3Where Volcanoes OccurVolcanoes occur most frequently at plate boundaries.Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called hot spots.The greatest number of volcanoes occur on the ocean floor along spreading ridges spreading ridges.Over 80% of those on land occur at edges of continents, or subduction zones, where one plate dives, or subducts, under another plate.
6Why Volcanoes OccurTemperatures in the mantle are hot enough to melt rock into magma.Less dense than the solid rock around it, magma rises and some of it collects in magma chambers.As the magma rises, pressure decreases allowing trapped gases to expand and propel the magma through openings in the Earth’s surface causing an eruption.•Erupted magma is called lava.
7How Volcanoes EruptEruptions are described as explosive or effusive (loosely flowing).How explosive an eruption is depends on the magma’s chemical composition and gas content, which affect the magma’s stickiness, or viscosity.Magma’s low in silica are low in viscosity (mafic)Magma’s high in silica are high in viscosity (felsic)•If magma is fluid, gases can escape rapidly and lava flows; if magma is viscous the gases can not escape and pressure builds inside the magma until the gases escape, sometimes violently.
8Magma vs. Lava Magma vs. Lava Magma is molten rock beneath the surface.Lava is erupted magma. There are 2 types of lava:A a (ah ah) is largely solidified rock that gets pushed forward.– Pahoehoe (pahhoyhoy) is flowing “liquid” with a ropy, billowy surface.
11Types of Lava FlowsLava flows are superheated streams of molten rock that flow at 1 –50 mph.Pyroclastic flows are avalanches of hot ash, rock fragments, and gases that flow at speeds greater than 100 mph.Landslides are avalanches of rock, snow and ice on slopes of volcanoes (loosened and tumbling due to seismic activity).Lahars(mud flows) are a mixture of volcanic ash and water (like wet concrete)
12Volcano Terms A vent is an opening through which eruptions take place. A crater is a basin like depression over a vent, at the summit of a volcanoA caldera is a depression larger than the original crater (>1km. Diameter) that forms when the summit is blown off, or when the volcano collapses into the empty magma chamber.
14Types of VolcanoesRepeated volcanic eruptions build volcanic mountains of three basic types three basic types, or shapes, depending on the composition of the materials deposited by the eruption.Shield volcanoesStratovolcanoesCinder cones
15Shield VolcanoesShield volcanoes are broad gently sloping volcanic mountains slowly formed by layer over layer of solidified lava.Shield volcanoes are formed by effusive eruptions of fluid lava.These can become very large as the low viscosity lava spreads widely and thickly.Examples:Kilauea, Hawaii and Mt. Etna, Italy
17Stratovolcanoes (Composite) • Stratovolcanoes formed from both explosive and effusive eruptions.•Layers of tephra alternating with layers of viscous lava flows create steep sided,often symetrical cones that can be very large.•Formed over long spans of time as periods of 100,000+ yrs. separate periods of a few years of intense activity.Examples: Aconcagua, Andes (22,825’) and Mt. St. Helens
19Cinder Cone Volcanoes• Cinder cones are the smallest volcanoes (< 500’), formed by explosive eruptions of formed by explosive eruptions of lava.Blown violently into the air, the erupting lava breaks apart into fragments called cinders that fall and accumulate around the vent.• Cinder cones are temporary geologic features as they are easily eroded. They have short life spans as gas causing violent eruptions is quickly depleted.Example: Paricutin, Mexico
21Monitoring and Predicting Eruptions Volcanic activity is monitored using several observations:Land deformation: Rock structures bend outward as gas build up.Gas Emissions: Higher release of Carbon Dioxide and Sulfur Dioxide gases.Tremors- As magma moves under the volcano and gas pressure builds, mild earthquakes will occur in the area.– Rock Fall: Due to movement of land, loose rocks will begin to slide.