Download Ch.6 Volcanoes 6.1 – Volcanic Eruptions 1. Nonexplosive Eruptions

Ch.6 Volcanoes
6.1 – Volcanic Eruptions
1. Nonexplosive Eruptions
a. Magma that flows onto the Earth’s surface is called
lava
b. Most common type
c. Relatively calm flow of lava and large amounts of it
d. Vast areas of Earth’s surface (like most of seafloor &
Northwest region of USA) are covered by lava from
these type of volcanoes
2. Explosive Eruptions
a. Can be 10,000 times stronger than the first atomic bomb
exploded during World War II
b. Can turn an entire mountain into a cloud of ash and rock in a
matter of seconds
c. More rare than non-explosive eruptions
d. Instead of lava produce ash which is molten rock blown into
tiny particles that harden in the air
i. Can circle the Earth for years
ii. Larger pieces fall closer to the volcano
3. What is inside a Volcano?
a. Magma Chamber – body of molten rock
deep underground that feeds a volcano
i. Vents - Magma rises through cracks
in the Earth’s crust during an
eruption
4. What Makes up Magma?
a. The composition of magma affects how
explosive a volcanic eruption will be
(water, silica, and gas content)
i. Water and Magma are an Explosive Combination
1. Water content high – more explosive eruptions
a. Water is dissolved under intense pressure in the magma underground until it
moves to the surface. When the pressure suddenly decreases the water and
other components like Carbon Dioxide become gases.
b. Like opening a can of soda pop
c. Pumice - Some lava is so frothy with gas when
it reaches the surface that its solid form
with float in water
ii. Silica-Rich Magma Traps Explosive Gases
1. High in silica – more explosive eruptions
a. Rocks more stiff causes magma to flow slowly and harden in a volcano’s vents
like a plug
b. Cause more pressure to build and make it harder for gases to escape
5. What Erupts from a Volcano?
a. Types of Lava – liquid rock flowing from vent (check out p. 159)
i. Blocky lava & pahoehoe (puh HOY hoy) flow slowly and have a high viscosity – like a
milkshake
ii. Aa (ah ah) and pillow lava flow quickly and have a low viscosity – glass of milk
b. Types of Pyroclastic Material – magma blasted into the air and hardens
i. Sizes range from boulders the size of houses and tiny particles that circle the Earth
for years
ii. Volcanic bombs, Volcanic blocks, lapilli, and volcanic ash (check out p.160)
c. Pyroclastic Flows – enormous amounts of hot ash, dust and gasses are ejected from a
volcano
i. Can race downhill 200 km/hr (faster than most hurricane winds)
ii. Can exceed temperatures of 700C
6.2 – Effects of Volcanic Eruptions
6. Volcanic Eruptions and Climate Change
a. 1816 – year without a summer in New
England (snowed in June)
b. Mt Tambora exploded in 1815
i. 12,000 people died as a direct result
ii. 80,000 people died from resulting
hunger and disease
iii. Enough volcanic ash and gasses
ejected into the upper atmosphere to
block sunlight and cause global
temperatures to drop – resulting in food shortages in North America and Europe
iv. Mt. Pinatubo eruption caused global temperatures to drop .5C
7. Different Types of Volcanoes (check out p.163)
a. Shield Volcanoes
i. Layers of lava from many nonexplosive eruptions
ii. Lava runny and spreads over a large area
iii. Volcano has gently sloping sides
iv. Can be very large
v. Hawaii’s Mauna Kea – from seafloor to tip largest mountain on Earth (taller than Mt.
Everest.)
b. Cinder Cone Volcanoes
i. Made of pyroclastic material
ii. Moderately explosive eruptions
iii. Steep slopes, small volcanoes, erupt for short time
iv. Often occur in clusters on the sides of other volcanoes
v. Erode quickly because pyroclasitc material is not cemented together.
c. Composite Volcanoes
i. Stratovolcanoes
ii. Most common type
iii. Explosive pyroclastic eruptions followed by quieter lava flows
iv. Japan’s Mt. Fuji
v. Broad bases and sides that get steeper toward the top
vi. USA - Mt. Hood, Mt. Rainier, Mt. Shasta, and Mt. St. Helens
8. Other Types of Volcanic Landforms
a. Craters
i. A funnel-shaped pit centered around the vent at the top of many volcanoes
1. Lava flows and pyroclastic material pile up around the vent creating a cone with a
central crater, when eruption stops the lava left in the crater can drain back
underground. If lava remains will be blasted away during next eruption widening
the crater
b. Calderas
i. Larger than craters
but appear similar
ii. When the chamber
that supplies magma to
a volcano partially
empties the chamber’s
roof collapses and the
ground sinks
iii. Yellowstone park –
three large calderas
from volcano 1.9 million
and .6 million years ago
c. Lava Plateaus
i. Rifts – long cracks in Earth’s
crust that lava flows from
1. Most lava on Earth’s surface
comes from rifts
ii. Columbia river plateau in
northwest region of USA – lava
plateau from 17 – 14 million
years ago (some places 3 km
thick)
6.3 – Causes of Volcanic Eruptions
9. The Formation of Magma
a. Pressure and Temperature
i. Upper mantle is made of very hot puddle-like rock that flows slowly (remains solid
because of pressure inside of Earth from all the weight of the rock above)
ii. Rock melts when its temperature increases or pressure decreases (the mantle
presses atoms so close that the rock cannot
melt)
b. Magma Formation in the Mantle
i. Temperature of the mantle is fairly consistent
ii. Decrease in pressure is the most common cause
of magma formation
iii. Often forms at the boundary of separating tectonic plates where pressure is
decreased
iv. Once magma is formed it is less dense than the mantle and will slowly bubble towards
the surface
10. Where Volcanoes Form
a. Many lie directly on boundaries
between tectonic plates
b. Ring of Fire – plate boundaries
surrounding the Pacific ocean
has many volcanoes (check out
p.167)
c. 80% of volcanoes – plates collide
d. 15% of volcanoes – plates
separate
e. 5% of volcanoes – far from plate
boundaries
11. When Tectonic Plates Separate
a. Divergent boundary
b. Rift zone – deep set of cracks where mantle rock rises to fill the gaps and form new
crust
c. Mid-Ocean Ridges Form at Divergent Boundaries
i. Most volcanic activity occurs here
ii. Most underwater
iii. Iceland – hot springs and volcanoes
1. In 1963 a new island “Surtsey” was created
when enough lava poured out of the MidAtlantic Ridge
12. When Tectonic Plates Collide
a. Convergent boundary
b. Subduction Produces Magma
i. As the descending oceanic crust scrapes past
the continental crust the temperature and
pressure increase and release the trapped water. The water mixes with mantle rock,
lowers its melting point, and causes it to melt
1. Can rise and form a volcano
c. Hot Spots – volcanically active places far from plate boundaries
i. Not all magma develops on tectonic plate boundaries
ii. Hawaiian Islands
iii. Two theories:
1. Above mantle
plumes – columns of
rising magma
2. Results of cracks in the
Earth’s crust
iv. Often produce a long chain of
volcanoes
1. Two theories:
a. Mantle plume stays in same spot as crust
moves
b. Volcanoes are forming along the cracks
d. Predicting Volcanic Eruptions
i. Extinct Volcanoes – not erupted in recorded history and probably not erupt again
ii. Dormant Volcanoes – currently not erupting, but record of past eruptions suggest
they will erupt again
iii. Active Volcanoes – currently erupting or show signs or erupting in the near future
1. Measuring Small Quakes and Volcanic Gases
a. Most active volcanoes produce small earthquakes as magma works its way up to
the surface
b. Monitoring the quakes is the best way to predict an eruption
c. Studying the volume and composition or volcanic gases can predict and
eruption (ratio of sulfur dioxide to carbon dioxide)
i. Changes indicate changes in the magma chamber below
iv. Measuring Slope and Temperature
1. Earth’s surface swells as the magma rises upward prior to an eruption
2. Tiltmeter – detects small changes in the angle or a volcano’s slope
3. GPS – detect changes in a volcano’s slope
4. Satellite images – infrared images record changes in surface temperature and
gas emissions over time
a. if site is getting hotter – magma rising