18.3: TYPES OF VOLCANOES SWBATS:

ANATOMY OF A VOLCANO Below Earth’s lies the magma chamber. Hot, less dense magma rises to the surface at a volcanic vent. Vent: opening in Earth’s crust through which lava erupts and flows out onto the surface. Lava cools and solidifies around the vent and can eventually form a mountain. The crater is located on the top of volcano Crater: Bowl-shaped depression, usually less than 1 km in diameter, that forms around the central vent at the summit of a volcano. Caldera: large crater, up to 50km in diameter that can form when the summit or side of a volcano collapses into the magma chamber during or after an eruption.

3 MAIN TYPES OF VOLCANOES SHIELD VOLCANO CINDER CONE VOLCANO COMPOSITE

SHIELD VOLCANO DESCRIPTION/DEFINITION: Broad volcano with gently sloping sides built by nonexplosive eruptions of basaltic lava that accumulates in layers. TYPE OF MAGMA/LAVA: Basaltic magma TYPE OF ERUPTION: Quiet/Nonexplosive Low viscosity Small amounts of gases Low silica content EXAMPLE OF THIS VOLCANO: Mauna Kea in the Hawaiian Islands Iceland

CINDER-CONE VOLCANO DESCRIPTION/DEFINITION: Steep-sided, generally small volcano that is build by the accumulation of tephra around the vent. TYPE OF MAGMA/LAVA: Andesitic More water and silica than shield volcanoes Medium viscosity Lots of gases TYPE OF ERUPTION: More explosive than shield volcanoes EXAMPLE OF THIS VOLCANO: Izalco Volcano, El Salvador Paricutin Volcano in Mexico

COMPOSITE/STRATOVOLCANO DESCRIPTION/DEFINITION: Large, sloping volcano built by violent eruption of volcanic fragments and lava that accumulate in alternating layers. TYPE OF MAGMA/LAVA: Volcanic fragments (tephra) with alternating layers of lava Large amounts of silica, water and gases Rhyolitic/Granitic TYPE OF ERUPTION: Violent eruptions EXAMPLE OF THIS VOLCANO: Cascade Range – Mt. St. Helens and Mt. Rainier. Mt. Shasta in CA Mt. Fuji in Japan

SIZE AND SLOPE OF A VOLCANO Shield volcanoes – largest size, gentle slopes Cinder-cone volcanoes – smallest size, steep slopes Composite volcanoes – medium size, moderate slopes Size and slope are a result of: Materials that make up each volcano Vegetation that grows on the volcanoes slope Local climate Eruptive history of the volcano

VOLCANIC MATERIAL dust = less than 0.25 mm in diameter Tephra: rock fragments, classified by size, that are thrown in o the air during a volcanic eruption and fall to the ground. Types of Tephra – smallest to largest dust = less than 0.25 mm in diameter ash = larger than dust but less than 2 mm in diameter lapilli “little stones” = larger than 2mm but less than 64mm in diameter volcanic blocks = size of a car or small building volcanic bombs = large blobs of lava forcefully ejected from a volcano

PYROCLASTIC FLOW Pyroclastic flow: swift-moving, potentially deadly clouds of gas, ash, and other volcanic material produced by a violent eruption. Pyroclastic flows are so deadly because they: Can travel close to 200km/h May contain hot, poisonous gases. Temperatures can exceed 700 degrees C. Most deadly pyroclastic flow: 1902 – Mount Pelee – Island of Martinique in Caribbean. More than 29,000 people killed (suffocation or burned)

WHERE DO VOLCANOES OCCUR? CONVERGENT PLATE BOUNDARIES Plates moving together to create subduction zones Oceanic plate descends into the mantle – eventually melts Magma is then forced upward through the plate above – creating a volcano EX: Circum-Pacific Belt “Ring of Fire” Mediterranean Belt

WHERE DO VOLCANOES OCCUR? DIVERGENT PLATE BOUNDARIES Plates are moving apart. Magmas is forced upward to fill in the cracks as plates separate Areas of faults and fractures are called rift zones Usually occurs underwater – forming new seafloor EX: Mid-Atlantic Ridge Iceland (volcano at sea level)

WHERE DO VOLCANOES OCCUR? HOT SPOTS Volcanoes occur far from plate boundaries Regions of Earth’s mantle that contain high temperature plumes of magma Magma rises to the surface and forms volcanoes Plumes do not move laterally – only vertically Plate moves over the top of the plume Leave a trail of progressively older volcanoes EX: Hawaiian Islands

CIRCUM-PACIFIC AND MEDITERRANEAN BELTS Circum-Pacific Belt: Pacific Ring of Fire – along west coasts of North and South America, across the Aleutian Islands and down the eastern coast of Asia. Example of Volcanoes: Cascade Range – Mt. Shasta, Mt. Lassen, Mt. St. Helens), Mount Pinatubo in Philippines Mediterranean Belt: Smaller belt found in Europe Examples of volcanoes: Mount Etna and Mount Vesuvius (both composite volcanoes)

HOW THE HAWAIIAN ISLANDS WERE FORMED Pacific plate moved over a hot spot. Oldest island – Kauai – inactive now because it no lover over the hot spot. Youngest island – Hawaii – Most active volcano – Kilauea – currently over the hot spot. Loihi is another volcano forming currently on the seafloor east of the big island of Hawaii – May eventually forma a new island. Studying the volcanoes helps to study the rate at which the plate moves

OLDEST AND YOUNGES VOLCANOES IN THE HAWAIIAN-EMPEROR VOLCANIC CHAIN Part of the Hawaiian-Emperor volcanic chain (5800km long) Hawaiian Islands – youngest Oldest seamount - Meiji is about 75-80 million years old Daikakuji Seamount shows a bend in the chain = Change in direction of the Pacific Plate about 43 million years ago.

FLOOD BASALTS FLOOD BASALT: erupt from fissures rather than a centeral vent and can form flat plains of plateaus rather than volcanic mountains Ex: Colombia River Basalts in Northwestern US cover 170,000 km3 of basalt Decan Traps in India estimate to be 521,000km3