Question

Compare and contrast the thickness of the material that makes up ocean basins and continents and how they sit on the mantle. (Which one sits higher on the mantle and why?) 

Answer 1

The difference between ocean basins and continents is

* Ocean basins are made of thin basaltic material that is denser. Continents are made of thick granitic material that is less dense.

* Continents are the relatively stable portions than ocean as new crust is being created at the mid ocean rides.

* Continents are much older than oceans.

* Oceanic crust will always be subducted under continental crust. continental crust is less dense than oceanic crust.

* The oceanic crust has been forming on the Earth for over a 4 billion years period, all the sea floor is normally older than about 200 million years and it has been recycled by the plate tectonics. Continental crust is not subducted and has not been destroyed like the oceanic crust so very old continental rocks have just survived.

Which one sits higher on the mantle and why

       The density of basalt is around 3 grams per cubic centimeter. In contrast, the mantle is made of peridotite, which has a density of about 3.3 grams per cubic centimeter. Therefore, when the mantle partially melts and re-crystalizes as basalt, the oceanic crust will sit on top of the mantle.

Question 28

Solution:

   In the process of a magma cooling and crystallizing, if the original melt containes large amounts of iron than it is not quartz a ferromagnesium mineral because  as a magma begins to crystallize, early-forming minerals like olivine will remove iron from the melt and lock it away in solid crystals, thus preventing later minerals from using iron in their molecular structures.