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VOLCANOES-CH. 13
Vocabulary
• Magma- liquid rock produced under Earth’s surface
• Volcanism- any activity that involves movement of magma to Earth’s
surface
• Lava- magma that flows onto Earth’s surface
• Volcano- VENT or FISSURE in Earth’s surface through which magma
and gases are expelled (ejected;erupted)
• Vent- opening in the Earth in which lava erupts
• MELTING POINT- The temperature at which rock will melt.
How does Magma form?
• The mantle is solid because of the large amount of pressure.
This causes rock in the lower mantle to stay BELOW its
melting point.
• All rock is composed of minerals. These minerals have a
specific melting point.
3 ways magma can form
1. The temperature of the rock rises above the melting point of the
minerals in the rock, they will begin to melt.
2. When enough pressure is removed from rock, the melting point of
the minerals in the rock lower and the rock will melt (has the ability
to melt at a lower temperature). (DIVERGENT BOUNDARIES)
3. When fluid (such as water) is added, it also lowers the melting point
of the minerals in the rock and cause the rock to melt.
(SUBDUCTION ZONES).
VOLCANISM
• Magma rises up through crust because it is LESS
DENSE than surrounding rock
• Bodies of magma get LARGER because 1) they melt
rock around it adding to the magma, and 2) as magma
rises through cracks it breaks off surrounding rock,
some melts and is added to body of magma
Where are zones of volcanism found?
• Plate boundaries (convergent and divergent)
• SUBDUCTION ZONES:
• OCEANIC- CONTINENTAL• As oceanic plate subducts, a deep trench forms. Fluids combine with the crust and
mantle and decrease the melting point (this means the rock will melt at a lower
temperature);magma rises because less dense than crust and erupts onto earth’s surface
forming a line of volcanic mountains
• OCEANIC-OCEANIC
• Same as above, but deep ocean trench forms. Line of volcanic Islands form, instead of
mountains.
Marianas Trench
• httpvideo.nationalgeographic.com/video/news/deep-sea-challenge/cameronhow-deep://
Mid-Ocean Ridges
• Form a major zone of volcanic activity
• Pillow lava is a volcanic rock that forms here
• https://www.youtube.com/watch?v=hmMlspNoZMs
• Most underwater volcanoes are unnoticed by humans
• Exception: Iceland- Part of the Mid-ocean ridge above sea level; one
half of Iceland is on the North American Plate moving westward and
the Eurasian plate moving eastward.
Hot Spots
Pluton
ERUPTIONS
• There are 2 types of magma
• MAFIC- contains magnesium and iron/ dark in color
• FELSIC- rich in silica/ light in color
• Quiet- mafic w/ low viscosity. Eruptions from ocean volcanoes.
• Explosive- felsic lava of continental volcanoes.
PYROCLASTIC MATERIAL
• Classified according to size
• Less than 0.25 mm = volcanic dust
• Less than 2 mm = volcanic ash
• Less than 64 mm = lapilli (luh PIL ie)
• Large clots of lava that cool as ejected = volcanic bombs
• Solid rock blasted from the vent = volcanic blocks
TYPES OF VOLCANOES
• Lava and pyroclastic material build up around a vent to form a
volcanic cone
• CRATER- the funnel shaped pit at the top of the cone.
SHIELDformed from quiet eruptions; lava flows out;
hardens; and builds up to form the cone. Broad
base; gentle sloping sides
CINDER CONEsteep slopes; only a few meters high; made from
build up of pyroclastic material.
COMPOSITE
made of alternating layers of hardened lava flows
and pyroclastic material; largest volcanic
mountains.
CALDERA