GEO Plates, Plate Boundaries, and Driving Forces

True/False Earthquakes along divergent boundaries occur only at shallow depths of 0 to 33 km below the earth's surface.

True Divergent boundaries are the mid-ocean ridges characterized by shallow earthquakes and minor lava flows. Only shallow earthquakes (depths of 0 to 33 km) are recorded at the spreading ridges.

Why did the distribution of earthquakes and volcanoes help confirm plate tectonic theory?

-Earthquakes and volcanoes were different along different types of boundaries. Plate tectonics provided an explanation of why earthquakes and volcanoes followed certain patterns. Earthquakes are shallow at spreading ridges where plates are thin and deeper inland where plates are deep beneath the continents. With volcanoes, most major eruptions take place near subduction zones, while eruptions that occur along spreading ridges are much gentler.

Why are we able to measure past plate motion using hotspots?

-Hotspots are relatively stationary, whereas plates move. Since hotspots are relatively stationary, their tracks can be used to trace plate motion history. Hotpots are stationary plumes of hot magma that act like Bunsen burners as plates move over them, creating volcanic islands and seamounts (islands eroded below sea level). Chains of volcanic islands show the motion of a plate over a mantle plume.

Most earthquakes are located

-along plate boundaries. Earthquakes are concentrated along plate boundaries. In fact, plate boundaries are the scene of not only earthquakes but of other geologic activity like volcanic eruptions and the formation of mountain ranges.

The driving forces behind plate tectonics, a subject that plagued Wegener, continue to be a matter of debate today. Which of the following is NOT a possible cause of plate motion?

-earthquake activity Earthquakes occur along plate boundaries and are a result of plate motion. Gravity and mantle convection are two driving forces for the movement of plates.

When oceanic crust meets oceanic crust along a convergent boundary, which plate is most likely to be subducted?

-the plate with the oldest crust When oceanic crust meets oceanic crust, the older crust is usually subducted because it is colder and slightly denser.

Continental crust is _______________________ than oceanic crust.

-thicker and less dense Continental crust is thick and buoyant, while oceanic crust is thin and dense.

Which of the three main boundary types is least common?

-transform Most boundaries are either convergent or divergent. However, transform boundaries occur in a few places to accommodate lateral motion, in which plates move horizontally past one another. Transform boundaries are rare on the continents, though they are more common in the oceans.

The San Andreas Fault in California is an example of what kind of boundary?

-transform The San Andreas Fault is a continental transform boundary. Transform boundaries accommodate lateral motion, in which plates move horizontally past one another. This type of boundary is very rare on continents, and is characterized by frequent, shallow earthquakes.

The Himalayas occur along a convergent boundary where

continental crust meets continental crust. When two pieces of continental crust come together, a great pileup of continental material results because both pieces are buoyant and therefore not easily subducted. The Himalayan mountain range is an example of continental convergence.