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Wind, Water, Ice, and more!

http://www.nationalgeographic.org/encyclopedia/erosion/

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Wind, Water, Glaciers, Tectonic Plates

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The North American and Eurasian Plates are moving away from each other along the line of the Mid Atlantic Ridge. The Ridge extends into the South Atlantic Ocean between the South American and African Plates. The ocean ridge rises to between 2 to 3 km above the ocean floor, and has a rift valley at its crest marking the location at which the two plates are moving apart.

The Mid Atlantic Ridge, like other ocean ridge systems, has developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American Plates. As the mantle rises towards the surface below the ridge the pressure is lowered (decompression) and the hot rock starts to partially melt. This produces basaltic volcanoes when an eruption occurs above the surface ( Eyjafjallajökull in Iceland) and characteristic basalt " pillow lava" in underwater eruptions. In this way, as the plates move further apart new ocean lithosphere is formed at the ridge and the ocean basin gets wider. This process is known as " sea floor spreading " and results in a symmetrical alignment of the rocks of the ocean floor which get older with distance from the ridge crest.

Evidence for this process comes from the magnetic properties of the erupted basalt. The Earth's magnetic field has been shown to "flip" occasionally so that the North and South magnetic poles reverse with time. Basalt contains minute magnetic minerals that take on the direction of the Earth's magnetic field at the time of eruption.
Image courtesy of the Naked Science Society
These polarity reversals are therefore recorded in the rocks forming at the Mid Atlantic Ridge every time the Earth's magnetism changes from a normal field (as it is today) to a reversed field (as it was about 0.78 million years ago). When these variations in the magnetic rocks are mapped on the ocean bed they are seen to line up in a series of alternating " magnetic stripes ", rather like barcodes.

The significance of the magnetic stripes was only revealed when their ages were discovered from dating magnetic reversals in volcanic rocks accessible on land. The dates revealed that the Atlantic Ocean was opening by seafloor spreading from the Mid Atlantic Ridge at a rate of about 0.02 metres per year.

This means that North America and Europe are moving away from each other at about the rate it takes for your fingernails to grow.

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The North American and Eurasian Plates are moving away from each other along the line of the Mid Atlantic Ridge. The Ridge extends into the South Atlantic Ocean between the South American and African Plates.

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Many rift valleys are part of " triple junctions," a type of divergent boundary where three tectonic plates meet at about 120° angles. Two arms of the triple junction can split to form an entire ocean. The third, "failed rift" or aulacogen, may become a rift valley.

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Carribeand Islands

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The South American Plate is moving westwards due to sea floor spreading at the Mid Atlantic Ridge. Where it meets the Caribbean Plate, it descends (subducts) beneath it. This is because the oceanic lithosphere of the South American Plate is cooler and denser than that of the Caribbean Plate. The subduction causes low density ocean floor sediment to be scraped off the surface of the South American Plate and thrust onto the Caribbean Islands as accretionary wedges, in a process called obduction. The line of subduction is marked by the deep sea Puerto Rico Trench.

As the South American Plate descends, it drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone.

Continued subduction of the South American Plate brings sea water, locked in the ocean crust, deep into the mantle. As the plate heats up the water is liberated, lowering the melting point of the mantle and causing partial melting. This produces magma, which rises and may be erupted explosively as andesite at the surface.

Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000 o C. It is viscous, trapping gases as it rises. The water and gases in andesitic magma account for the explosive activity of andesitic volcanoes, which typically lie dormant for many hundreds or thousands of years. These volcanoes typically produce ash and pyroclastic flows, as well as small amounts of andesitic lava.

The eruptions on Montserrat during the 1990s are a good example of this type of activity.

The Caribbean volcanic islands form a curved linear chain or 'volcanic island arc' parallel and to the west of the Puerto Rico Trench.

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An online resource from the Geological Society, outlining the three types of plate boundary and the activity that characterises them.

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Volcanoes

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The simplified version of plate tectonics!

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Earthquakes

S an Andreas Fault