1. Earth Science 10.1 Volcanoes and Plate Tectonics

2. Origin of Magma
Starting in 1999, the 5000 meter high Tungurahua volcano in Ecuador began to erupt. It spewed chunks of lava, volcanic ash, and gases into the sky. Than on August 2006, streams of hot gas and rock exploded down the volcanoes sides. Villagers fled as ash buried their homes and fields. Tungurahua volcano is one of the 800 active volcanoes on average that occur on Earth each year.

3. Origin of Magma
Recall that magma is molten rock from beneath Earth’s surface. Magma is a complex mixture that contains partly melted mineral crystals, dissolved gases, and water. Magma forms in the crust and upper mantle when solid rock partially melts. The formation of magma depends on several factors; including heat, pressure and water content.

4. Origin of Magma: Heat
What source of heat is sufficient to melt rock? At a depth of 100 kilometers, the temperatures of the mantle ranges between 1400 degrees centigrade and 1600 degrees centigrade. At these temperatures, the solid rock of the lower crust and upper mantle is close to it’s melting point.

5. Origin of Magma: Heat
The additional heat needed to produce magma comes from three sources
Friction generated by slabs of lithosphere sliding past each other
The mantle itself heats the subducting slabs
Hot mantle rock rising into the cooler lithosphere

6. Origin of Magma: Pressure
We have learned that pressure increases with increased depth inside the Earth.
Increasing pressure raises the melting point of rock deep inside the Earth.
Decreasing pressure, decreases rock’s melting point.
When pressure drops enough; decompression melting occurs.
Example: As hot yet solid mantle rock rises, the pressure on the rock decreases. As the decreasing pressure lowers the rock’s melting point, pockets of hot magma form.

7. Origin of Magma: Water Content
The water content of rock also lowers the rock’s melting point.
Because of the, “wet rock” deep beneath the surface melts at a much lower temperature than does “dry rock” of the same composition and under the same pressure.
Lab studies have shown that the melting point of basalt can be lowered by up to 100 C by adding only 0.1 percent water.

8. Volcanoes and Plate Boundaries
Fortunately for us, hot magma only reaches the surface in certain areas. What determines where volcanoes form?
Most volcanoes form along divergent and convergent plate boundaries.
Some volcanoes form far from plate boundaries above “hot spots” in the crust.

9. Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
Volcanoes form at convergent plate boundaries where slabs of oceanic crust are pushed down into the mantle.
As a slab sinks deeper into the mantle, the increase in temperature and pressure drives water from the oceanic crust.

10. Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
Once the sinking slab reaches a depth of about 100 to 150 kilometers, this water reduces the melting point of mantle rock low enough for melting to begin.
The magma formed slowly migrates upward forming volcanoes.

11. Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
Volcanoes form at convergent boundaries where two oceanic plates meet and oceanic lithosphere is subducted beneath another oceanic plate.

12. Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
This process results in a chain of volcanoes being formed on the ocean floor. Eventually these volcanic mountains grow large enough to rise above the surface and are called volcanic islands.

13. Volcanoes and Plate Boundaries
Convergent Boundary Volcanism:
Several volcanic island arcs, such as the Tonga Islands and the Mariana island arcs, lie on the eastern side of the Pacific ocean

14. Volcanoes and Plate Boundaries
Together with other volcanoes bordering the pacific, they form the Ring of Fire.
The Ring of Fire is the long belt of volcanoes that circles much of the Pacific Ocean.

15. Volcanoes and Plate Boundaries
Volcanism may also occur at convergent plate boundaries where a continental plate meets an oceanic plate and slabs of oceanic lithosphere are subducted under continental lithosphere.
The result is a continental volcanic arc.
The process is basically the same as for an island arc.

16. Intraplate Volcanism
Kilauea volcano in Hawaii is earth’s most active volcano. But Kilauea is in the middle of the pacific plate, thousands of kilometers away from any plate boundary.
Kilauea is an example of intraplate volcanism; volcanic activity that occurs within a plate.

17. Intraplate Volcanism
Most intraplate volcanism occurs where a mass of hotter-than-normal mantle material, called a mantle plume, rises toward the surface.
Once the plume nears the top of the mantle, decompression melting forms magma.

18. Intraplate Volcanism
The result may be a small volcanic region a few hundred kilometers across called a hot spot.
More than 40 hot spots are known. Most of these hot spots have lasted for millions of years.

19. Intraplate Volcanism
The volcanic mountains that make up the Hawaiian Islands have formed as the Pacific plate moves over a hot spot.
The age of each volcano indicates the time when it was over the hot spot. Kauai is the oldest while the most recently formed island has two volcanoes; Mauna Loa and Kilauea.