Magma Chamber

A magma chamber is an area beneath the Earth’s surface where magma (molten rock) collects in a pool of molten rock. Magma forms in subduction zones where an oceanic plate is overridden by another crustal plate. The descending plate begins to melt and the molten rock collects in these underground chambers. The magma is hotter than the surrounding rocks and is under great pressure.

The melting plates continues to add molten rock to the chamber increasing the pressure until it breaks the solid rock apart. The hot liquid rock is lighter than the surrounding rock so the magma forces its way upward where it can erupt during a volcanic eruption.

Pinnacles near Crater Lake, Oregon, Photo by Myrna Martin

Pinnacles near Crater Lake, Oregon

Magma chambers are detected using earthquake waves. P waves travel slower when traveling through liquids than through solid rock. Scientists use the speed of these waves to determine the shape and size of these chambers. Most known chambers of molten rock are between one and ten miles deep. Little is known about deeper chambers because they are hard to detect.

Some magma travels through two chambers on its way to the Earth’s surface. The bottom chamber feeds an upper chamber near the summit of a volcano. In the Hawaiian Islands scientists study harmonic tremors, special earthquake waves that indicate movement of magma into a chamber near the summit of Kilauea and Mauna Loa volcanoes on the island of Hawaii. Harmonic tremors often precede a volcanic eruption.

Magma that remains in a chamber for a long period of time begins to cool near the walls of the chamber. Olivine is one of the first minerals to crystallize in a magma chamber. The elements that make up the mineral are taken from the melt as gases are released from the crystallizing elements.

Olivine and other heavy elements settle near the bottom of the chamber. Lighter elements and gases move upward forming a stratified chamber that can erupt creating a massive eruption. The upper part of the chamber contains gases and felsic minerals that are more explosive when they erupt. Mafic minerals that collect near the bottom of the chamber are not as viscous.

The Mount Mazama eruption that formed Crater Lake had a stratified magma chamber beneath the mountain when it erupted. The mountain first spewed out light colored pumice located in the upper portion of the chamber. As the eruption continued dark gray pumice formed layers on top of the light pumice. When the eruption was over so much material had erupted that the summit collapsed forming a caldera that is 2,148 feet (655 meters) deep.

The caldera has filled with snow melt and rain water. It is the deepest lake in the United States. The Pinnacles formed in Sand Canyon near Crater Lake are fossil fumaroles that were exposed by erosion. They clearly show the stratification of the magma chamber prior to the climatic eruption of Mt. Mazama.


More Volcano Links

Yellowstone Caldera The Yellowstone Caldera is the location of three supervolcano eruptions.

Iceland Volcanoes  Find out more about Grimsvotn volcano, Hekla volcano and Katla volcano which are the most dangerous volcanoes on Iceland. 

Composite Volcano  Composite volcanoes are volcanic mountains that form on the continental side of subduction zones. 

Super Volcanoes  Super volcanoes are places where massive volcanic eruptions occur. These eruptions can alter the weather and cause mini ice ages.


Dome Volcano  A lava dome is small and often forms inside the caldera of a stratovolcano. 

Volcano Facts  Find out lots of fascinating facts and trivia about volcanoes on this webpage.

Home Page  The Science Site contains information on our planet, volcanoes, science projects, earthquakes and much more.



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