Volcano rocks are igneous rocks that form during a volcanic eruption. They were all once magma which is molten rock beneath the Earth's surface. There are four major groups of the igneous rocks: basalt, andesite, dacite and rhyolite. Each of these different rock groups form different types of volcanoes.
Volcanic rocks forming during eruption of Kilauea, USGS
Rhyolite magma cooling underground forms granite. Granite has large interlocking crystals that can be easily seen if you hold the rock at arm's length. Rhyolite is associated super volcanoes and explosive volcanic eruptions that create calderas.
Basalt rocks are formed from dark heavy lava that usually forms in the upper mantle. Pillow basalt covers all the ocean floors. It forms when lava erupts on the ocean floor at spreading ridges where two crustal plates are separating.
Andesite lava forms in subduction zones where an oceanic plate is subducting beneath a continental plate. The lava erupts on the continental side of the subduction zone forming a large composite volcano.
Mt. Hood, Mt. Rainier and Mt. Spurr. These great composite volcanoes usually have layers or tephra alternating with layers of thick lava flows. The lava was named for the Andes Mountains in South America.
Dacite contains less quartz than rhyolite lava and more than andesite lava. The lava when it flows out a a volcano vent is so "sticky" that it piles on top of a volcano's vent forming a dome. Lava domes often form in the craters of stratovolcanoes like Mount Saint Helens. The lava also forms small volcanoes with rough crumbly surfaces.
Hawaiian Volcanoes The Hawaiian volcanoes are the largest volcanoes on Earth.
Dome Volcano Dome volcanoes form on the sides and in the calderas of large stratovolcanoes.
Alaskan Volcanoes The Alaskan volcanoes frequently erupt all along the coast of Alaska.
Lahars-Volcanic Mudflows Lahars are volcanic mudflows triggered by pyroclastic flows.
Mount Pinatubo The Mount Pinatubo eruption was the second largest volcanic eruption of the 20th century.
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Volcano Rocks There are four major groups of volcano rocks.
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