Volcanoes

Volcanoes

Volcanoes occur most frequently at plate boundaries. Some volcanoes occur in the interior of plates in areas called hot spots. Most of Earth’s volcanoes are hidden from view on the ocean floor.

How explosively a volcano erupts is determined by the chemical composition and gas content of the magma. The amount of silica, a substance formed from the elements oxygen and silicon, determines how thick magma is. The more silica the thicker the magma.

If the magma is thin and fluid, then the lava will flow out. If the magma is thick, then the pressure inside of the volcano builds and it will erupt violently.

Paricutin Volcano in Mexico

A cinder cone volcano is a steep sided cone formed of cinders, small pieces of hard lava, and ash from explosive volcanic eruptions. Paricutin Volcano in Mexico is an example.

Cinder Cone Volcanoes

Belknap Volcano in Oregon

A shield cone volcano is formed from non-explosive volcanic eruptions. Hot lava flows from the crater and other places in the volcano creating gently sloping sides. Mauna Loa in Hawaii is an example.

Shield Volcano

A composite cone volcano is formed of layers of cinders and lava from explosive and non-explosive eruptions. The volcano erupts explosively and then quietly at different times. Mt. St. Helens is an example.

Mt. St. Helens in Washington

Composite or Stratovolcano

An active, or live, volcano is one that is erupting or has shown signs that it may erupt in the near future.A dormant, or sleeping, volcano is a volcano that is expected to awaken in the future and become active. There may be 1000s of years between eruptions.An extinct or dead, volcano is unlikely to erupt again.

ActiveDormant

Extinct

Pahoehoe is a fast-moving hot lava. Aa is a cooler, slower-moving lava.

Pahoehoe

Aa