Volcanoes

1. Volcanoes Sections 7.1, 7.2

2. Section 7.1Volcanoes and Plate Tectonics • The asthenosphere has very high temperatures, but because it has so much pressure around, it remains rock. • There are some areas where it melts to form Magma or liquid rock. • Any activity where magma moves toward the surface is called volcanism.

3. Section 7.1Volcanoes and Plate Tectonics • Magma slowly moves to the crust because it is less dense than solid rock • Most magma forms near the edges of plates • Magma that erupts to the earth’s surface is called lava

4. Section 7.1Volcanoes and Plate Tectonics • The opening where molten rock flows to the surface is called a vent. • The structure that forms and builds up around a vent is called a volcano.

5. Section 7.1Volcanoes and Plate Tectonics • Like earthquakes, most volcanoes occur near both convergent and divergent boundaries • When there is a subduction of one plate under another, the plate that goes underneath will melt and create a volcano about 100 miles inland

6. Section 7.1Volcanoes and Plate Tectonics • The greatest amounts of magma comes to the surface where the plates are moving apart. • Large volcanoes are formed here • One of those is the country of Iceland • Half of Iceland is moving toward Europe and the other half is moving toward North America

7. Section 7.1Volcanoes and Plate Tectonics • Not all volcanoes form on plate boundaries • Some form over Hot Spots. • Hot spots are just pockets of magma in the middle of a plate • The Hawaiian Islands were formed this way

8. Section 7.2Volcanic Eruptions • There are 2 different types of lava. • The first type is dark colored when it hardens and is rich in magnesium and iron • It is called mafic lava. • This forms much of the oceanic crust.

9. Section 7.2Volcanic Eruptions • The second kind of lava is lighter in color and is made of silica and lesser amounts of iron and magnesium • It is called felsic lava • Felsic lava found in continental crust

10. Section 7.2Volcanic Eruptions • When mafic lava cools quickly, a crust forms on top, but the bottom still flows. The rock wrinkles forming pahoehoe • If the rocks break into jagged chunks, it is called aa • Rounded blobs of hardened lava on the ocean floor are called pillow lava

11. Section 7.2Volcanic Eruptions • Lava that has a lot of trapped gases will produce large violent eruptions out of continental volcanoes. (like Mt. St. Helens) • Oceanic Volcanoes produced less violent volcanoes with very hot and thin mafic lava (like Hawaii)

12. Section 7.2Volcanic Eruptions • Felsic lava explodes out of a volcano throwing Pyroclastic material • Volcanic ash is small particles, they are less than 2mm in diameter • Volcanic dust is the smallest particles, they are less than 0.25mm in diameter

13. Section 7.2Volcanic Eruptions • Dust and Ash can be carried all around the world by wind • Large pyroclastic material up to 64mm in diameter is called lapilli • Sometimes lava gets thrown into the air while it is still hot, that’s called a volcanic bomb • If it is solid when it is blasted, it’s called a volcanic block

14. Section 7.2Volcanic Eruptions • There are 3 kinds of volcanoes. • Shield volcanoes are very wide volcanoes that are made from layers of lava • Cinder Cones are taller but are made from ejected pyroclastic material • Composite Volcanoes are the tallest and look like mountains. They are made with layers of lava and pyroclastic material.

15. Section 7.2Volcanic Eruptions • Craters are formed on volcanoes when a large explosion blows the top off. • If the magma chamber below a volcano is empty, the volcano can fall in. The hole that is left in the ground is called a caldera.