Unit 6-1: An Introduction to Volcanoes

Volcanoes! Out of the many happenings on the earth, few can match the raw strength and power of a volcano. Out of the many happenings on the earth, few can match the raw strength and power of a volcano. –Where does the molten rock come from? –How does it get inside a volcano? –Why are there different types of volcanoes? –Why do volcanoes erupt when they do?

Magma Magma is melted, or molten, rock that exists underground. Magma is melted, or molten, rock that exists underground. –It will form whenever temperatures are high enough to melt rock (around 700°C to 1300°C) –The astenosphere is where temperatures are high enough to melt rock. –Solid rock must be brought down into the astenosphere in order to melt.

Magma Rock from the crust is pulled down into the astenosphere at subduction boundaries. Rock from the crust is pulled down into the astenosphere at subduction boundaries. –As this rock goes deeper into the earth:  It heats up steadily  It eventually melts –This process results in most of the magma that creates volcanoes around subduction boundaries

Magma Since magma is molten rock, it is less dense than solid rock. Since magma is molten rock, it is less dense than solid rock. –It rises towards the surface. –It also has more volume, which results in a higher pressure. –If there is an opening in the Earth’s crust, called a volcano, the magma escapes.

Magma The movement rate of magma is determined by its silica content. The movement rate of magma is determined by its silica content. –Silica is a major component of all magma. –The more silica there is in magma, the thicker it is. –This is called felsic magma –Low silica content is called mafic magma.

Magma Mafic: Mafic: –Low viscosity (relatively thin) –Low silica content (<50%) –High temperatures (1300°C) –Water can easily escape from this type of magma –Results in gentle, steadily flowing eruptions

Magma Felsic: Felsic: –High viscosity (very thick) –High silica content (around 70%+) –Relatively low temperature (around 900°C) –Water cannot escape easily from this magma. –Results in highly explosive, violent, and sporadic eruptions.

Gases in Magma Magma contains dissolved gases which are given off during eruptions. Magma contains dissolved gases which are given off during eruptions. –The most important are water vapor, carbon dioxide, and sulfur gases. –Minor gases include hydrogen and carbon monoxide. –Sulfur will combine with hydrogen to make hydrogen sulfide –Sulfur will combine with oxygen to make sulfur dioxide.

Gases in Magma Many of these gases are dangerous to humans. Many of these gases are dangerous to humans. –The gases can continue to pour out of the volcano even months after the eruption –After the Mount St. Helens eruption of 1980, scientists had to wear gas masks.  The gases emitted by the volcano rotted clothing, burned eyes, corroded the finish of radios, ethed prism reflectors, and turned the steel rims of eyeglasses green.

Gases in Magma The amount of gas in magma determines the type of eruption. The amount of gas in magma determines the type of eruption. As magma reaches the surface, there is less and less pressure holding the gas in. As magma reaches the surface, there is less and less pressure holding the gas in. Gas in solution begins to come out as bubbles. Gas in solution begins to come out as bubbles. –Bubbles expand rapidly. –If the magma is too thick, then the bubbles may escape explosively.

Lava Magma that reaches the surface is called lava. Magma that reaches the surface is called lava. The composition is different than magma because lava has less dissolved gas in it. The composition is different than magma because lava has less dissolved gas in it. The composition is also different because it picks up new material along the way as well. The composition is also different because it picks up new material along the way as well.

Lava Like magma, lava is classified as being felsic or mafic. Like magma, lava is classified as being felsic or mafic. Felsic lavas are thicker and more explosive. Felsic lavas are thicker and more explosive. Mafic lavas are thinner and generate a more gentle eruption. Mafic lavas are thinner and generate a more gentle eruption.

Lava Fragments Explosive eruptions produce solid fragments of lava called tephra. Explosive eruptions produce solid fragments of lava called tephra. –The smallest pieces of tephra are less than 2mm in diameter.  These pieces are called ash. –Pieces up to 64 mm are called lapilli –The largest fragments are greater than 64mm  These are called blocks and bombs.

What does ash look like?

What does lapilli look like?

Lava Fragments The difference between blocks and bombs: The difference between blocks and bombs: –Blocks are ejected from the volcano as a solid piece of rock. –Bombs, on the other hand, are ejected as liquid. –Bombs harden into a solid as they fly through the air and cool.

What does a lava block look like?

What does a lava bomb look like?

Lava Fragments In some explosive eruptions, tephra combines with gas to form a highly dense, superheated cloud. In some explosive eruptions, tephra combines with gas to form a highly dense, superheated cloud. –The cloud may move more than 100km/h –This cloud can flatten forests –Bury buildings (if it doesn’t destroy them) –Overwhelm any living creatures unfortunate enough to be in its path.

What does a pyroclastic flow look like?