A volcano is a mountian that forms when molten rock (magma) is forced to the earth’s surface
What causes volcanoes? Tectonic Plate Boundaries!!! ~75% world’s active volcanoes in Ring of Fire
Where do volcanoes occur? • Divergent boundaries • Convergent boundaries • Hot spots
Volcanoes form around ventsthat release lava, which is the magma that reaches the earth’s surface.
Non-explosive Eruptions • Quiet eruption in which magma flows easily • Usually gas dissolved in magma escapes easily • Thin, runny lava oozes quietly from vent • Mafic lava has low silica (silicon dioxide) content, so it is not viscous • Example: Hawaiian Islands
Lava: Pohoehoe • Flows slowly, like wax dripping from a candle. • Forms glassy surface with rounded wrinkles ‘ropey’ appearance.
Lava: A’a • A’a is slightly stiffer. • Pours out quickly and forms a brittle crust which is torn into jagged pieces as molten lava underneath continues to move.
Lava: Block • Cooler and stiff • Oozes from volcano • Forms jumbled heaps of sharp-edged chunks. Mexico’s Colima Volcano
Lava: Pillow • Forms when lava erupts underwater at sea floor spreading zones. • Forms rounded lumps the size and shape of pillows.
Explosive Eruptions • Thick and sticky magma will not flow out of a volcano. Instead, it plugs the vent like a cork in wine bottle. • Felsic lava: high silica content results in high viscosity (resistance to flow) • Dissolved gasses and water cannot escape the thick magma, build up pressure that contributes to explosive eruptions. • Example: Mt. St. Helens
Explosive Eruptions • Clouds of hot debris and gasses shoot out from the volcano, often at supersonic speeds. • Molten rock is blown into millions of pieces that harden in the air. • Dust-sized particles can circle the globe for years in the upper atmosphere. • Larger pieces fall closer to the volcano. • Can blast millions of tons of solid rock and, in just a few minutes, can demolish rock formations that took hundred or thousands of years to accumulate. • Volcanoes can actually shrink due to repeated eruptions.
What Erupts from an explosive Volcano? • Pyroclastic material - rockfragments created by explosive eruptions • magma explodes from volcano and solidifies in the air • existing rock is shattered by powerful eruptions Volcanic ash Volcanic bombs Volcanic blocks Lapilli
Pyroclastic Material: Size • Volcanic blocks: solid rock blasted out of volcano • Volcanic bombs: large blobs of magma that harden in air • Lapilli: “little stones” (Italian) – pebble-like bits of magma that become solid before hitting the ground. • Ash: forms when gases in stiff magma expand rapidly & walls of gas bubbles explode into tiny glasslike slivers.
Types of Volcanoes Shield volcano Cinder cone volcano Composite volcano
Shield Volcanoes are produced by the accumulation of basalt lava flows. • Broad, dome shape • Frequent non-explosive eruptions • low silica = low viscosity
Composite Volcanoes • Switches between quiet eruptions of flowing lava and violent eruptions of thick, gas-rich lava (pyroclastic material). This type of volcano has the most powerful eruptions of all. • Composite volcanoes, also called stratovolcanoes, form tall conical mountains. • Example: Cascade Volcanoes.
Cinder Cone Volcanoes • Built up from ashes, cinders, and rocks that burst from Earth during a violent eruption. • The rocks fall back to Earth near the opening, to form a cone. Ash covers a larger surrounding area. • Erupt for a short period of time, so most are not taller than 300m (about 1000 ft) • Occur in clusters or on sides of shield and composite volcanoes. • Erode quickly due to pyroclastic material not being cemented together by lava. Mt. Etna, Italy
Craters, Calderas, and Lava Plateau • Crater • From explosions of material out of the vent and the collapse of material back into vent • Caldera • Much larger depression that forms when magma chamber empties and its roof collapses • Lava Plateau • Forms when lava erupts from long cracks, or fissures, and spreads out evenly (thousands of km)
Crater Lake is an example of a caldera, created when Mt. Mazama erupted an collapsed around 5600 BC