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Unit 6-1: An Introduction to Volcanoes

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. Where does the molten rock come from? How does it get inside a volcano? Why are there different types of volcanoes?

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Unit 6-1: An Introduction to Volcanoes

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  1. Unit 6-1:An Introduction to Volcanoes

  2. Volcanoes! • 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?

  3. Magma • 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.

  4. Magma • 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

  5. Magma • 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.

  6. Magma • 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.

  7. Magma • 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

  8. Magma • 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.

  9. Gases in Magma • 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.

  10. Gases in Magma • 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.

  11. Gases in Magma • 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. • Gas in solution begins to come out as bubbles. • Bubbles expand rapidly. • If the magma is too thick, then the bubbles may escape explosively.

  12. 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 also different because it picks up new material along the way as well.

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

  14. Lava Fragments • 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.

  15. What does ash look like?

  16. What does lapilli look like?

  17. Lava Fragments • 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.

  18. What does a lava block look like?

  19. What does a lava bomb look like?

  20. Lava Fragments • 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.

  21. What does a pyroclastic flow look like?

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