Transcript Volcanoes

Volcanoes
Main topics to be covered:
• Magma
• Intrusive Activity
• Volcanoes
Magma
How does magma form?
• Magma forms when temperatures are high
enough to melt rock.
• Rocks usually melt between 800°C and
1200°C
• Pressure can also effect the formation of
magma…greater pressure requires higher
temperatures to melt rock. (p. 472 Fig. 18-1)
• Presence of water allows rock to melt at a
lower temperatures!
Types of Magma
• Basaltic Magma
• Andesitic magma
• Rhyolitic magma
Do these magma types have familiar names?
What makes these magma types different?
Magma Types
What makes magma different?
1. Viscosity – resistance to flow. (temp + comp)
2. Gas content – percent of trapped gasses in the
magma.
3. Silica content – percent of silica in magma.
4. Explosiveness – how explosive the magma is.
Magma Types
viscosity
Gas content Silica
explosiveness Location of
content
magma
Basaltic
magma
Andesitic
magma
low
1-2%
~50%
least
Oceanic and
continental
crust
Intermediate 3-4%
~60%
intermediate
Subduction
zone at
continential
margins
Rhyolitic
magma
high
~70%
greatest
Continential
crust
4-6%
Intrusive Activity
• Magma will rise upward in the Earth, coming in
contact with or intruding into the overlying
crust. Why does magma rise?
Because it is less dense than surrounding rock!
• Magma can cool inside the Earth forming
plutons.
• Plutons are large areas of coarse-grained
igneous rock.
Types of Plutons
• Batholiths – largest type of pluton, irregular
shaped mass of coarse-grained igneous rock.
(greater than 100 km2)
• Stocks – similar to batholiths, but smaller.
• Laccoliths – mushroomed shaped pluton, cause
by rocks bowing upward due to heat and
pressure. (much smaller)
• Sill – pluton that forms when magma intrudes
into parallel layers of rock. (Palisades Sill)
• Dike – pluton that cuts across existing rock.
Intrusive Activity Diagram
Palisades Sill
Anatomy of a Volcano
Vent – opening in the crust that allows magma
to come out on the surface.
Crater – bowl-shaped depression that connects
the magma chamber to the vent, usually near
the top of the volcano.
Magma chamber – large pocket of magma
below the surface.
Caldera – large depressions that are formed
when the summit of a volcano collapses into
the magma chamber. (Figure 18-11)
3-Main Types of Volcanoes
1. Shield Volcano – a volcano with broad,
gently sloping sides and a nearly circular
base.
2. Cinder-Cone Volcano – Volcano with
steep sides as a result of material being
ejected and being piled up around the
vent.
3. Composite Volcano – volcano with
alternating layers of lava and volcanic
fragments.
•Numerous layers of basaltic lava build up.
•Low viscosity and less explosive…lava flows for great
distances.
• Contains andesitic lava
• Magma is more viscous and contains more water and gasses.
• More explosive than shield volcanoes.
•Contains mostly rhyolitic lava.
•Lava contains large amounts of silica, water and gasses.
•Violently explosive!
Volcanic Material
Tephra – rock fragments thrown into the air
by a volcano, classified by size below.
dust
<0.25 mm dia.
smallest
ash
0.26-2.00 mm dia.
larger
lapilli
2.01-64.00 mm dia.
larger
blocks
>64 mm dia.
bombs
>64 mm dia.
biggest
(angular)
Biggest
(rounded)
Pyroclastic Flows
• Hot material that is ejected during violent
eruptions.
• Contain poisonous gasses, dust, ash, and
other material that move down hill at
amazing speeds.
• Can reach speeds of 200 km/h
• Can reach temperatures of greater than 700°C
Locations of Volcanoes
•
•
•
•
Mostly occur at plate boundaries.
80% occur at convergent boundaries.
15% at divergent boundaries.
~5% at other locations not at boundaries,
such as hot spots.
• Most of Earth volcanoes occur in 2 main
belts:
1. Circum-Pacific Belt
2. Mediterranean Belt.
Hot Spots
• Volcanoes that are far away from plate
boundaries are usually over hot spots.
• These are unusually hot area of the mantle,
where molten material rises to the surface.
• The heat melts rock of the crust, and the
magma rises upward…forming volcanoes.
• Hawaii is located over a mantle hot spot.
• As the plates move over the hot spot,
volcanoes are formed.
http://www.geo.wvu.edu/~donovan/geol101/a
nimations/55.swf
Anatomy of a Volcano
Video
Clips:/www.cnn.com/interactive/nature/020
1/volcano/volcano.swf
• http://dsc.discovery.com/convergence/pomp
eii/videogallery/videogallery.html