Where Do Volcanoes Begin?
Volcanoes begin deep inside the earth. The earth’s crust is made up of giant pieces of rock called plates. Volcanoes are located where two plates meet. As the plates press together, pressure and heat melt their edges. This produces hot, liquid rock called magma.
A volcanic eruption occurs when magma and hot gases force their way up through cracks in the earth’s crust, blowing a hole in the surface.
The tons of volcanic ash and red-hot rock, called lava, form a cone-shaped mountain around the hole. In this way, a volcano is built. Over time as the volcano continues to erupt, it will get bigger and bigger.
There are more than 1500 active volcanoes on the Earth. We currently know of 80 or more which are under the oceans. Active volcanoes in the U.S. are found mainly in Hawaii, Alaska, California, Oregon and Washington.
Volcanoes are grouped into four types: cinder cones, composite volcanoes, shield volcanoes and lava volcanoes.
Cinder Cones: Cinder cones are circular or oval cones made up of small fragments of lava from a single vent that have been blown into the air, cooled and fallen around the vent.
Composite Volcanoes: Composite volcanoes are steep-sided volcanoes composed of many layers of volcanic rocks, usually made from high-viscosity lava, ash and rock debris. Mt. Rainier and Mount St. Helens are examples of this type of volcano.
Shield Volcanoes: Shield volcanoes are volcanoes shaped like a bowl or shield in the middle with long gentle slopes made by basaltic lava flows. Basalt lava flows from these volcanoes are called flood basalts. The volcanoes that formed the basalt of the Columbia Plateau were shield volcanoes.
Lava Volcanoes: Lava domes are formed when erupting lava is too thick to flow and makes a steep-sided mound as the lava piles up near the volcanic vent. The eruption of Mount St. Helens in 1980 was caused in part by a lava dome shifting to allow explosive gas and steam to escape from inside the mountain.