UCSB Science Line
Divergent boundaries -- where new crust is generated as the plates pull away from Transform boundaries -- where crust is neither produced nor . Among the three techniques, to date the GPS has been the most useful for. As these zones of shear link other plate boundaries to one another, Because new crust is formed, divergent margins are also called constructive margins. .. as well as their complex relationships with subduction processes. Dating Rocks Using Fossils . The plates are made up of crust and the lithospheric part of the mantle (Figure ), and even though they are Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from Some of the processes taking place in this setting include.
The volcanoes are known as a continental arc. The movement of crust and magma causes earthquakes.
New Ocean Crust May Form Slower Than Thought
The Andes Mountains, which line the western edge of South America, are a continental arc. The volcanoes are the result of the Nazca plate subducting beneath the South American plate Figure 6.
Subduction of an oceanic plate beneath a continental plate forms a line of volcanoes known as a continental arc and causes earthquakes. Helens, which erupted explosively on May 18,is the most famous and currently the most active of the Cascades volcanoes.
The Juan de Fuca plate forms near the shoreline at the Juan de Fuca ridge. Sometimes the magma does not rise all the way through the continental crust beneath a volcanic arc.
This usually happens if the magma is rich in silica. These viscous magmas form large areas of intrusive igneous rock, called batholiths, which may someday be uplifted to form a mountain range.
The Sierra Nevada batholith cooled beneath a volcanic arc roughly million years ago Figure 6. Similar batholiths are likely forming beneath the Andes and Cascades today. The granite batholith of the Sierra Nevada Mountain range is well exposed here at Mount Whitney, the highest mountain in the range at 14, feet 4, meters and the second highest mountain in North America.
A convergent plate boundary subduction zone between two plates of oceanic lithosphere. Melting of the subducting plate causes volcanic activity and earthquakes. When two oceanic plates converge, the older, denser plate will sink beneath the other plate and plunge into the mantle. As the plate is pushed deeper into the mantle, it melts, which forms magma. As the magma rises it forms volcanoes in a line known as an island arc, which is a line of volcanic islands Figure 6. The Japanese, Indonesian, and Philippine islands are examples of island arc volcanoes.
The volcanic islands are set off from the mainland in an arc shape as seen in this satellite image of Japan Figure 6. Japan is an island arc composed of volcanoes off the Asian mainland, as seen in this satellite image. When two plates of continental crust collide, the material pushes upward forming a high mountain range.
The remnants of subducted oceanic crust remain beneath the continental convergence zone. When two continental plates collide, they are too thick to subduct.
Just like if you put your hands on two sides of a sheet of paper and bring your hands together, the material has nowhere to go but up Figure 6. Some of the world's largest mountains ranges are created at continent-continent convergent plate boundaries. In these locations, the crust is too thick for magma to penetrate so there are no volcanoes, but there may be magma. Metamorphic rocks are common due to the stress the continental crust experiences.
As you might think, with enormous slabs of crust smashing together, continent-continent collisions bring on numerous earthquakes. The world's highest mountains, the Himalayas, are being created by a collision between the Indian and Eurasian plates Figure 6. The Appalachian Mountains are the remnants of a large mountain range that was created when North America rammed into Eurasia about million years ago. The high peak in the center is world's tallest mountain, Mount Everest 8, meters; 29, feet.
Transform Plate Boundaries Figure 6. At the northern end of the picture, the transform boundary turns into a subduction zone. Transform plate boundaries are seen as transform faults. At these earthquake faults, two plates move past each other in opposite directions. Where transform faults bisect continents, there are massive earthquakes.
The world's most notorious transform fault is the 1, kilometer mile long San Andreas Fault in California Figure 6. This is where the Pacific and North American plates grind past each other, sometimes with disastrous consequences. California is very geologically active. A transform plate boundary creates the San Andreas Fault. A convergent plate boundary between an oceanic plate and a continental plate creates the Cascades volcanoes. Just offshore, the Juan de Fuca ridge is subducting beneath the North American plate at a divergent plate boundary.
Earth's Changing Surface Geologists now know that Wegener was right when he said that the continents had once been joined into the supercontinent Pangaea and are now moving apart. Most of the geologic activity that we see on the planet today is due to the interactions of the moving plates. Where plates come apart at a divergent boundary, there is volcanic activity and small earthquakes.
If the plates meet at a convergent boundary, and at least one is oceanic, there is a chain of volcanoes and many earthquakes. If both plates at a convergent boundary are continental, mountain ranges grow.
If the plates meet at a transform boundary, there is a transform fault. These faults do not have volcanic activity but they have massive earthquakes. If you look at a map showing the locations of volcanoes and earthquakes in North America, you will see that the plate boundaries are now along the western edge.
This geologically active area makes up part of the Pacific Ring of Fire. California, with its volcanoes and earthquakes, is an important part of this region.
The eastern edge of North America is currently mostly quiet, although mountain ranges line the area. If there is no plate boundary there today, where did those mountains come from? Remember that Wegener used the similarity of the mountains in eastern North America, on the west side of the Atlantic, and the mountains in Great Britain, on the eastern side of the Atlantic, as evidence for his continental drift hypothesis.
These mountains were formed at a convergent plate boundary as the continents that made up Pangaea came together. So about million years ago these mountains were similar to the Himalaya today Figure 6.
The Appalachian Mountains of eastern North America were probably once as high as the Himalaya, but they have aged since the breakup of Pangaea. Before the continents collided they were separated by an ocean, just as the continents rimming the Pacific are now.
That ocean crust had to subduct beneath the continents just as the oceanic crust around the Pacific is being subducted today. Subduction along the eastern margin of North America produced continental arc volcanoes. Ancient lava from those volcanoes can be found in the region. Currently, Earth's most geologically active area is around the Pacific. The Pacific is shrinking at the same time the Atlantic is growing. But hundreds of millions of years ago, that was reversed: Scientists think that the creation and breakup of a supercontinent takes place about every million years.
Intraplate Activity While it is true that most geological activity takes place along plate boundaries, some is found away from the edges of plates. This is known as intraplate activity. The most common intraplate volcanoes are above hotspots that lie beneath oceanic plates.
What occurs at plate boundaries that are associated with seafloor spreading?
There are three types of boundaries that can occur if you consider the relative movement of two spherical caps on a sphere: Converging where one plate dives beneath the other. An example is what happens off the coast of Japan where the pacific plate dives underneath the Asian plate 2. In this case, material from below rushes up to fill the crack. This is where the two plates slide past each other --an example is the san Andras fault.
It represents the boundary between the North American and pacific plates. In about 40 million years LA will be in the Aleutian Islands and will dive underneath back into the earth interior.
Oceanic crust is very dense compared with continental crust, so it sinks beneath continental crust. Ocean crust can be pulled in two opposite directions because this sinking of oceanic crust beneath continental crust. Wide zones of deformation are usually characteristic of plate boundaries because of the interaction between two plates. The three boundaries are characterized by their distinct motions. The first sort of plate boundary is called a divergent boundary, or spreading center.
At these boundaries, two plates move away from one another. As the two move apart, mid-ocean ridges are created as magma from the mantle upwells through a crack in the oceanic crust and cools.
This, in turn, causes the growth of oceanic crust on either side of the vents. As the plates continue to move, and more crust is formed, the ocean basin expands and a ridge system is created. Divergent boundaries are responsible in part for driving the motion of the plates. As you can imagine, the formation of the new crust on either side of the vents would act to push plates apart, as we see at the Mid-Atlantic Ridge, which helps to move North America and Europe further and further apart.
Mid-ocean ridges are vast mountain chains in the ocean and are as tall if not taller than mountain chains on land. The process which actually drives the motion at these ridges is known as convection. Magma is pushed upwards through the ridge cracks by convection currents. As some magma erupts out through the crust, the magma which does not erupt continues to move under the crust with the current away from the ridge crest.
These continual convection currents, called convection cells, help to move the plates away from each other to allow more crust to be created and the sea floor to grow. This phenomenon is known as sea-floor spreading.