10.1 Pt.1

10.1: Volcanoes and Plate Tectonics

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.

Origin of Magma

Multiple Choice

What is Magma?

• A type of cloud formation
• Molten rock beneath the earth's surface
• Frozen water on the earth's surface
• Lava on the earth's surface

• At a depth of 100 km, the temperature of the mantle rock ranges from 1400C to 1600 C.

• At these temperatures, the solid rock of the lower crust and upper mantle is near, but not quite at, its melting point.

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Heat

• The additional heat needed to produce magma comes from three sources:

  • First, friction generates heat as huge slabs of lithosphere slide past each other in subduction zones.

  • Second, the mantle itself heats these subducting slabs.

  • Third, hot mantle rock can rise and intrude into the cooler lithosphere, heating it.

Heat

Pressure

Pressure increases with depth inside Earth!

• Increasing pressure raises the melting point of rock deep beneath the surface.

• Decreasing pressure, in contrast, lowers rock's melting point.

• Decompression melting- occurs when rock rises and melts due to reduced pressure.

Decompression Melting

• This process typically takes place as hot yet solid rock rises because it is less dense than the surrounding rock.

• As the rock rises, pressure on the rock decreases.

Multiple Choice

What is decompression melting?

• Decompression melting is the process of melting that occurs as a result of increased pressure on a rock.
• Decompression melting is the process of melting that occurs due to extreme cold temperatures on a rock.
• Decompression melting is the process of melting that occurs as a result of chemical reactions within a rock.
• Decompression melting is the process of melting that occurs as a result of a decrease in pressure on a rock.

• Increasing water content of rock also lowers the rock's melting temperature.

• Because of this, "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.

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Water Content

• Fortunately, magma only reaches the surface in certain areas.

• Most volcanoes form along divergent and convergent plate boundaries.

• Some volcanoes form far from plate boundaries above "hot spots" in the crust.​

Volcanoes and Plate Boundaries

Divergent Boundary Volcanism

• At divergent boundaries, volcanic activity occurs where the plates pull apart.

• Mantle rock rises to fill the gap between plates.

• As the rock rises, decompression melting occurs.

• This forms magma, which erupts through a spreading center.

Divergent Boundary Volcanism

• Although most spreading centers are located along mid-ocean ridges, some are not.

• The Great Rift Valley in Eastern Africa is an area where continental crust is being pulled apart along a divergent boundary.

• Mount Kilimanjaro in Tanzania is one of many volcanoes that have formed near the rift valley.

Mount Kilimanjaro