continental rift is right here in the United States along the Rio Grande Rift near Taos, New Mexico. As the rift valley extends from the stretching and thinning of the continental crust, water fills in the lower region, and eventually forms a shallow sea. At the rifts center, at the bottom of the sea, a mid-ocean ridge develops. As a result, new oceanic crust forms in this region, and eventually a full-fledged mid-ocean ridge forms, and with the birth of a new ocean. This is exactly how the Atlantic Ocean formed around 55 million years ago.
Convergent Plate Boundary:
Convergent plate boundaries occur where two plates move toward each other and collide. That is, they converge. Recall that these plates are driven by heat convection in the mantle. At these boundaries, large sections of the plate subduct (descend) into the deeper mantle and are destroyed. So, at convergent plate boundaries, plates are destroyed and later recycled in the mantle; while at divergent plate boundaries, new sea floor forms. Along the boundary, a deep trench forms in the ocean floor.
The subducting plate slides down with violent jerking, causing frequent earthquakes. And, as the plate descends (taking ocean water with it), the temperature increases, and the continental rocks near the subducting oceanic plate heat up and melt. The melt (magma) rises upward through cracks in the rock and forms deep magma chambers. The erupted magma (lava) forms a chain of volcanoes. Such inland volcanoes are known as volcanic arcs. The figure to the left represents one of three types of convergent plate boundaries called Oceanic-Continental Convergence. The other two types of convergent plate boundaries are Continental-Continental and Oceanic-Oceanic.
The subducting plate slides down with violent jerking, causing frequent earthquakes. And, as the plate descends (taking ocean water with it), the temperature increases, and the continental rocks near the subducting oceanic plate heat up and melt. The melt (magma) rises upward through cracks in the rock and forms deep magma chambers. The erupted magma (lava) forms a chain of volcanoes. Such inland volcanoes are known as volcanic arcs. The figure to the left represents one of three types of convergent plate boundaries called Oceanic-Continental Convergence. The other two types of convergent plate boundaries are Continental-Continental and Oceanic-Oceanic.
Transform Plate Boundaries:
Transform boundaries are zones where the edges of two plates move horizontally past each other. This movement can create a great deal of friction, and as it moves, it jerks along causing earthquakes. There are two basic types of transform plate boundaries: submarine transform and continental transform. A few transform boundaries exist on land.
Continental Transform Boundary:
One good example of a continental transform boundary is that of the San Andreas Fault that runs through the state of California. Along this boundary, portions of continental lithosphere belonging to the the Pacific and North American plates grind past each other in opposite directions, and as a result, cause frequent and intense earthquakes in the region. As represented in the figure to the left, the lower western part of North America has three smaller plates that are also causing havoc on the continent.
These individual plates colliding and moving past North America are responsible for creating the Cascade Range, which contains several composite volcanoes, and the Basin and Range Province of the Colorado Plateau. The majority of the transform boundaries occur on the ocean floor; however, submarine transform boundaries consist of relatively short fault lines in oceanic plates perpendicular to the mid-ocean ridges. Like continental transform plate boundaries, parts of oceanic lithosphere move past each other in opposite directions causing submarine earthquakes.
These individual plates colliding and moving past North America are responsible for creating the Cascade Range, which contains several composite volcanoes, and the Basin and Range Province of the Colorado Plateau. The majority of the transform boundaries occur on the ocean floor; however, submarine transform boundaries consist of relatively short fault lines in oceanic plates perpendicular to the mid-ocean ridges. Like continental transform plate boundaries, parts of oceanic lithosphere move past each other in opposite directions causing submarine earthquakes.
Hot Spots:
There are a few locations in the uppermost mantle that appear to be the source of oddly large amounts of energy. As this energy percolates to the surface, it causes a high degree of volcanic activity. Such locations are known as "hot spots". There are two main hypotheses as to why and how hot spots form, but the older theory seems to "stand the test time". The theory states that hot spots are the result of mantle plumes, which are narrow flows of hot, semi-molten rock rising up from the core-mantle boundary to particular spots under the Earth's lithosphere. The figure to the left illustrates an ocean hot spot where the Pacific plate is passing over a stationary hot spot and creating the Hawaiian Islands.
Notice how the magma plume stretches and thins out as it attempts to follow the older volcanic island which once resided over the magma plume. Hot spots often exist under oceanic lithosphere, however, they can and do sometimes occur on land. The Yellowstone hotspot in the US is one of the best known continental hot spots. Yellowstone has a large and deep magma chamber, geysers, and hot springs at the surface with large and infrequent volcanic eruptions. Yellowstone's last supervolcanic eruption occurred around 640,000 years ago and is overdue by 40,000 years for another super eruption. Volcanologists (geologists who study volcanoes) believe that the next supervolcanic eruption of Yellowstone will be 2,500 times greater than the 1980 Mount St. Helens eruption.
Notice how the magma plume stretches and thins out as it attempts to follow the older volcanic island which once resided over the magma plume. Hot spots often exist under oceanic lithosphere, however, they can and do sometimes occur on land. The Yellowstone hotspot in the US is one of the best known continental hot spots. Yellowstone has a large and deep magma chamber, geysers, and hot springs at the surface with large and infrequent volcanic eruptions. Yellowstone's last supervolcanic eruption occurred around 640,000 years ago and is overdue by 40,000 years for another super eruption. Volcanologists (geologists who study volcanoes) believe that the next supervolcanic eruption of Yellowstone will be 2,500 times greater than the 1980 Mount St. Helens eruption.
Hawaiian Islands:
About three-quarters of identified hot spots around the world are oceanic, where they exist under oceanic lithosphere and often times occur far from plate boundaries. At these hot spots, are large amounts of magma in the crust rising upward and erupting on the seafloor as lava. Continuous eruptions of lava create submarine volcanoes, and eventually they lead to form volcanic islands. The Hawaiian Islands (left satellite photo) represent an ocean hot spot. Each island formed by passing over the stationary hot spot and later became inactive as the plate continued to move off of the hot molten region.
The inactive volcanic islands are now eroding down through weathering processes. Through the passing of thousands of years, the older, inactive volcanoes will once again submerge beneath the surface waters.
Earthquakes:
Along the boundaries of tectonic plates, rocks are squeezed and stretched like a spring by the huge forces inside Earth. Nearer the surface, rocks are sufficiently cold and strong that they eventually break along faults. If this occurs suddenly, the rocks snap, generating the violent vibrations of an earthquake. The Earth can shake so violently that buildings collapse, cracks open up in the surface, and mountainsides tumble down. Yet these terrifying events, occurring without warning, are part of the natural workings of our planet.
