The theory of plate tectonics was developed in the 1960’s and states that the earth is broken into a number of shifting plates. Plate tectonics also provides geology with a comprehensive theory of how the earth’s crust moves. In this essay I will explain the different types of plate movements, how the plates move and how this movement affects earthquakes and volcanoes. To begin, there are three different types of plate movement; divergent boundaries, convergent boundaries, and transform boundaries.
At a divergent boundary the lithosphere breaks apart on parallel faults that move outward from each other. The plates break apart the block between the two faults sink into the asthenosphere creating a rift and pushing magma upward to fill in the crack. With a convergent boundary the plates move toward each other with one plate subducted under another plate. At the location where the plate sinks under another plate is called the subduction zone. Finally, with a transform boundary two plates slide horizontally past each other rubbing (Murck, Skinner, McKenzie).
To follow, each of the different boundaries move in different ways but mostly all move at the same speed. The plates are said to move relative to one another, moving just a few inches per year. The relative motion between the plates can range from three to ten centimeters per year. At five centimeters per year it would add up to thirty miles in one million years and some the earth’s plates have been moving for one hundred million years (Murck, Skinner, McKenzie).
To finish, the different types of plate movements cause different reactions in whether a certain boundary movement will cause a large earthquake or maybe not an earthquake at all. Divergent boundary earthquakes frequently shake the mid-ocean ridge as a result of faults that form as two plates separate. Mostly shallow earthquakes occur at divergent boundaries because at this boundary the asthenosphere is too hot and plastic to fracture. When there is a convergent boundary there is a rise of earthquakes because the subducting plate slips past the plate above giving off continuous jerks.
The earthquakes happen along the upper part of the sinking plate where it scrapes against the opposite plate. With a transform plate boundary the plates move past each other in three different ways and each way has a different effect of an earthquake. The first type of movement is a fault creep where the rocks move past each other at a continuous but slow rate, this movement occurs without violent earthquakes due to the slow movement. The second movement the plates pass each other with small hops causing many small earthquakes.
Finally with the third case, when the rocks near the faults deform and store the elastic energy the elastic energy exceeds friction the rocks slip along the fault and snaps back into its original position causing large destructive earthquakes (Murck, Skinner, McKenzie). On May 18, 1980 Mount Saint Helens erupted, the eruption blew off more than 1,300 ft. of the mountains top. Moreover, the eruption sent ash and debris over twelve miles into the air covering three states; Washington, Oregon, and Idaho.
The Mount Saint Helens eruption killed sixty two people and destroyed forests and lakes which resulted in three billion dollars in damage. Thousands of acres of timber were destroyed and lakes were clogged with mud and silt. Spirit Lake was turned into a mud littered with timber. The massive Mount Saint Helens eruption was attributed to interactions between the Pacific Plate, North American Plate, and the Juan de Fuca Plate, this area is known as a triple plate junction (Diggles). In conclusion, the tectonic plates all move in different directions and at different speeds.
There are three different types of plate movement; divergent boundaries, convergent boundaries, and transform boundaries. Each boundary moves in its own way, however, they all move at the same velocity. Lastly, each boundary has a different affect on earthquakes around the world.