The main vent is the main outlet for the magma to escape. Secondary vents are smaller outlets through which magma escapes. The crater is created after an eruption blows the top off the volcano
Volcanoes are formed when magma from within the Earth's upper mantle works its way to the surface. At the surface, it erupts to form lava flows and ash deposits. Over time as the volcano continues to erupt, it will get bigger and bigger.
It tells you that there used to be an ocean in the area the rock was found.
How many do you need? because there is more then one.
