Causes of Volcanoes

1. Causes of Volcanoes Pgs. 156 - 161

2. Formation of Magma • The key to understanding why volcanoes form is understanding how magma forms. • Magma collects in the deeper regions of the Earth’s crust and upper layer of the mantle. • When it collects it is a zone of intensely hot and pliable rock beneath the crust. • Think of silly putty as a reference to the pliability of the magma.

3. Pressure and Temperature of Magma • Why doesn’t the temperature of the mantle make all the rock melt? • Pressure. • The pressure of the rock above the mantle forces particles of the rock to pack together tightly, preventing them from becoming liquid. • Magma will form when the temperature increases enough or the pressure decreases enough. • Since the mantle’s temperature is fairly constant, decreases in pressure is the main cause of magma formation.

4. Density and Magma • Once magma is formed, it will rise toward the surface because its density is less than the rock above it. • Magma will contain both liquid and solid minerals, but is still less dense. • It will rise to the surface like bubbles in a pot of boiling water.

5. Where Volcanoes Form • A large number of volcanoes lie directly on plate tectonic boundaries. • There are so many plate boundaries in the Pacific Ocean that the active volcanoes surrounding it gave it the nickname the Ring of Fire. • Where plates collide or separate are places where magma can travel upward through the crust easiest. • This is why plate boundaries have so many volcanoes.

6. Separating Tectonic Plates • At divergent boundaries, plates separate and form a rift where the mantle rises to fill the gap. • When the mantle is exposed to the surface, pressure drops and magma will form. • Magma is less dense than the rock around and it will rise to the surface. • As it continues to be exposed, magma stays liquid because the pressure continues to drop with less rock on top of it. • Separation usually occurs on the ocean floor creating mid-ocean ridges.

7. Colliding Tectonic Plates • At convergent boundaries between oceanic and continental crust, the denser oceanic crust will move underneath the continental. • This movement is called subduction and the area where this happens is called a subduction zone. • The oceanic crust sinks into the mantle and gets heated to melt. • The magma is now less dense than the rock around it and will rise to the surface under the continental crust.

8. Hot Spots • Some magma doesn’t form around tectonic boundaries. • The Hawaiian Islands formed from what’s called a hot spot. • These are areas directly above columns of rising magma, called mantle plumes. • Deep within the Earth is where mantle plumes originate and rise toward the surface. • Scientists don’t know the exact cause of them, but believe the heat from the core and radioactive elements keep them rising.

9. Effects of Hot Spots • Hot spots will produce chains of volcanoes over time. • The mantle plume stays in its spot, but the tectonic plates above it will continue to move. • As they move, the mantle plume will continue allow magma to rise and form volcanoes.

10. Predicting Volcanic Eruptions • Scientists have classified volcanoes on how likely they will erupt based on their histories of eruptions. • Extinct volcanoes: those that have not erupted in recorded history and may never erupt again. • Dormant volcanoes: those that are not currently erupting, but have in recorded history. • Active volcanoes: those that are in the process of erupting or show signs of erupting in the near future.

11. Measuring Small Quakes • Using a seismograph, scientist can try to determine when a volcano will erupt. • Most active volcanoes produce small earthquakes as magma moves up toward the surface. • The frequency and intensity of the quakes increases just before an eruption.

12. Measuring Slope • Scientists can help predict eruptions using a tiltmeter. • Some volcanoes will bulge in areas, affecting its slope, before an eruption. • A tilt meter can be used to detect these changes on the volcano’s slope.

13. Measuring Volcanic Gases • Scientists can use the ratio of sulfur dioxide to carbon dioxide to help predict eruptions. • When the ratio of these two gases changes, an eruption is coming, because this indicates a change in the magma chamber below. • Collecting these gases can be dangerous, because it requires you to climb the volcano and collect poisonous material. • Appropriate precautions must be taken.

14. Measuring Temperature from Orbit • Satellite images can even help predict volcanic eruptions. • Using infrared radiation can pick up temperature changes of active volcanoes. • Drastic increases in temperature indicate that magma is rising to the surface.