earths tectonic plates

how plates move

If seafloor spreading drives the plates, what drives seafloor spreading? This goes back to Arthur Holmes idea of mantle convection. Picture two convection cells side by side in the mantle, similar to the illustration in Figure 1.2. 1. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust. 2. The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor. 3. The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches. 4. The material sinks to the core and moves horizontally. 5. The material heats up and reaches the zone where it rises again.

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what is a plate

What portion of Earth makes up the plates in plate tectonics? Again, the answer came about in part due to war. In this case, the Cold War. During the 1950s and early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. These seismographs also recorded all of the earthquakes around the planet. The seismic records were used to locate an earthquakes epicenter, the point on Earths surface directly above the place where the earthquake occurs. Why is this relevant? It turns out that earthquake epicenters outline the plates. This is because earthquakes occur everywhere plates come into contact with each other. The lithosphere is divided into a dozen major and several minor plates (Figure 1.1). A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both. The movement of the plates over Earths surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, about the same rate fingernails grow.

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plate boundaries

Plate boundaries are the edges where two plates meet. How can two plates move relative to each other? Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. The features found at these plate boundaries are the mid-ocean ridges, trenches, and large transform faults (Figure 1.3). Divergent plate boundaries: the two plates move away from each other. Convergent plate boundaries: the two plates move towards each other. Transform plate boundaries: the two plates slip past each other. The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there. We can visit each of these types of plate boundaries on land or at sea.

instructional diagrams

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questions

if two plates at a plate boundary are sliding past each other, what type of boundary is it?

a) divergent

-->  b) transform

c) convergent

d) normal

the top of a convection cell moves away from the ridge axis along with the seafloor.

-->  a) true

b) false

when two continents split apart, this forms.

a) a mountain range

b) a transform fault

c) a volcanic arc

-->  d) a mid-ocean ridge

how does the sinking side of a convection cell affect the seafloor?

a) it creates seafloor at a mid-ocean ridge.

b) it creates a transform fault in the seafloor.

-->  c) it drags seafloor into the mantle at a trench.

d) none of the above

earthquakes occur at all plate boundaries.

-->  a) true

b) false

a single plate can only be made of either continental or oceanic crust.

a) true

-->  b) false

plates move at a rate of a few ______ a year.

a) millimeters

b) centimeters

-->  c) meters

d) kilometers

how do scientists determine where the plate boundaries are?

-->  a) earthquake epicenters outline the plates.

b) most plate boundaries are at the edges of the continents.

c) most plate boundaries are in the center of the ocean basins.

d) volcanoes are only found at plate boundaries.

seafloor is created from and then recycled back into the mantle.

-->  a) true

b) false

diagram questions

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