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. ___ basaltic ___ andesitic ___ rhyolitic B A C 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 false ______ 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.