Volcanoes

Main topics to be covered:

• Magma

• Intrusive Activity

• Volcanoes

MagmaHow 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 Typesviscosity Gas content Silica

contentexplosiveness Location of

magma

Basaltic

magma

low 1-2% ~50% least Oceanic and continental crust

Andesitic

magma

Intermediate 3-4% ~60% intermediate Subduction zone at continential margins

Rhyolitic

magma

high 4-6% ~70% greatest Continential crust

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 VolcanoVent – 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 Volcanoes1. 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. biggest

(angular)

bombs >64 mm dia. 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/animations/55.swf

Anatomy of a Volcano

Video Clips:/www.cnn.com/interactive/nature/0201/volcano/volcano.swf

• http://dsc.discovery.com/convergence/pompeii/videogallery/videogallery.html