Download 10.1 Volcanoes and Plate Tectonics Lecture Outline Origin of

10.1 Volcanoes and Plate Tectonics
Lecture Outline
Origin of Magma
Magma forms in the crust and upper mantle when solid rock particles melt.
The Formation of magma depends on several factors.
1. Heat
2. Pressure
3. Water content
Heat
At a depth of 100 kilometers, the temp of the mantle ranges between 1400 0 C
and 16000C.
At these temperatures, the solid rock of the lower crust and upper mantle is near,
but not at the melting point.
The additional heat needed to produce magma comes from:
1. Friction generates as huge slabs of lithosphere slide past each other in
the subduction zone
2. Mantle itself heats these subducting slabs
3. Hot mantle rock can rise and intrude into the cooler lithosphere, heating
it.
Pressure
Increasing pressure raises the melting point of rock deep beneath the surface.
Decreasing pressure lowers rock’s melting point.
When pressure drops enough, decompression melting occurs.
This process takes place as hot yet solid mantle rock rises because it is less
dense than the surrounding rock.
As the rock rises, pressure on the rock decreases.
This decrease lowers the rock’s melting point.
Water Content
The water content of rock also lowers the rock’s melting point.
“Wet” rocks deep beneath the surface melts as a much lower temperature
than “dry” rocks.
Volcanoes and Plate Boundaries
Most volcanoes form along divergent and convergent plate boundaries.
Some volcanoes form far from plate boundaries above “hot spots” in the
crust.
Divergent Boundary Volcanism
At divergent boundaries, volcanic activity occurs where the plates pull apart.
1. Mantle rock rises to fill the gap between the plates
2. As the rock rises, decompression melting occurs.
3. This forms magma, which erupts along the axis of the spreading center.
Spreading centers are located along mid-ocean ridges, some are not
1. The Great Rift Valley in East Africa is where continental crust is being
pulled apart along a divergent boundary.
2. Mount Kilimanjaro in Tanzania is a volcano that formed near the rift
valley.
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 increased in temperature and
pressure drives water from the oceanic crust.
Once the sinking slab reaches a depth of about 100 to 150 kilometer, this
water reduces the melting point of hot mantle rock low enough for melting to
begin.
The magma formed slowly migrates upward, forming volcanoes.
Volcanoes from as convergent boundaries where two oceanic plates meet and
where the oceanic lithosphere is subducted beneath another oceanic plate.
This process results in the formation of a chain of volcanic mountains on
the ocean floor.
Eventually this volcanic mountain grow large enough to rise above the
ocean surface.
Called Volcanic Island Arcs.
Ring of Fire is the long belt of volcanoes that circles the Pacific Ocean.
Volcanism may also occur at convergent plate boundaries where the continental
plate meets an oceanic plate.
Result the oceanic plate is subducted under the continental lithosphere.
This forms a Continental Volcanic Arc.
Intraplate Volcanism
Volcanic activity that occurs within a plate. Example : Yellowstone
National Park.
Most Intraplate volcanisms occurs where hotter - than – normal mantle material,
called a mantle plume, rises toward the surface.
This may create a small volcanic region called a hot spot. The more
recently formed island of Hawaii Island has two active volcanoes-Mauna Loa
and Kilauea.