Earth’s Creators and Destroyers Volcanoes Earth’s Creators and Destroyers

Structure of the Earth The earth is composed of layers: Inner Core – solid Outer Core – liquid Mantle – solid Upper Mantle – plastic Crust – solid

Diagram of Earth’s Layers

Tectonic Plates The lithosphere is the Earth’s hard, outermost shell that is divided into a mosaic of 16 major slabs, or tectonic plates. These plates float on the upper mantle. As the plates move about they spread apart, collide, or slide past each other

Why Plates Move: Convection Cells

Earth’s Tectonic Plates

Where Volcanoes Occur Volcanoes 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. Over 80% of those on land occur at edges of continents, or subduction zones, where one plate dives, or subducts, under another plate.

Why Volcanoes Occur Temperatures 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 gasses to expand and propel the magma through openings in the Earth’s surface causing an eruption. Erupted magma is called lava.

How Volcanoes Erupt Eruptions 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. 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.

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. Pahoe hoe (pah hoy hoy) is flowing “liquid” with a ropy, billowy surface.

Lava – Pahoe hoe

Lava - Aa

Types of Lava Flows Lava 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)

Volcano 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 volcano A 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. Example: Crater Lake atop Mt. St. Helens.

Types of Volcanoes Repeated volcanic eruptions build volcanic mountains of three basic types, or shapes, depending on the composition of the materials deposited by the eruption. Shield volcanoes Stratovolcanoes Cinder cones

Shield Volcanoes Shield 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

Kilauea, HI

Kilauea

Mt. Etna, Italy

Mt. Etna

Mt. Etna

Stratovolcanoes (Composite) Stratovolcanoes are 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

Aconcagua, Andes

Aconcagua

Mt. St. Helens

Mt. St. Helens

Cinder Cone Volcanoes Cinder cones are the smallest volcanoes (< 500’), 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

Paricutin, Mexico

1950 1944 1966 1946 1948

Evolution of Paricutin

Monitoring and Predicting Eruptions Volcanic activity is monitored using several observations: Land deformation Ash clouds Tremors- measured by seismic data Volcanic Tremors (VT) Rockfall (RF)

Active,or Recently Active, Volcanoes Name Type Last Erupted Bezymianmy Stratovolcano 1993 Erebus 1980 Kilauea Shield 1995 La Palma 1954 Mt. Etna Mt. St. Helens 1986 Nevada del Ruiz 1991 Ol Doinyo Lengai Paricutin Cinder cone 1952 Pinatubo 1992