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Volcanoes. A volcano is an opening in the Earth that erupts gases, ash and lava. Kilauea is the world’s most active volcano. Lava.
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Volcanoes • A volcano is an opening in the Earth that erupts gases, ash and lava. • Kilauea is the world’s most active volcano.
Lava • When the magma reaches the surface, it is called lava. The place in the Earth’s surface through which magma and other materials reach the surface is called a volcano. In some places, lava can build up to form a cone-shaped mountain. • The opening from which lava erupts is the vent. Volcanoes often have more than one vent.
Formation of a Volcano • The opening where lava flows out is a vent. • The steep walled depression around the vent is a crater.
Volcanoes • Volcanoes can be both destructive and constructive forces.
Destructive Effects of Eruptions • Lava destroys everything in its path. Ash can cause lung damage. • If ash and debris run down the side of the mountain, this is called a pyroclastic flow. Temperatures in this flow can be hot enough to ignite wood. • Sulfurous gases can mix with water vapor and form acid rain that kills plants and animals.
Constructive Effects of Eruptions • The soil formed from volcanoes is very fertile and will eventually lead to abundant plant growth. • The sulfur in volcanic vents along the ocean floor allow life to exist without sunlight • Volcanoes recycle earth’s minerals
Where do Volcanoes occur? • Volcanoes form along divergent plate boundaries, convergent boundaries and hot spots.
Divergent Plates • Volcanoes often form where plates are moving apart, such as the Surtsey in Iceland. • Iceland is located on the Mid Atlantic Ridge.
Convergent Plate Boundaries • Soufriere Hills On the Island of Monserrat is an example of a volcano that formed when one plate slide under another and melted. • The magma was forced up and formed a volcano.
Hot Spots • Some volcanoes form over hot spots in the mantle. • These are not on a plate boundary but in the center of a plate. • The Hawaiian Islands formed over a hot spot in the Pacific Plate.
Magma Deep within the Earth, under tremendous pressure and at great temperatures, rock exists as a hot liquid called magma. This molten rock is found in pockets called magma chambers.
What Controls Eruptions? • Eruptions are controlled by • 1) the amount of trapped gas and the water vapor • 2) how much silica is present in the magma
Trapped Gas As magma reaches the surface, it is under less pressure. Gas can escape. Gas escapes easily during quiet eruptions. • If gas build up to high pressure this will cause an explosive eruption.
Amount of Water Vapor • The magma at some convergent boundaries contains a lot of water vapor. • This is because oceanic plate material and some of its water slide under other plate material at some convergent plate boundaries. • The trapped water vapor in the magma can cause explosive eruptions.
Viscosity of Magma • Magma is divided into two major types: silica rich and silica poor • The greater the silica content, the more viscous or thick the magma.
Basaltic Magma • Magma that is low in silica is called basaltic magma. It is fluid and produces quiet flows such as those at Kiluea. • Basaltic lava flows under water produce pillow lava formations.
Basaltic Lava • Pahoehoe forms a ropelike structure. • If it flows at a lower temperature it is stiff and slow moving and called aa.
Granitic Magma • The second type of lava is light in color. This lava, has high water content and high silica content. Light-colored lava causes explosive eruptions. Silica hardens in the vents and form rocks. Steam and new lava build up under the rocks. When the pressure becomes great, a violent explosion occurs. When this type of lava cools on the surface it forms the igneous rock, rhyolite, which resembles granite.
Andesitic Magma • Andesitic magma is more rich in silica than basaltic magma but it has less silica than granitic magma. • It forms at convergent boundaries where one plate slides under the other. • It is more explosive than basaltic magma. • Krakatau was mainly andesitic in composition. The word andesitic comes from the Andes Mountains.
Gaseous Lava • When lava that contains large amounts of gases such as steam and carbon dioxide hardens, it forms rocks with many holes in them, due to the gas bubbles. • Pumice and scoria are igneous rocks formed from this type of lava.
Types of Volcanoes • Different types of volcanic eruptions form different types of volcanoes. • Cinder cones • Shield volcanoes • Composite volcanoes
Cinder Cones • Volcanoes made mostly of cinders and other rock particles that have been blown into the air are called cinder cones. • Cinder cones form from explosive eruptions. Because the material is loosely arranged, the cones are not high. The have a narrow base and steep sides such as Paricutin in Mexico.
Tephra • Explosive eruptions throw lava and rock high into the air. Bits of rock or solidified lava dropped from the air are called tephra. • The smallest particles are called volcanic dust. (less than 0.25 mm) • Volcanic Ash ( 0.25 -5mm) falls to the Earth and forms small rocks. • Volcanic bombs (a few cm to several meters) are molten and harden as they travel through the air.
Shield Volcanoes • Volcanoes composed of quiet flows are called shield volcanoes. Because it is rummy, the lava flows over a large area. After several eruptions, a dome-shaped mountain is formed such as Mauna Loa (4km over sea level) in the Hawaiian Islands.
Flood Basalts • Basaltic lava can also flow onto Earth’s surface through large cracks called fissures. • This type of eruption forms flood basalts • Columbia Plateau was formed as the lava spread out over the ground.
Composite Volcanoes • Volcanoes built up of alternating layers of rock particles and lava are called composite volcanoes. During the formation of a composite volcano, a violent eruption first occurs, hurling volcanic bombs, cinder and ash out of the vent. Then a quiet eruption, produces lava flow that covers the rock particles. After alternating eruptions, a cone-shaped mountain forms such as Mount Vesuvius, Mount Rainier or Sofriere HIlls.
Mount Vesuvius • Images of victims in eruption of Vesuvius in 79 AD. Most died as a result of suffocation.
Krakatau • One of the most violent eruptions in recent times occurred on an island in the Sunda Straits neat Indonesia in 1883. • 36,000 people were killed. • The island fell into the magma chamber.
Crater • There is often a funnel-shaped pit or depression at the top of a volcanic cone. This pit is called a crater. If the crater becomes very large as a result of the collapse of its walls, it is called a caldera. A caldera may also form when the top of a volcano explodes or collapses.
Intrusive Igneous Rock Features • Most magma never reaches the surface. • Much of it hardens in the earth. • Sometimes the hardened magma becomes exposed at the surface. • These rock bodies are called intrusive igneous rock features. • Examples are batholiths, sill, dikes and volcanic necks.
Batholiths • The largest intrusive igneous rock bodies are called batholiths. ‘They form when magma bodies that are being forced upward cool and solidify before reaching the surface.
Sills and Dikes • Magma sometimes squeezes into cracks below the surface. • Magma that cuts across rock layers and hardens is called a dike. • Magma that is forced into a crack parallel to the rock layers and hardens is a sill.
Volcanic Neck • When a volcano stops erupting magma will harden inside the vent. Erosion wears away the outside of the volcano and the solid igneous rock in the vent remains. • Ship Rock in New Mexico was formed this way.
Calderas • Sometimes after an eruption, the top of a volcano can collapse. This produces a large depression called a caldera. • Crater Lake in Oregon is a caldera that filled with water and is now a lake.
Volcanic Activity • Volcanoes are rather unpredictable . Some erupt regularly, others have not erupted in modern history. Scientists classify them as active, dormant or extinct.
Active Volcanoes • An active volcano is one that erupts wither continually or periodically such as Mount Katmai in Alaska and Mount St. Helens in the Cascade Range.
Dormant Volcano • A volcano that has been known to erupt within modern times but is now inactive is classified as a dormant volcano. Mount Rainier in Washington state are example of dormant volcanoes in the United States.
Extinct Volcano • A volcano not known to have erupted within modern history is classified as an extinct volcano. They have been worn away almost to the level of their magma chamber. Scientists can be wrong. Mount St. Helens was considered to be dormant but erupted after long periods of inactivity.
Volcano and Earthquake Zones • Most major earthquakes and volcanic eruptions occur in three zones of the world. Scientists believe that there is a great deal of movement and activity in the Earth’s crust in these three zones.
Ring of Fire • One major earthquake and volcano zone extends nearly all the way around the edge of the Pacific Ocean. This zone goes through New Zealand, the Philippines, Japan, Alaska and along the western coasts of North and South America. The San Andreas fault is part of this zone.
Mediterranean Zone • A second Major earthquake and volcano zone is located near the Mediterranean Zone and extends across Asia into India. Many countries in the zone, including Italy, Greece and Turkey, have violent earthquakes. Many volcanic eruptions also occur in this zone.
Mid-Atlantic Ridge Zone • The third major earthquake and volcano zone extends through Iceland and to the middle of the Atlantic Ocean. There is under the ocean a long range of volcanic mountains called the Mid-Atlantic Ocean Range. Scientists believe that the volcano and earthquake activity are due to the formation of new parts of the Earth’s crust along the ridge. The volcanic island of Iceland is part of this zone.
