Volcanoes
Section 1 Notes: Volcanoes and Plate Tectonics
· Volcano: is a weak spot in the crust where molten material, or magma comes to the surface
· Magma: is a molten mixture of rock-forming substances, gases, and water from the mantle
· Lava: when magma reaches the surface
· Ring of Fire: formed by the many volcanoes that rim the Pacific Ocean
· Volcanic belts form along the boundaries of Earth’s plates
o At plate boundaries, huge pieces of the crust diverge (pull apart), or converge (push together). As a result, the crust often fractures, allowing magma to reach the surface
· Island arc: an volcano that is created in a string of island
· Hot Spot Volcanoes
o Hot spot: is an area where material from deep within the mantle rises and then melts, forming magma
o A volcano forms above a hot spot when magma erupts through the crust and reaches the surface
o Some hot spots lie in the middle of plate far from any plate boundaries, but other hot spots occur on or near plate boundaries
o A hot spot in the ocean floor can gradually form a series of volcanic mountains
Section 2 Notes: Properties of Magma
· Physical and Chemical Properties
o Element: is a substance that cannot be broken down into other substances
§ Carbon, hydrogen, and oxygen are examples of elements
o Compound: is a substance made of two or more elements that have been chemically combined
o Each substance has a particular set of physical and chemical properties. These properties can be used to identify a substance or to predict how it will behave.
o Physical property: is any characteristic of a substance that can be observed or measured without changing the composition of the substance
§ Examples of physical properties include density, hardness, melting point, boiling point, and whether a substance is magnetic
o Chemical Properties: is any property that produces a change in the composition of matter
§ Examples of chemical properties include a substance’s ability to burn and its ability to combine, or react, with other substances.
o Viscosity: the physical property of liquids
o Because liquids differ in viscosity, some liquids flow more easily than others
o The greater the viscosity of a liquid, the slower it flows
§ Example: honey is thick, sticky liquid with high viscosity; honey flows slowly
§ The lower the viscosity, the more easily a liquid flows
o Viscosity of magma depends upon its silica content and temperature
o Silica: is made up of particles of the elements oxygen and silicon
o Silica is one of the most abundant materials in Earth’s crust
o The amount of silica in magma helps to determine its viscosity
o The less silica magma contains, the lower its viscosity
o Different types of lava
§ Pahoehoe: is fast-moving, hot lava that has low viscosity
· The surface of a lava flow formed from pahoehoe looks like a solid mass of wrinkles, billows, and ropelike coils
§ aa: lava that is cooler and slower-moving
· Aa has higher viscosity than pahoehoe
· When aa hardens, it forms a rough surface consisting of jaded lava chunks
Section 3 Notes: Volcanic Eruptions
· A volcano is more than a large, cone-shaped mountain. Inside a volcano is a system of passageways through which magma moves
· Magma chamber: beneath a volcano, magma collects in a pocket
· The magma moves upward through a pipe, a long tube in the ground that connects the magma chamber to Earth’s surface
· Vent: molten rock and gas leave the volcano through an opening
· Lava flow: is the area covered by lava as it pours out of a vent
· Crater: is a bowl-shaped area that may form at the top of a volcano around the central vent
· When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the lava
· Geologists classify volcanic eruptions as quiet or explosive
o Quiet eruptions: a volcano erupts quietly if its magma is low in silica; low silica magma has low viscosity and flows easily; the gases in the magma bubble out gently; lave with low viscosity oozes quietly from the vent and can flow for many kilometers
o Explosive eruptions; a volcano erupts explosively if its magma is high in silica; high silica magma has high viscosity, making it thick and sticky; the high-viscosity magma does not always flow out of the crater
· Pyroclasic flow: occurs when an explosive eruption hurls out a mixture of hot gases, ash, cinders, and bombs
· Volcano hazards
o During a quiet eruption, lava flows from vents, setting fire to, and then burying, everything in its path
o During an explosive eruption, a volcano can belch out hot clouds of deadly gases as well as ash, cinders, and bombs
· Geologists often use the terms active, dormant, or extinct to describe a volcano’s stage of activity
o Dormant: or sleeping volcano is likely to awaken in the future and become active
o Extinct: or dead volcano is unlikely to erupt again
Section 4 Notes: Volcanic Landforms
· Volcanic eruptions create landforms made of lava, ash, and other materials. These landforms include shield volcanoes, cinder cone volcanoes, composite volcanoes, and lava plateaus.
· Shield volcanoes: lava that flows gradually build a wide, gently sloping mountain
o Shield volcanoes rising from a hot spot on the ocean floor created the Hawaiian Islands
· Cinder cone: these materials build up around the vent in a steep, cone-shaped hill or small mountain
o Example: Paricutin in Mexico erupted in 1943 in a farmer’s cornfield
· Composite volcanoes: are tall, cone shaped mountains in which layers of lava alternate with layers of ash
o Examples: Mount Fuji in Japan, and Mount St. Helens in Washington State
· Caldera: the huge hole left by the collapse of a volcanic mountain
· How does a caldera form: enormous eruptions may empty the main vent and the magma chamber beneath a volcano
· Features formed by magma include volcanic necks, dikes, and sills, as well as bathliths and dome
· Batholiths: is a mass of rock formed when a large body of magma cools inside the crust
· Dome mountain forms when uplift pushes a batholiths or smaller body of hardened magma toward the surface
· Geothermal activity: magma a few kilometers beneath Earth’s surface heats underground water
· Hot springs and geysers are types of geothermal activity that are often found in areas of present or past volcanic activity
· Hot springs forms when groundwater is hearted by a nearby body of magma or by hot rock deep underground.
· Geyser: is a fountain of water and steam that erupts from the ground