Volcanoes and Earth’s Moving Parts

What is a Volcano? A volcano is an opening in the Earth that erupts gases, ash, and lava. Volcanic mountains form when layers of lava, ash, and other material build up around these openings.

Kilauea is the world’s most active volcano. It has been erupting for centuries although not explosively. Its lava moves very slowly.

Iceland is also famous for its active volcanoes Iceland is also famous for its active volcanoes. It sits on an area where the Earth’s plates move apart. ( a ------boundary).

How do volcanoes form? Deep inside the earth, heat, and pressure cause rock to melt, forming magma (liquid rock). Magma is forced upward because it is less ------- than the rock above it, so it is forced toward the Earth’s surface. After thousands or millions of years, the magma reaches the Earth’s surface and flows out of an opening called a vent.

volcano

Where do volcanoes occur? Divergent plate boundaries Convergent plate boundaries Hot spots -Hawaii

What controls eruptions? Some eruptions are quiet while others are explosive. Two factors control if an eruption is quiet or explosive.

One factor is the amount of water vapor and trapped gases in the magma. The second factor is the viscosity of the magma – how quickly or slowly the magma flows (determined by how much silica is present).

Basaltic Magma is relatively low in SILICA has a low viscosity. It is fluid and produces quiet, non-explosive eruptions (like those at Kilauea.)

Kilauea

Explosive Eruptions Silica-rich or GRANITIC magma produces explosive eruptions. It has a high viscosity. This magma sometimes forms where the Earth’s plates are moving together where one plate slides under another – (AKA ------)

Shield Volcanoes Cinder Cone Volcanoes Composite Volcanoes Forms of Volcanoes Shield Volcanoes Cinder Cone Volcanoes Composite Volcanoes

Shield volcanoes are volcanic mountains built up by the eruption of fluid, basaltic lava flowing out of a central vent. They have broad bases with very gentle slopes, and are the largest volcanoes on Earth. The volcano gets its name from its resemblance to the slightly domed shape of a warrior's shield.  

Shield volcanoes do not erupt violently Shield volcanoes do not erupt violently. Rather, fluid, basaltic lava erupts onto the surface and flows freely across the ground for great distances forming a broad cone. Lava tubes can form beneath the surface of the lava flow which allows the lava to flow for tens of kilometers (miles) without cooling. Each successive flow increases the size of the shield volcano. Sheild volcanoes have magma with a low viscosity.  

Hot Spots Weak areas in the earth’s crust where magma plumes punch through. Volcanoes eventually form islands chains like the Hawaiian Islands.

Cinder cone volcanoes consist of steep, loosely packed material.

Cinder Cone

Cinder Cone

Cinder cones are the simplest type of volcano Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava (tephra) ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone.

Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings. Cinder cones are numerous in western North America as well as throughout other volcanic terrains of the world. Cinder cone volcanoes have lava with high viscosity.

Composite Volcano

Structure of a Composite Volcano

Composite volcanoes are also known as strata volcanoes. They have alternating layers of tephra and lava. They have a mountainous form, and usually form at subduction zones. Mount St. Helen’s is a composite volcano.

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