How long does it take seamounts to form

Hey! can anyone me out with my homework? it will be very much appreciated! and !

How does rain water cause erosion and deposition? a. when acidic water soaks into the ground, water dissolves limestone, resulting in the formation of caves.b. when a river overflows it moves downhill resulting in the formation of caves.c. when it rains, water flows into rivers and streams and moves back to land, resulting in the formation of caves.d. when water empties into lakes, sediment is deposited, resulting in formation of caves.

An igneous rock that contains mostly pyroxene and olivine has what type of composistion