18.3: TYPES OF VOLCANOES

1. 18.3: TYPES OF VOLCANOES • SWBATS:

2. 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. 3 MAIN TYPES OF VOLCANOES • SHIELD VOLCANO • CINDER CONE VOLCANO • COMPOSITE

4. 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

5. 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

6. 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

7. SIZE AND SLOPE OF A VOLCANO Size and Slope • 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

8. VOLCANIC MATERIAL 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

9. 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)

10. 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

11. 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)

12. 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

13. 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)

14. 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

15. 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.

16. 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