1. Volcanoes Main topics to be covered: Magma Intrusive Activity

2. 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!

3. Types of Magma Basaltic Magma Andesitic magma Rhyolitic magma
Do these magma types have familiar names?
What makes these magma types different?

4. Magma Types What makes magma different?
Viscosity – resistance to flow. (temp + comp)
Gas content – percent of trapped gasses in the magma.
Silica content – percent of silica in magma.
Explosiveness – how explosive the magma is.

5. Magma Types Basaltic magma Andesitic Rhyolitic viscosity Gas content
Silica content
explosiveness
Location of magma
Basaltic
magma
low
1-2%
~50%
least
Oceanic and continental crust
Andesitic
Intermediate
3-4%
~60%
intermediate
Subduction zone at continential margins
Rhyolitic
high
4-6%
~70%
greatest
Continential crust

6. Because it is less dense than surrounding rock!
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.

7. 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.

8. Intrusive Activity Diagram

9. Palisades Sill

10. 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)

11. 3-Main Types of Volcanoes
Shield Volcano – a volcano with broad, gently sloping sides and a nearly circular base.
Cinder-Cone Volcano – Volcano with steep sides as a result of material being ejected and being piled up around the vent.
Composite Volcano – volcano with alternating layers of lava and volcanic fragments.

12. Numerous layers of basaltic lava build up.
Low viscosity and less explosive…lava flows for great distances.

13. Contains andesitic lava
Magma is more viscous and contains more water and gasses.
More explosive than shield volcanoes.

14. Contains mostly rhyolitic lava.
Lava contains large amounts of silica, water and gasses.
Violently explosive!

15. Volcanic Material
Tephra – rock fragments thrown into the air by a volcano, classified by size below.
dust
<0.25 mm dia.
smallest
ash
mm dia.
larger
lapilli
mm dia.
blocks
>64 mm dia.
biggest
(angular)
bombs
Biggest
(rounded)

16. 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

17. 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:
Circum-Pacific Belt
Mediterranean Belt.

18. 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.

19. Anatomy of a Volcano
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