Answer to Question 1
Intermediate and felsic continental crust cannot form by simple mantle melting. Modern processes at subduction zones do not produce enough felsic-igneous rocks to account for the volume of continental crust. Therefore, the first continental crust likely formed by processes that are absent or unusual today. There are two hypotheses for the formation of continental crust. (1) Partial melting of subducted basaltic oceanic crust in the hot early Earth may have generated magmas with continental-crust composition. (2) The alternative hypothesis suggests that the early formed basaltic crust resulted from partial melting of mantle peridotite at mantle plumes. Once this initial oceanic crust formed, it was subjected to further partial melting when later plume magma heated the crust from below. This melting of the basaltic crust, by rising plumes rather than near convergent plate boundaries, could then generate the more silica-rich igneous rocks that are most abundant in continental crust.
Answer to Question 2
C
Continental drift
Approximately 1 pound (0.45 kg) of ground elk meat formed into patties; note the relatively small fa
Basin and Range Province—Crust Extended
Humid Continental Climates with Severe Winters (D)
Passive Continental Margin
Erosion and Resulting Isostatic Adjustment of the Crust