Composite cones are famous for their conical shapes with steep ...

[lracut11]

Composite cones are famous for their conical shapes with steep summits, sloping flanks, and broad base.
 
  How might the composition of eruptive materials have changed for the volcano to transition from something with a broad base to something with a smaller base and steeper slopes?
  What will be an ideal response?

Question 2

Volcanic island arcs and continental volcanic arcs are both associated with convergent plate boundaries. However, the lavas that their volcanoes erupt are different. Where does the lava come from for each type of volcanic arc?
 
  What will be an ideal response?

[Juro]

Answer to Question 1

Answer: Early eruptive stages usually consist of lava flows that flow greater distances from the vent than later in the volcano's history. This process creates the broader base. As the volcano matures, lavas become more viscous, creating shorter, thicker flows that can create steeper slopes in the summit area. When pyroclastic materials are erupted, coarser materials fall closest to the crater and get finer farther away. This process helps to flatten the cone's flanks.

Answer to Question 2

Answer: Volcanic island arcs are constructed on oceanic lithosphere, which is basaltic. When oceanic lithosphere is subducted and dehydrates, the melt derived from flux melting only interacts with the overlying oceanic lithosphere and remains basaltic. However, continental volcanic arcs will see rising mantle-derived magma assimilate continental material that is more silica-rich. This produces andesitic or even rhyolitic magmas.

[lracut11]

Excellent

[pangili4]

Gracias!