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 Assessment1. Match the magma types with their characteristics.

___ basaltic

___ andesitic

___ rhyoliticA. 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

B

A

C

Section Assessment2. 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.

______ It is unlikely that Mount Kilauea in Hawaii will explosively erupt.

______ Wet granite will melt at a lower temperature than dry granite.

______ A liquid with a high viscosity will also have a high flow rate.

______ Major eruptions of Mount St. Helens in Washington state and Mount Fuji in Japan would probably be similar in nature.

Section Assessment3. Identify whether the following statements

are true or false.

true

true

false

true