Why does oceanic lithosphere almost always sink beneath continental lithosphere at convergent plate boundaries?
* oceanic lithosphere is denser/heavier
Short answer questions:
When did the direction of the Pacific plate change, and what direction was it moving before this change?
* changed at 43 million years.
* Before this change, it was moving north-northwest.
If the ocean floor is getting wider, why is the Earth not becoming larger?
* old ocean floor sinks at subduction zones (trenches)
Distinguish between Earth’s crust and mantle.
* Crust overlies mantle. It is basalt composition under the ocean basins, granitic composition in the continents
Roughly how many tectonic plates move around on the surface of the Earth?
* 10 or 12
What characteristics of tectonic plates distinguish them from deeper Earth materials?
* They are rigid.
Do tectonic plates consist of crust, mantle, or some combination of crust and mantle, and if so, what part of parts of each?
* all of the crust and part of the upper mantle.
What keeps the Appalachians standing as a mountain range even though they have been continuously eroding since they formed hundreds of millions of years ago?
* Isostacy. As material is eroded off the range, it floats higher, just as removing ice from the top of an iceberg causes it to float higher.
What is the basis for the “scientific method?”
* tentative hypotheses are tested using new observations and experiments
Why do many oceanic volcanoes occur as long lines of volcanoes that are active at only one end? How does the process work?
* They are hotspot volcanoes. A hot plume of magma rising through the mantle burns through the overlying lithosphere to erupt as a volcano. Since the lithosphere moves over the deeper mantle, new volcanoes form progressively in a line over the deep plume.
Distinguish between Earth’s lithosphere and asthenosphere in terms of both location and properties.
The lithosphere overlies the asthenosphere. It is rigid and the asthenosphere deforms plastically.
What is the difference between the Earth’s lithosphere and asthenosphere?
* lithosphere is more-rigid; asthenosphere is more-plastic/easily deformed.
* It is on top of the asthenosphere.
What does the oceanic crust consist of and how thick is it?
* basalt, about 7 km thick
What does the continental crust consist of and how thick is it?
“granite” composition (whole range of continental rocks, especially granite, gneiss, schist) 30 to 60 or 70 km thick
What are the main types of lithospheric plate boundaries, in terms of relative motions? Provide a real example of each (by name or location)
* convergent (continent – ocean plate collision): Cascadia subduction zone (Pacific Ocean floor against North America)
What was the primary reason why Alfred Wegener’s theory of continental drift rejected?
* He had no mechanism for moving continental crust through strong basaltic oceanic crust.
The “Ring of Fire” refers to what and where is it?
* the ring of active volcanoes surrounding the Pacific Ocean.
Convergent plate boundaries are especially noted for which two types of natural hazards?
* earthquakes and volcanoes +/- tsunami
In the line of arc volcanoes, like the Cascades of Washington and Oregon, over an active subduction zone, a series of events leads to the magmas that erupt at the surface. What magma forms first and where and how does it form?
* Basalt magma forms at (just above) the subduction zone below the volcano, when water formed by dehydration of hydrous minerals in the descending slab causes melting of the mantle peridotite above.
How is the magma formed that feeds the volcanoes in Hawaii?
* The lithosphere carrying Hawaii slowly moves over a mantle hotspot causing melting to form basalt magma..