GL 107 Earth Systems Science

K. Hannula

Discussion questions for 10/3/07

1.  Define the following terms:

• sea floor spreading:  process by which new seafloor (and new oceanic crust) is formed at divergent plate boundaries

• mid-ocean ridge:  elevated portion of the sea floor which makes up the longest topographic feature of Earth's surface.  The location of divergent plate boundaries.

• subduction zone:  convergent plate boundaries at which one plate slides underneath the other, to be consumed in the mantle

• trench: deepest points on the ocean floor, found where two converging plates meet and one plate slides beneath the other

• volcanic arc:  chain of volcanoes found on the over-riding plate, above subduction zones.  Most commonly andesite in composition.  Formed due to addition of water to the mantle above the subducting plate.

2.  Give an example of at least one place on earth where you can find:

• a divergent plate boundary:  Mid-Atlantic Ridge, East Pacific Rise

• an ocean-continent convergent boundary: Peru-Chile Trench (west coast of South America)

• an ocean-ocean convergent boundary:  Aleutian Trench, Japan Trench, Mariana Trench, Tonga Trench, Kermadec Trench (north of New Zealand), Philippine Trench, Java (Sunda) Trench (Indonesia)

• a continent-continent convergent boundary:  Himalayas

• a transform boundary:  San Andreas Fault

3.  How does the process of sea-floor spreading explain the topography of ocean floors?  Why are slow-spreading ridges and fast-spreading ridges different? (Not discussed in class)

Sea-floor spreading takes place at mid-ocean ridges.  The hot, rising asthenosphere and hot, newly-formed oceanic crust are lighter than cold rock would be, and therefore have a higher elevation.  Fast-spreading ridges are very broad, because the hot, new oceanic crust moves rapidly away from the plate boundary, meaning that a large area is underlain by hot, new, high-elevation crust.  Slow spreading ridges are very narrow, because the rock has plenty of time to cool and sink before it travels very far away from the ridge.

4.  Describe the events that occur when a continent rifts apart.

See Figure 7.11 (p. 203 in textbook).

1) Continental crust is uplifted and stretched above hot rising mantle rock.

2) A rift valley (like the East African Rift) forms where the continent is beginning to split apart.  Volcanoes and earthquakes occur along the forming rift.

3) The continental plate begins to break in two, and new oceanic crust forms between the rifting continents.  (The Red Sea is currently at this stage.)

4) Spreading becomes concentrated at a mid-ocean ridge, and new oceanic crust continues to form in the middle of the growing ocean.  (The Atlantic Ocean is at this stage.)