Download Chapter 18 - Volcanoes

CHAPTER 18 VOLCANOES
I. MAGMA
A.
Formation
1. Temperature increases with
depth
2. Increased pressure increases
melting point
3. Increased water content
decreases melting point
B. Types of Magma
1. Basaltic – rich in iron & magnesium,
melts around 1000o C. Quiet
eruptions
Oceanic crust
2. Rhyolitic – high silica content; high
water and gas content; explosive!
Continental crust
3. Andesitic – mixture of basaltic &
rhyolitic, found along continental
margins
C. Viscosity
1. Definition – resistance to flow
2. Basaltic=low viscosity (runny)
3. Rhyolitic=high viscosity (does
NOT flow well)
II. VOLCANOES
A.
Anatomy of a volcano
1.
2.
3.
Vent- opening in crust; magma
leaves earth and becomes lava
Crater- as lava accumulates,
mountain forms. The bowl-shaped
depression at the top is the crater
Caldera- once the magma chamber
empties out, the mountain may
collapse.
B. Volcanic Landforms
1. Shield
a. broad, gently sloping sides
b. basaltic lava
c. Example; Mauna Kea in Hawaii
2. Cinder
a. steep sides, short
b. rhyolitic lava; very explosive
c. Example; Izalco in El Salvador
3. Composite
a. steep top, gently sloping base
b. andesitic lava
c. Example; Mt. St Helens