Volcanoes
Volcano refers both to the opening in Earth’s crust through which molten rock, gases, and ash erupt and to
the landform that develops around this opening. A volcanic eruption occurs when magma rises to the surface.
The following three conditions allow magma to form:
 A decrease in pressure can lower the melting temperatures of materials in the asthenosphere. This
takes place along a mid-ocean ridge rift valley where the lithosphere is thinner.
 An increase in temperature can cause materials in the asthenosphere to melt. This occurs at a hot
spot, such as Hawaii.
 An increase in the amount of water in the asthenosphere can lower the melting temperatures of
materials there. This occurs at subduction boundaries.
Volcanoes are formed at subduction (convergent) boundaries:
 Water in the subducted rock is released into the asthenosphere.
 Water lowers melting temperatures leading to magma formation.
 Magma rises because it is less dense than surroundings.
 Some of the magma reaches Earth’s surface, and volcanoes form on the overriding plate.
Volcanoes are formed at divergent boundaries:
 Below a rift, mantle material rises from deeper, hotter regions within the Earth
 Because of the rift, the pressure is lower than it is elsewhere in the mantle, lowering melting temps.
 Combination of high temperature and low pressure causes large amounts of magma to form
 Magma rises because it is less dense than surrounding materials
Most of the magma that reaches Earth’s surface does so at divergent boundaries, along mid-ocean ridges.
Most of Earth’s volcanic activity takes place under the oceans.
In Iceland, the Mid-Atlantic Ridge rises above the surface.
Thule, Iceland
Volcanoes are formed over hot spots.
 A hot spot is an area of volcanic activity that results from plumes of hot solid material that have risen
from deep within the Earth’s mantle.
 As the material rises, it melts at areas of lower pressure.
 A hot spot remains in the same place even as a lithospheric plate moves across it.
Characteristics and types of magma.
Characteristic
Silica %
Basaltic
Andesitic
Rhyolitic
Least (about 50%)
Intermediate (About 60%)
Most (About 70%)
Gas Content
Least
Intermediate
Most
Viscosity
Least viscous
Intermediate
Most
Eruption Type
Rarely explosive
Sometimes explosive
Usually explosive
Melting Temperature
Highest
Intermediate
Lowest
Location
Rifts, oceanic hot spots
Subduction boundaries
Continental hot spots
(Resistance to flow)
Example
Mount St. Helens
Kilauea
Yellowstone caldera
Largest volcanic system in North
America
1. Fill in the blanks:
a. Magma that reaches the Earth’s surface is called lava.
b. Lava with smooth, ropelike surfaces is called pahoehoe.
c. Lava with rough jagged edges is called aa.
d. When lava cools quickly underwater it forms rounded lumps called pillow lava.
e. Solid fragments released from a volcano when trapped gas is ejected are called pyroclastic material.
f.
Pyroclastic material is classified by size. The smallest fragments are called ash, intermediate pieces are
lapilli, and the largest pieces are called blocks and bombs.
g. A dense superheated cloud of pyroclastic material flowing downhill with amazing speed is a pyroclastic
flow. The eruption of Mount Vesuvius in A.D. 79 produced a pyroclastic flow that buried the Roman city
of Pompeii.
Volcano Type
Volcano Shape
Shield
Broad base can
support enormous
height
Cinder Cones
Composite
Description of lava
discharge
Basaltic lava flows
long distances before
hardening due to low
viscosity
Cone shaped
mound with an oval
base that tend to be
smaller than other
types
Molten lava thrown
into air hardens
before it hits the
ground
Layers of materials
from successive
explosive eruptions
accumulate around
a vent
Magma, water and
gas explode in a
massive cloud of
superheated ash and
stones
Sketch of Volcano
Fill in the table below to describe the formation of Crater Lake (a caldera).
1. A volcanic eruption partially The top of the cone collapsed inward, Over time, rainwater filled the caldera,
emptied the magma chamber. forming a basin known as a caldera
forming Crater Lake