At the top of most volcanic mountains is a crater, which marks the upper end of the vent. A large crater, or caldera, may be formed by the collapse of the mountain top after the withdrawal of magma. Crater Lake in Oregon occupies 20 sq mi of a caldera left by the collapse of Mounth Mazama, an ancient extict volcano. Underwater eruptions are common, especially in the seas near island arcs. In Japan in 1952, a underwater eruption of Myozin-San sunk a ship with all on board.
Most new volcanoes start on the flanks of older ones, forming adventive cones. Paricutin was born on Feb. 20 1943 in the state of Michoacan, Mexico. Gases and hot ashes began to emerge from a fissure in a cornfield, and soon later a cone 30 ft high had been built around the vent. In a week it had grown to 450 ft and the roar of the eruption was heard 200 mi away. When its activity ceased nine years later, the volcano had built a cone 1,300 ft high and its lava had buried two towns.
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