Wednesday February 24, 2010

Volcanic Activity

Basic Information

All volcanoes are fueled by magma deep beneath Earth’s surface

Magma = • Mixture of molten rock, suspended

mineral grains and dissolved gases deep beneath Earth’s surface.

• Formed when temperatures are high enough to melt the rocks involved.

Lava = Magma that has reached the Earth’s surface

Volcanic Gases

The expansion of gases brings the magma closer to the surface and drives eruptions.

The interaction between the viscosity and temperature of the magma and the gas content determines if an eruption will be effusive or explosive.

http://volcano.oregonstate.edu/education/gases/index.html will be effusive or explosive.

Magma is less dense than its surrounding rock. This density difference causes magma to move

upward (float) and eventually come in contact with (intrude) the crust

Intruding magma can affect the crust in several ways• Force the crust apart• Break blocks of rock off which sinks into the

magma• Melt the rocks that it intrudes with (IGNEOUS)

Magma Meets Crust

Vent

Crater

Volcano

Magma Chamber

http://www.msnucleus.org/membership/html/k-6/pt/volcanoes/2/images/pt2v01_.gif

Anatomy of a Volcano

Vent = Opening in the crust where lava exits. Volcano

• As lava erupts it flows out onto the surface where it cools and solidifies around the vent

• Over time the lava accumulates to form a mountain, aka the volcano

Crater = Bowl shaped depression at the top of the volcano around the vent

Magma Chamber = Storage area for magma that fuels the volcano

Emitted Volcanic Material 1

Ash Cloud = • Volcanic ash consists of tiny jagged

pieces of rock and glass. Ash is hard, abrasive, mildly corrosive, conducts electricity when wet, and does not dissolve in water. Ash is spread over broad areas by wind.

• Falling ash can turn daylight into complete darkness

Volcanic Ash

http://volcanoes.usgs.gov/ash/ashfall.html

Emitted Volcanic Material 2

Tephra• Rock fragments thrown into the air

during a volcanic eruption

Volcanic bombs • Blobs of lava that can harden in the air

or hit the ground, flatten then solidfy

http://facweb.bhc.edu/academics/science/harwoodr/GEOG102/Study/images/volc1.jpg

http://www2.sfu.ca/soils/people/tephra.jpg

http://www.ngdc.noaa.gov/seg/image/titlethumb/73900210.jpg

Pyroclastic flows Mixtures of hot gas, ash and tephra traveling very

quickly down the slopes of volcanoes. 

They are one of the most dangerous hazards posed by volcanoes.

Pyroclastic flows are so hot and choking that if one is caught in one the person will certainly be killed.  Because these flows are very fast they cannot be out run! 

Pyroclastic flows can travel at 200 km/hr and exceed 700ºC!

Types of Volcanoes

Based on • The material that

forms the volcano• Type of Eruption

that occurs

Shield Volcanoes Cinder-cone

Volcanoes Composite

Volcanoeshttp://www.ugs.state.ut.us/education/tc/images/volcanos.gif

Shield Volcano• The widest in size • Gentle sloping sides• Least explosive• Form as layers of lava accumulate from non-

explosive eruptions Cinder Cone Volcano

• Smallest in size, often found next to large volcanoes

• Steepest sides• Intermediate explosiveness• Form when tephra falls back to Earth and piles

around the vent

Composite• Tallest• Steep sides• Most dangerous; very explosive• Form when layers of volcanic fragments

alternate with lava

Composite

http://home.flash.net/~alanm52/VolcanoTypes.jpg

Where Do Volcanoes Occur

Most form at plate boundaries• Convergent and divergent boundaries - 95%

Hot Spots – 5%• Unusually hot regions of the mantle that rise to

the surface, melting everything in its path all the way through the crust

• Far from plate boundaries• The magma moves vertically but NOT laterally..• As a result, a trail of progressively older

volcanoes forms as the plate moves over a hot spot

http://www.oceansonline.com/hotspots.htm