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Ch.6, Sec.1 - Volcanic Eruptions
 volcano: a vent or fissure in the Earth’s surface through
which magma and gases are expelled
- molten rock in volcanoes is called magma & lava is molten
rock on Earth’s surface
 Nonexplosive Eruptions
- these eruptions are the most common type and they
produce calm flows of lava
 Explosive Eruptions
- these eruptions are less common and blow dust-size ash
particles into the atmosphere with millions of tons of lava
Ch.6, Sec.1 - Volcanic Eruptions
Mount St. Helens before the
eruption of May 18, 1980
Mount St. Helens 30 years
after the eruption in May, 2010
Ch.6, Sec.1 - Volcanic Eruptions
 What Is Inside a Volcano?
Popocatepetl, Mexico (17,802ft.)
 magma chamber: the
body of molten rock that
feeds a volcano
 vent: an opening at the
surface of the Earth
through which volcanic
material passes
Ch.6, Sec.1 - Volcanic Eruptions
 What Makes Up Magma?
- the key to whether an eruption will be explosive lies in the
silica, water, and gas content of the magma
- if the water content is high, an explosive eruption is likely to
occur due to high pressure in the rock, once it rises to the
surface and turns gaseous (think about shaking a can of soda)
- if the silica content is high, an explosive eruption is likely to
occur due to high pressure in the vents caused by built-up,
hardened magma
Ch.6, Sec.1 - Volcanic Eruptions
 What Erupts from a Volcano?
- explosive eruptions produce pyroclastic material (ash, blocks
of rock, bombs of rock)
- nonexplosive eruptions produce liquid magma called lava
(Pillow, Blocky, Aa, Pahoehoe)
 Lava
1. Pillow Lava – forms when
lava erupts underwater (low
viscosity, so flows quickly)
Ch.6, Sec.1 - Volcanic Eruptions
2. Aa Lava – pours out quickly
& forms a brittle crust (low
viscosity, so flows quickly)
3. Blocky Lava – usually oozes
out slowly & forms sharpedged chunks (high viscosity,
so flows slowly)
4. Pahoehoe Lava – flows very
slowly with a glassy surface
(high viscosity, so flows slowly)
Ch.6, Sec.1 - Volcanic Eruptions
 Pyroclastic Material
1. Ash – forms when the gases
in stiff magma expand rapidly
and the walls of the gas bubbles
explode into tiny particles
2. Lapilli – forms from magma
that has hardened before hitting
the ground (also called cinders)
Ch.6, Sec.1 - Volcanic Eruptions
3. Volcanic Bomb – large blobs
of magma that harden & spin in
the air before landing
4. Volcanic Block – the largest
pieces of pyroclastic material are
made from solid rock
Ch.6, Sec.1 - Volcanic Eruptions
 Pyroclastic Flows
- produced when enormous amounts of ash, dust, and gases
are ejected from a volcano
- these flow at speeds of up to 125mph with temperatures
reaching 1300°F!
- these flows can
create lahars, which
are massive watersaturated mudflows,
which have the
consistency of wet
cement