Molten material that comes out from the ridge is called magma. Magma is a mixture of molten and semi-molten rock found beneath the surface of the Earth. At a mid-ocean ridge, mantle material partially melts as it rises in response to reduced pressure. This melted rock, or magma, may collect in a reservoir a few kilometers below the seafloor, awaiting eruption. When magma pressure builds up enough to force its way out to the seafloor, eruption occurs. Dikes are magma-filled cracks and are the conduits that magmas flow through to reach the surface. A typical ridge eruption leaves behind a dike that is 10s of centimeters up to 2 meters in width, extending between the crustal magma chamber and the eruptive fissure at the surface. Lavas pour from the fissure across the surface of the volcanic seafloor, adding a thin coat of new lava (typically <10 m thick) with each eruption.