Where does magma come from? The mantle! Image taken from http://www.windows2universe.org/earth/interior/how_plates_move.html. We begin this lecture by asking where the magma comes from that feeds the volcanoes. Lead the students into thinking about what happens beneath the surface that causes volcanoes to erupt.

Solid mantle rock can melt to form magma in either of these cases: Decrease in pressure Occurs at hotspots and spreading centers Addition of water Occurs at subduction zones Photograph taken from http://www.hawaiiretreatsandtours.com/hawaii_lava_walks.php. The mantle is made of solid rock. So how does it melt into magma that can burst through several kilometers of crustal rock? There are two ways that mantle rock can melt to form magma. One way to create magma is to decrease the pressure on the rock. Pressures inside the Earth are so strong that even at very high temperatures rock can remain solid. If the pressure is released, the rock will melt. This happens at hot spots and spreading centers. As mantle rocks rise, the pressure decreases and they melt.   Another way to form magma is by adding water or carbon dioxide. This is very common at subduction zones. As one slab is pushed underneath another, water and carbon dioxide sweat out and are added to the overlying mantle. This changes its melting point, causing it to melt.

Anatomy of a Volcano Photograph taken from http://geology.com/usgs/redoubt-volcano-photos/. “The Redoubt eruption cloud as seen from the Kenai Peninsula. The mushroom-shaped plume rose from avalanches of hot debris (pyroclastic flows) that cascaded down the north flank of the volcano. A smaller, white steam plume rises from the summit crater. Photograph by R. Clucas, April 21, 1990.” Now that magma has formed, where does it go? How does it get to the surface?

Intrusion vs. Extrusion Magma Molten rock can cool beneath the surface of the Earth Extrusion Lava Volcanic eruption When magma rises and reaches the crust, it “intrudes” or pushes into the crust. Sometimes the magma makes it all the way through the crust and erupts. This process is extrusion. Sometimes the magma only makes it part of the way into the crust. It cools and hardens beneath the surface. This process is intrusion. When magma reaches the surface and erupts, the magma changes its name to lava. Modern research suggests that the interior of the volcanoes themselves is actually made of such intrusions and we over-estimate the amount of extrusions because that is all we see as we are walking across a volcano.

Active volcano Diagram taken from http://www.enchantedlearning.com/subjects/volcano/. When a volcano is active, it still has a chance of erupting even if it hasn’t erupted in 1,000 or more years. While it is active, the names labeled on this diagram are used to describe places in the volcano. The magma that pools beneath the volcano until the pressure is high enough to erupt, resides in the magma chamber. When it erupts, the magma travels through the conduit and the throat. The top of the volcano is called the summit and most times it has a crater at the top, which is the vent for the magma. When the magma comes out, it becomes a lava flow or an ash cloud if it is fragmented into pieces. Magma does not always erupt from the summit though. It can break through the side, or “flank,” of the volcano creating a “side vent” or a “parasitic cone.” The magma that rises in the volcano forms a “dike” and if the magma is trapped between two layers and spreads out laterally, it forms a “sill.” This image, like most textbook images of volcanoes, does not do justice to the amount of intrusives in a volcano. They are actually more common than depicted here. This is a diagram of a composite volcano meaning that it is made up of several layers of lava and ash.

Extinct volcano Diagram taken from http://www.indiana.edu/~geol105/1425chap5.htm. “Rock bodies that cool beneath the surface are generally described as Plutons.   A batholith is a large former magma chamber, often many miles across. A sill is a sheetlike injection of magma between layers of sedimentary rock. A dike is a sheetlike body that fills a fracture that cuts across other rocks. A laccolith is a small magma chamber at shallow depth (roughly lens shaped). Volcanic cones and lava flows are surface expressions (see volcanic landforms for pictures). All of the subsurface igneous rock bodies will eventually be exhumed by erosion and can be seen at the surface. Erosion may reveal the solidified magma plug in the base of volcanoes, a so called volcanic neck.” When a volcano becomes extinct, the magma that is left beneath the surface cools and hardens. The extinct magma chamber becomes a pluton or, in this case, a batholith. The conduits of cooled magma are dikes and the magma that spread laterally between layers is called a sill. If the magma spread laterally between two layers and lifted the surface it is called a laccolith. All of these solidified bodies of magma are called plutons. The mountain that formed is called a volcanic cone and the lava that flowed on the surface is called a lava flow.

Half Dome in Yosemite is a Batholith Picture taken from http://en.wikipedia.org/wiki/Batholith. “Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada batholith.” After a very long time, the rain and the wind erode away the mountain. The plutons are often much sturdier than the rest of the volcano so they may remain even after the rest of the volcano has disappeared. This is a photograph of a batholith that is left over from an ancient volcano. It is located in Yosemite, California and it is called Half Dome.

Cross-cutting dikes in Alaska Photograph taken from http://www.britannica.com/EBchecked/media/153652/Dike-set-along-the-Baranof-Cross-Island-Trail-Baranof-Island. “Dike set along the Baranof Cross-Island Trail, Baranof Island, Alaska, U.S.” This is a photograph of a dike (dark gray). The surrounding rock has a light gray color.

Sills in Canada Image taken from http://en.wikipedia.org/wiki/Sill_(geology). “Mid-Carboniferous dolerite sill cutting Lower Carboniferous shales and sandstones, Horton Bluff, Minas Basin South Shore, Nova Scotia, Canada. The sill is the more resistant band.” This is a photograph of a sill (light orange). The surrounding rock has more of a brownish color.