Answer:
The magma that melts in the upper mantle and erupts at the mid-ocean ridge has a different composition (higher in Ca, Mg, Fe and relatively lower in Si and Al) than the composition of the magma (more Si, Al, Na, K rich in relative terms) that built the continents. Melting different degrees (2%, 5%, 30%) of a rock with a mantle-like composition generates magma of different Al, Si contents. Continents grow at their edges by partial melting of oceanic crust that is being subducted, generating magmas of a somewhat more Si, Al-rich composition than the subducting slab itself. The chemistry and physics of melting a mixture of minerals rather than one homogeneous substance has the wonderful outcome of creating two distinct types of crust:
with spreading rates of up to 13 cm/yr.
Question:
These large sheets of ice cover large parts of
Antarctica.
Answer:
Continental Glaciers
The Parthenon had a total of 69 columns. There were 46 outer columns and 23 inner columns.
