Volcanoes and Igneous Rock Types - Ms. Muir at Union Middle School -Ms. Muir at Union Middle School Photo of Mt. St. Helens by Ms. Muir

Two Types of Volcanoes Shield Volcanoes Shield Volcanoes – Typically found in the oceans (underwater) or on islands. – Magma made from oceanic crust, which is mainly composed of a rock called basalt. – Basalt is a more dense rock than granite, so its magma flows slower. Composite Volcanoes Composite Volcanoes – Typically found on continents. – Magma made from continental crust, which is mainly composed of a rock called granite. – Granite is a less dense rock than basalt, so its magma flows faster.

Eruption Characteristics Shield Volcanoes Shield Volcanoes – Lava is typically less viscous than Composite volcano lava. – Less viscous lava requires less pressure to erupt, so eruptions tend to ooze out of volcanoes, often flows like a river. Composite Volcanoes Composite Volcanoes – Lava is typically more viscous than Shield volcano lava. – More viscous lava requires more pressure to erupt, so eruptions tend to explode out of volcanoes, this creates fast, violent eruptions.

Eruption Movies Shield Volcano Video of Hawaiian Volcano Eruption Video of Hawaiian Volcano Eruption Composite Volcano Mt. St. Helens Eruption 1980 Mt. St. Helens Eruption 1980 Pyroclastic Flow Footage Pyroclastic Flow Footage

Basic Ways to Classify Igneous Rocks Extrusive Extrusive – Extrusive rocks cool above ground due to a volcanic eruption. This allows less time for minerals to grow. Igneous rocks with no visible minerals are classified as fine- grained, gas-filled or glassy. Intrusive Intrusive – Intrusive rocks cool underground where it is warmer. This allows more time for minerals to form and grow. We call igneous rocks with visible minerals coarse- grained.

Rocks From Each Type of Volcano Both Shield and Composite Volcanoes produce a rock type for each of the texture classifications from the previous page. To know which type of volcano an igneous rock comes from, you have to know a little bit about the density, color, texture or grain size of each rock you are looking at. These are clues to unravel the history of any igneous rock that you look at. Both Shield and Composite Volcanoes produce a rock type for each of the texture classifications from the previous page. To know which type of volcano an igneous rock comes from, you have to know a little bit about the density, color, texture or grain size of each rock you are looking at. These are clues to unravel the history of any igneous rock that you look at.

Clues from Each Type of Volcano Shield Volcano Shield Volcano – Rocks are typically dark in color due to the darker colored minerals that make up Oceanic Crust. – Rocks are typically more dense due to more dense minerals found in Oceanic Crust. Composite Volcano Composite Volcano – Rocks are typically light in color due to the lighter colored minerals that make up Continental Crust. – Rocks are typically less dense due to less dense minerals found in Continental Crust. *Color and density are only accurate when you compare it to the opposite type of rock from the other type of volcano.

Igneous Rock Identification Shield Volcano Shield Volcano – Coarse-Grained: Gabbro – Fine-Grained: Basalt – Gas-Filled: Scoria – Glassy: Obsidian Composite Volcano Composite Volcano – Coarse Grained: Granite – Fine-Grained: Rhyolite – Gas-Filled: Pumice – Glassy: Rhyolite Glass

Mt. St. Helens After Eruption in 1980 (Picture on right) Mt. St. Helens Prior to Eruption in 1980 (Picture on left)