1. CHAPTER 18: VOLCANOES SECTION 1: MAGMA
Objectives:
Standards:

2. CHAPTER 18: VOLCANOES SECTION 1: MAGMA
is a mixture of molten rock, suspended mineral grains, and dissolved gasses deep below Earth’s surface
Magma forms when:
Temperatures are high enough to melt rock (between 800 oC and 1200 oC )
Temperatures exist at the lithosphere/asthenosphere border
Magma is less dense than the surrounding rock
Forced upward

3. FACTORS THAT AFFECT MAGMA FORMATION
TEMPERATURE:
Temperatures increases with depth.
This is why the asthenosphere is plastic-like.
PRESSURE:
Pressure increases with depth due to weight of rock above.
Pressure explains why rocks in Earth’s lower crust and upper mantle do not melt even though the temps are high enough
WATER CONTENT:
Water can be found in the spaces of rocks.
A wet mineral or rock will melt at a lower temp than a dry rock.

4. CHARACTERISTICS OF BASALTIC MAGMA:
Rocks in upper mantle melt and rises to crust quickly
Low viscosity – fluid magma
Small amounts of dissolved gases
Small amounts of silica
Quiet/non-violent eruptions

5. CHARACTERISTICS OF ANDESITIC MAGMA:
Found along continental-oceanic subduction zones
Oceanic crust and/or oceanic sediments melt
60 % silica
Intermediate viscosity – thicker than basaltic and intermediate gas content
Intermediate eruptions

6. RHYOLITIC/GRANITIC MAGMA
CHARACTERISTICS OF RHYOLITIC/GRANITIC MAGMA:
Continental crust
Rich in silica and water content
High viscosity – thickest magma, slows movement
Large amounts of trapped gases
Very explosive eruptions

7. VISCOSITY AND HOW IT AFFECTS THE EXPLOSIVENESS OF MAGMA
Viscosity: A substance’s internal resistance to flow.
EX: water has a lower viscosity than honey and flows more easily.
Honey has a higher viscosity than water because it does not flow as easily.
Viscosity and explosiveness of magma:
The lower the viscosity, the more fluid-like the magma, the least explosive eruption. BASALT
Intermediate viscosity, intermediate magma, intermediate explosion. ANDESITIC
The higher the viscosity, the less fluid-like the magma – magma gets caught and pressure builds up causing a more explosive eruption. RHYOLITIC/GRANITIC

8. TABLE 8.1 – PG. 474. Basaltic Upper mantle Low 1-2% About 50% Least
Composition
Source Material
Viscosity
Gas content
Silica content
Explosive-ness
Location of Magma
Basaltic
Upper mantle
Low
1-2%
About 50%
Least
Both oceanic and continental crust
Andesitic
Oceanic crust and oceanic sediments
Inter-
mediate
3-4%
About 60%
Intermediate
Continental margins associated with subduction zones
Rhyolitic/
Granitic
Continental crust
High
4-6%
About 70%
Greatest

9. SILICA CONTENT AND VISCOSITY OF EACH TYPE OF MAGMA
Hotter magma or lava = lower viscosity
Basaltic: temp. between 1000 oC and oC – low viscosity
Rhyolitic: temp between 700 oC and 900 oC – high viscosity
High amounts of silca = higher viscosity
Basaltic: low amounts of silica = low viscosity
Rhyolitic: high amounts of silica = high viscosity
Andesitic: intermediate amounts of silica = intermediate viscosity