Volcanoes and Plate Tectonics: 13.1
Formation of Magma
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Even under the high temperature, most of the mantle stays solid due to
pressure, though it may melt to form magma
Magma - a liquid rock produced under Earth’s surface
Magma can form under 3 conditions
1. If temperature of rock rises above the melting point of the minerals the
rock is composed of, the rock will melt
2. If enough pressure is removed from the rock, the melting point will
decrease and the rock will melt
3. If fluids such as water are added to the rock, some of the minerals it
contains will have a lowering of their melting points and cause the rock
to melt
Subduction Zones
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Many volcanoes are located
along subduction zones, where
a tectonic plate moves under another
When oceanic lithosphere collides with continental lithosphere, the denser
oceanic lithosphere subducts, forming a deep trench. The plate with
continental lithosphere folds into a line of mountains on the continents’
edge, and the subducting plate melts into magma as fluids, such as water,
decrease the melting point of the rock and the magma then rises through
the lithosphere and forms volcanic mountains
When 2 oceanic plates collide, the denser one subducts, also forming a
trench, and causing magma to form and break through the overriding plate,
forming a string of volcanic islands, or island arcs
Volcanism
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Volcanism – any activity that includes the movement of magma towards or
onto Earth’s surface
Magma rises upward due to its lower density than surrounding rock, and
rising bodies of magma can become larger since 1) their heat melts some
surrounding rock and, 2) as it rises it is forced into cracks in surrounding
rock causing blocks of overlying rock to break off and melt
Lava – magma that flows onto Earth’s surface, and the rock that forms
when it cools; as it flows through an opening (vent) it may build up a
mountain
Volcano – vent on Earth’s surface through which magma and gases are
expelled
Major Volcanic Zones
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Most active volcanoes occur near convergent and divergent boundaries of
tectonic plates, with their locations forming a pattern across Earth’s surface
Pacific Ring of Fire – major zone of active volcanoes formed by subducting
plates along the Pacific Coasts of North America, encircles the Pacific
Ocean
3 tectonic settings where volcanoes can form are
subduction zones, mid-ocean ridges, and hot spots
Mid-Ocean Ridges
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Largest amount of magma comes to the surface at divergent plates, and
interconnected mid-ocean ridges form a major zone of volcanic activity
As plates move apart, magma flows up along the rift, adding new material
to the mid-ocean ridge, and creating new lithosphere along the rift
This magma erupts to form underwater volcanoes
Pillow lava - type of volcanic rock which forms underwater at mid-ocean
ridges
Hot Spots
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Hot spots – areas of volcanism within the interior of the lithospheric plates
Most form where columns of solid, hot material (mantle plumes) from
deep within the mantle rise and reach the lithosphere, where it spreads out
Magma rises to the surface, breaking through the crust, forming volcanoes
As a lithospheric plate moves over a
stationary hot spot, volcanic activity
ceases in the volcano and a new one is
formed over the hot spot
Volcanic Eruptions: 13.2
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Two types of lava are Mafic and Felsic
Mafic lava is magma/rock rich in magnesium
and iron, and is commonly dark
Felsic lava is rich in light-colored silicate and feldspar materials
Mafic rock commonly makes up the oceanic crust, while mafic and felsic
rock make up continental crust
Types of Eruptions
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A volcano’s eruption force is affected by the viscosity, or resistance to flow,
of magma
Magma’s viscosity is determined by its composition
Mafic magma produces runny lava with a low viscocity and therefore causes
quieter eruptions
Felsic magma produces sticky lava with a high viscosity and therefore
causes explosive eruptions
Magma with a lot of trapped gases is more likely to produce explosive
eruptions
Quiet Eruptions
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Oceanic volcanoes which form from mafic magma have quiet eruptions
because of mafic magma’s low viscosity and because gases can easily escape
from it
Types of Volcanoes
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Volcanic activity produces a variety of features that form during both
quiet and explosive eruptions
Lava and pyroclastic material ejected during eruptions build up around a
vent and form volcanic cones
Funnel shaped pit at the top of a volcanic vent is a crater
3 main types of volcanoes are shield, cinder, and composite
Shield Volcanoes
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Broad based with gently sloping sides and covers a wide area
Generally forms from quiet eruptions, and the layers of hot, mafic lava
flow out around the vent, harden, and slowly build up to form the cone
Cinder Cones
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Has very steep slopes with an angle up to 40 0 and are rarely over a few
hundred meters high
Form from explosive eruptions and are made of pyroclastic material
Explosive eruptions
Composite Volcanoes
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Continental volcanoes which produce felsic lava have explosive eruptions
because of the magma’s high viscosity, and because gases such as water
vapor and CO2 are trapped in it and cause the lava to explode in different
directions as it escapes from the lava in an eruption
This throws pyroclastic material, which consists of fragments of rock
formed during a volcanic eruption, into the air
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Made up of alternating layers of hardened lava flows and pyroclastic
material
Lava flows cover the sides of the cone during quiet eruptions and
pyroclastic material is deposited around the vent during explosive
eruptions
Composite volcanoes are also known as stratovolcanoes and commonly
develop to form huge volcanic mountains
Calderas
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Caldera – a large, circular depression that forms when the magma
chamber below a volcano partly empties and causes the ground above to
sink