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Once upon a time, a volcano erupted, and magma from deep inside the Earth became lava. The lava cooled outside rapidly to form obsidian, a black shiny igneous rock, without crystals. Some lava cooled slowly over time deep within the Earth, forming crystals inside a rock called granite. Over time, the igneous rocks weathered and eroded. As water carried them along the Earth's surface, they ended up in a river and eventually the ocean. The rocks became weathered and their edges became rounded from abrasion between the bouncing rocks and the sandy seafloor. As they settled on the sea floor, surrounded by sand and mud, over time, due to pressure and heat, they naturally cemented together to form conglomerate, a sedimentary rock. Eventually, a volcano on the seafloor erupted and spewed lava. Its heat caused the conglomerate rock to melt and eventually to cool again and recrystallize, forming a metamorphic rock called metaconglomerate. Picture a piece of bread that is toasted. Heat and/or pressure are capable of changing any type of rock into a metamorphic rock.
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