Geology: Inside the Earth Chapter 3 Volcanoes

Chapter 3: VolcanoesSee pp. 80-111

Big Idea:Forces Inside Earth

Essential Question:What causes volcanoes and how do they affect Earth’s surface?

As tectonic plates move due to convection currents in Earth’s mantle, stressbuilds up along the lithosphere’s plate boundaries and weak areas in Earth’s crust called hot spots. Less dense magma rises through the crust in these stressed places to form avolcano. Most volcanoes form due to the forces built up from subduction (at convergent boundaries) and sea-floor spreading (at divergent boundaries). The Earth’s surface is affected when lava cools to form new landforms like volcanic mountains, volcanic islands and lava plateaus. Volcanic debris from erupting volcanoes covers the surface and eventually becomes part of the rock record.

Section 1: Volcanoes and Plate Tectonics

Volcano: A vent (opening) in the Earth's surface through which magma erupts; it is also the landform that is constructed by the eruptive material.

Magma: Molten rock containing liquids, crystals, and dissolved gases that forms within the upper part of the Earth's mantle and crust. When erupted onto the Earth's surface, it is called lava.

Lava: The term used for magma once it has erupted onto the Earth's surface

Ring of Fire: a major belt of volcanoes that rims the Pacific Ocean

Hot Spot: An area in the middle of a lithospheric plate where magma rises from the mantle and erupts at the Earth's surface.

Island Arc: a string of islands formed by the volcanoes along a deep-ocean trench

Most volcanoes are found along the boundaries of Earth’s plates and over hot spots.

Section 2: Properties of Magma

Element: a substance that cannot be broken down into other substances by ordinary chemical or physical means

Compound: a substance made of two or more different elements that have been chemically combined

Physical Property: any characteristic of a substance that can be observed or measured without changing the composition of the substance

Chemical Property: any property that produces a change in the composition of matter

Silica:the major ingredient of magma; made of the elements oxygen and silicon; silica content ranges from 50 to 70% and affects viscosity.

Visocosity:Measure of the fluidity of a substance. Lower viscosity magma flows quickly and cools to form basalt. Higher viscosity magma flows poorly and cools to form rhyolite (like granite).

Pahoehoe:relatively hotter, faster flowing lava that has a smooth, hummocky or ropey surface

Aa: relatively cooler, slower flowinglava that has a rough, rubbly surface composed of broken lava blocks

Each substance has a particular set of physical and chemical properties that can be used to identify a substance or to predict how it will behave.

Viscosity is the resistance to flowing. Some liquids flow more easily than others (low viscosity).

The viscosity of magma depends upon the amount of silica and temperature. High silica increase viscosity. Low temperature increases viscosity.

Section 3: Volcanic Eruptions

Pyroclastic Flow: a ground hugging avalanche of hot ash, pumice, rock fragments and hot, toxic gas that rushes down the side of a volcano as fast as 100 km/hr. Temperatures can exceed 500° C.

Active or Live Volcano: A volcano that is currently erupting, or has erupted during recorded history.

Dormant or Sleeping Volcano: An active volcano that is in repose (quiescence) but is expected to erupt in the future.

Extinct or Dead Volcano: A volcano that is not expected to erupt again.

The forces indissolved, expanding gases push magma from the magma chamber through the pipe and out of the vent during a volcanic eruption.

Eruptions can be either quiet or explosive, depending on the physical properties of magma.

Quiet eruptions result from lowsilica, low viscosity magma, and can result in aa and pahoehoe.

Explosive eruptions result from high silica, high viscosity magma, which can plug the vent and cause a build up of gases. Explosive eruptions feature bombs and pyroclastic flow, and can result in rocks like pumice and obsidian.

Geologists monitor volcanoes for swelling, changes in tilt or elevation, seismic activity, ground water temperatures and escaping gases.

Section 4: Volcanic Landforms

Shield VolcanoCinder Cone VolcanoComposite or Stratovolcano

Caldera:Volcanic Neck:Batholith:

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