Volcanoes & Plate Tectonics

What is a Volcano? A volcano is a landform that is created by lava and/or pyroclastics Pyroclastics are the solid materials that are ejected from a volcano Magma is molten rock that is found BENEATH the earth’s surface When magma reaches the surface it is called lava

When lava cools & hardens it forms solid rock called an igneous rock Lava that is released during a volcanic eruption adds new rock to existing land and forms new islands – constructive force

Location of Volcanoes 600 active volcanoes on land, more underwater Volcanoes occur in belts Major volcanic belt in the Ring of Fire which rims the Pacific Ocean Volcanic belts form along plate boundaries

Volcanoes at Divergent Boundaries Divergent boundaries are also known as spreading centers Lithospheric plates along the ocean floor spread apart and allow lava to move upward This creates new oceanic crust Some of the lava creates a volcanic cone that may rise above the water Islands off the coast of Iceland

Volcanoes located within oceans extrude lava that is mafic Mafic lava is thin and runny and is dark in color Main rock that is created from mafic lava is basalt

Volcanoes at Convergent Boundaries Volcanoes at convergent boundaries occur at subduction zones 2 types of subduction zones Oceanic-continental convergence Oceanic crust subducts beneath continental crust and begins to melt – creating magma Magma slowly moves upward because it is less dense than the surrounding rock When magma reaches the surface, a volcano is formed

Oceanic-oceanic convergence Denser oceanic plate will be subducted creating a deep-ocean trench Subducting plate will begin to melt creating magma Magma moves upward and creates a chain of volcanic islands This is known as a volcanic island arc These occur 200-300 km from the deep-ocean trench Example: Aleutian Islands

Hot Spot Volcanoes A hot spot is an area in the middle of a continental or oceanic plate where volcanism occurs Not near a plate boundary Created by mantle plumes Areas of magma originating in the mantle Magma moves its way up to the crust and reaches the surface through cracks Hot spots do not move, plates move over them Examples: Hawaii and Yellowstone

http://www.nps.gov/yell/naturescience/tracking_hotspot.htm

Volcanic Activity How Magma Reaches Earth’s Surface Volcanic Eruption Magma rises because it is less dense than the surrounding rock It flows upward through any crack in the rock Volcanic Eruption Gas is dissolved in the magma Gas is under tremendous pressure As magma rises to the surface, pressure decreases The dissolved gases begin to separate and form bubbles

An eruption occurs when an opening develops in the crust Gases rush out and carry magma with them

Inside a Volcano Pocket beneath the volcano that holds the magma is the magma chamber Tube that connects the magma chamber to the Earth’s surface is called the pipe Magma and pyroclastics leave the volcano through an opening called the vent The bowl-shaped area at the top of the volcano is the crater

Viscosity Viscosity is a fluid’s resistance to flow Similar to thickness Low viscosity fluids flow more easily than high viscosity fluids The hotter a liquid, the less viscous it becomes http://www.seed.slb.com/en/scictr/lab/visco_exp/viscosity.htm

Type of Liquid Viscosity Water Syrup Shampoo

Characteristics of Magma Magmas differ in viscosity The hotter the magma, the more fluid the magma is Silica Content Material formed from oxygen and silicon The more silica there is in magma, the thicker the magma will be

Types of Magma Type of Magma Silica Content Viscosity Basaltic 50% Least Andesitic 60% Intermediate Granitic 70% Greatest

Types of Volcanic Eruptions Silica content of magma helps determine whether the eruption is quiet or explosive

Quiet Eruptions Basaltic lava – thin and runny 2 types of lava in quiet eruptions Pahoehoe: fast-moving, hot lava which resembles a braid or rope Aa: slow-moving, cooler lava that forms jagged chunks

Explosive Eruptions If magma is thick and sticky, a volcano erupts explosively Lava gets stuck in the volcano causing pressure to build up Explosion breaks lava into pieces – pyroclastics Ash: smallest pieces Cinders: pebble-sized particles Bombs: range in size from a baseball to a car Pyroclastic flow: highly heated mixture of pyroclastics moving down a volcano

Stages of a Volcano Active – “live” volcano that is erupting or has shown signs that it will in the near future Dormant – “sleeping” volcano that scientists expect to be active in the future Extinct – “dead” volcano, unlikely to erupt again

Other Types of Volcanic Activity Hot spring – superheated pool of water heated by a nearby body of magma

Geyser – fountain of water and steam that erupts from the ground

Geothermal energy – energy created by water heated by magma Used in Iceland

Monitoring Volcanoes Geologists can predict volcanic eruptions more successfully than earthquakes Geologists monitor changes in elevation, gases from the volcano, and earthquakes

Volcano Hazards Quiet Eruptions – lava pours out of the volcano setting everything in its path on fire Explosive Eruptions – emits hot, burning clouds of ash and cinders/bombs

Volcanic Landforms 3 types of volcanoes Shield volcanoes Wide, gently sloping mountains Formed by thin, runny basaltic magma with a low viscosity Located at divergent boundaries and hot spots Example: Mauna Loa

Cinder cone volcano Steep, cone-shaped hill or mountain Built from ejected lava fragments Fragments are ejected and begin piling up, creating the cone shape Example: Paricutin

Composite Volcano Tall, steep, symmetrical structure Composed of alternating layers of lava flows and pyroclastic deposits Example: Mt. St. Helen’s

Lava Plateau Flat, level areas where lava flows have traveled far and cooled Cooled lava flows pile up on one another

Caldera Roughly circular depression created by the collapse of the empty vent and magma chamber Greater than 1 km in diameter

Landforms from Magma Volcanic neck Forms when magma hardens in the volcano’s pipe Softer rock wears away around the pipe leaving the neck behind

Dike When magma forces its way across rock layers and hardens

Sill When magma squeezes its way between parallel rock layers

Batholith When a large body of magma cools and hardens beneath the surface

Dome Mountain When small bodies of rising magma are blocked by horizontal layers of rock Magma forces its way up and bends the rock layers into a dome shape

Lahar Volcanic mudflow generated during a volcanic eruption

Volcanoes in the Solar System Volcanoes on Venus Thousands of volcanoes Largest volcano is Theia Mons Mostly shield volcanoes

Volcanoes on Mars Shield volcanoes Cone-shaped volcanoes Lava flows Olympus Mons Largest mountain in the solar system Shield volcano Covers an area as large as Ohio

Volcanoes on Distant Moons Io – moon of the planet Jupiter Triton – moon of the planet Neptune