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Lava (which as you undoubtedly know, is partially molten rock erupted by volcanoes) typically comes from the mantle the Earth's middle layer, sandwiched between the crust and the core. Once it reaches the surface, lava quickly cools down and solidifies completely, creating new land.

Solidification of flows ranging 20 30 m (65 100 ft) thick could take about 2.5 6 years. The thickest LERZ flows on land, which are approximately 55 m (180 ft) thick, may take roughly 20 years to reach a completely solid state.