Volcanoes
Source: Photograph copyright Paul Hickson, 1980
Image by NPS
I. Volcanoes release magma
A.
Molten rock is less dense than solid rock
so it rises through fractures
Shield Volcano
B. Speed of magma depends on silica
content.
1. felsic magmamagmaa. High silica
b. Thick
c. LightLight-colored
d. SlowSlow-moving
e. violent eruptions
2.
Mafic magma
a. Low silica
b. Thin
c. dark
d. FastFast-moving
e. Eruptions are not violent
C. Gas content of magma
determines violence of eruptions
1.
2.
The more gas, the more violent
CO2, sulfur, H2, H2O vapor
High gas
Low
Gas
D. Lava composition are a little
different than their magma was.
Why?
1.
2.
Gas is released
New material is added
E. TephraTephra-solid fragments of lava
Tephra is classified by size
1. ash
ash-- < 2 mm in diameter
Ash
2. LapilliLapilli- 2-64 mm
3. BlocksBlocks- >64 mm
4. BombsBombs- >64 mm, but are
ejected as liquid & harden as
they fall
Pyroclastic flow- hot, fast, high-density
mixture of ash, pumice, rock fragments
and gas formed during explosive
eruptions.
Image from: U.S. Geological Survey, photo by B. Yount
II. 3 Types of Volcanoes
A.
B.
C.
Cinder Cones
Composite Volcanoes
Shield Volcanoes
A. Cinder Cones
1.
2.
3.
4.
5.
Steep-sided
Gas-charged, felsic lava
Single vent
Bowl-shaped crater at the summit
Rarely more than 1000 ft tall
Sunset Crater in Arizona
http://www.geology.sdsu.edu/how_volcanoes_work/
Cinder Cones
Cinder Cones
Cinder cone
B. Composite or Stratovolcanoes
1.
2.
3.
4.
5.
6.
Steep-sided symmetrical cones
Felsic lava
Explosive
Make up about 60% of Earth’s volcanoes
Layered structure consisting of layers of
lava & pyroclastic material
Large (1-10 km across)
Composite volcano
Mt. St Helens
Source:http://www.geology.sdsu.edu/how_volcanoes_work/
Composite Volcano
Mt Fuji in Japan
C. Shield Volcanoes
1.
2.
3.
4.
5.
Built of fluid basaltic lava flows
Lava pours out in all directions from a
central summit vent, or group of vents
Gently sloping sides like a “warriors shield”
Some of the largest volcanoes are shield
volcanoes (3-4 mi wide, 1,500-2,000 ft tall)
Usually no tephra
Shield Volcano
Layers of basalt lava build
over time.
Contrasting the 3 types of Volcanoes
Cinder
Cones
Composite
Volcano
Shield
Volcano
III. Causes of Volcanoes
A.
Rift eruptionseruptions-occur at fractures in the
crust on the ocean floor
1. Flow smoothly
2. Can form shield cones
3. Rift eruption at spreading center form pillow
lavas
4.
5.
On land, lava can spread out evenly over
large areas to form basalt plateaus.
plateaus.
Mafic magma
B. Subduction boundary eruptions
1.
2.
3.
Felsic magma
Explosive w/tephra
Form cinder cones
cones-- steep
steep--sided volcanic
mountain
C. Hot SpotsSpots-areas of volcanic
activity in the middle of plates
1.
2.
3.
ooze mafic magma through shield cones
Caused by concentrations of heat from
radioactive isotopes in the asthenosphere
They stay in the same place even though
plates move above them.
IV. Examples of Famous Volcanoes
A.
Eldfell
1.
2.
3.
4.
Off the coast of Iceland
Erupted in 1973
Villagers pumped sea water on lava
On the midmid-Atlantic ridge, but it is above sea
level.
5. Rift eruption
http://volcano.und.nodak.edu/vwdocs/volc_images/europe_w
est_asia/heimaey/heimaey.html
http://volcano.und.nodak.edu/vwdocs/volc_images/eu
rope_west_asia/heimaey/heimaey.html
B. Mount St. Helens in Washington
1.
2.
3.
4.
Cascade range
Erupted 1980
Subduction boundary eruption
Juan de Fuca plate slides under the North
American plate
Mt. St Helens
C. KilaueaKilauea-Hawaii
1.
2.
Has erupted every year since 1952
Hot spot
Kilauea
Low Gas
V. Plutonic Activity
A.
Pluton (igneous intrusion)
intrusion)- rock masses
that form when magma cools inside other
rocks.
B. Volcanoes Underground
1.
Dikes – sheets of igneous rock that cut
across the rock layers they intrude
2.
Sills – sheets of igneous rocks
parallel to the layers they intrude
3. Laccolith – domelike masses that form
when magma that intrudes between rock
layers is unable to flow easily. As a result,
you get a bulge. The rock layers above
push into a dome.
4. Batholith – largest igneous intrusions
that form at the cores of many
mountain ranges
5. Volcanic neck – a plug of hardened
magma left in the vent from which lava
flowed. Found when an extinct volcano is
almost completely eroded.
Test Monday!!!
Pelee