The eruption of volcanoes is one factor in changing the Earth's crust. When two tectonic plates move away from each other, magma flows up from the mantle, filling the space between the two plates. This is how volcanoes are created (many of them underwater). When the molten lava cool, the surrounding rock and crust contributes to a buildup of crust on the ocean floor. this hot liquid called contributes to the shape of the ocean floor. Another factor is the general movement of continental plates. When volcanoes cool, the crust on the ocean floor is pushed further and further apart. the pacific ocean is getting smaller as the north and south America move plates move west. Conversely, the Atlantic Ocean is getting larger as north and South America move away from Europe and Africa.