Volcanic
Activity
Chapter 18.1
Magma
• Magma is a mixture of molten rock,
minerals and gasses.
• Magma forms when rock melts at 800°C to
1200°C
Molten Core with Magma plumes
What determines when rock will melt?
• Pressure- Rocks
under pressure melt
slower.
• Water- Wet rocks
will melt at lower
temperatures.
Viscosity
• The thickness of a liquid.
– Magmas and lavas high in silica have higher
viscosities than magmas and lavas low in
silica.
– Hot lavas have more dissolved silicas and are
therefore more viscous.
Three Types of Magma
• Basaltic
• Andesitic
• Rhyolitic
Basaltic Magma
•
•
•
•
•
Upper mantle rocks melt 50% silica
Rises rapidly to surface
Low viscosity
Low amounts of gasses and silica
Slow Eruption
Andesitic Magma
• 60% silica
• Intermediate viscosity
• Intermediate eruptions
Rhyolitc Magma
•
•
•
•
70% silica
High Viscosity
Large volumes of gas trapped
Explosive eruptions
Section Assessment
1. Match the magma types with their
characteristics.
B basaltic
___
A andesitic
___
C rhyolitic
___
A. intermediate viscosity
content, forms from oceanic
crust and oceanic sediments
B. low viscosity and gas content,
forms from rocks in the upper
mantle
C. high viscosity, forms from
continental crust materials
Section Assessment
2. What would be the likely effect if the volcano
at Yellowstone National Park were to erupt?
Why?
It would most likely be a devastating eruption
because it would be fueled by rhyolitic magma,
which has a very high viscosity and gas content.
Section Assessment
3. Identify whether the following statements
are true or false.
true It is unlikely that Mount Kilauea in Hawaii will
______
explosively erupt.
true
______ Wet granite will melt at a lower temperature than dry
granite.
false A liquid with a high viscosity will also have a high
______
flow rate.
true Major eruptions of Mount St. Helens in Washington
______
state and Mount Fuji in Japan
would probably be similar in nature.