Volcanoes 11.1 What Causes Volcanoes?

Volcanoes and You Volcano – opening in Earth’s surface Often forms a mountain from layers of lava and volcanic ash Most are dormant, not currently active More than 600 currently active Mount St. Helens http://volcanoes.usgs.gov/volcanoes/st_helens/st_ helens_geo_hist_102.html http://www.history.com/videos/mount-st-helens- erupts http://volcanoes.usgs.gov/volcanoes/st_helens/st_ helens_geo_hist_100.html

Eruptions of the Century Active Volcanoes Kilaua volcano in Hawaii www.cbsnews.com/video/watch/?id=50133863n www.youtube.com/watch?v=GX0yPGCEtLk Iceland known for active volcanoes http://www.cbsnews.com/video/watch/?id=63390 52n Eruptions of the Century Mount Katmai, June 6, 1912 on the Alaska Peninsula 30 times greater volume than Mount St. Helens Mount Pinatubo, June 1991, Philippines Killed nearly 900 people Millions of tons of sulfur dioxide and ash into Earth’s upper atmosphere Mount Unzen, June 1991, Japan 44 people died

How do Volcanoes Form? Heat and pressure cause rock to melt and form magma. Magma Forced Upward Magma is less dense than rock, so it is pressed upward. Vent – opening through which magma flows out As lava flows out, it cools quickly and becomes sold, forming layers of igneous rock Crater – steep-walled depression around a volcano’s vent

Where do Volcanoes Occur? Volcanoes are directly related to Plate movement and hot spots. Plates Moving Apart Iceland has volcanic activity because it sits on top of the Mid-Atlantic Ridge. Mid-Atlantic Ridge – plates moving apart forming rifts Magma flows from rifts and is cooled by seawater As more lava flows, it builds up It can form islands such as Iceland

Plates Moving Together Soufriere Hills volcano on the island of Montserrat Formed because North American and South American Plates are being forced under the less dense Caribbean plate Magma forms because the forced under plates get hot enough to partially melt Magma is forced up forming the volcanoes of the Lesser Antilles Hot Spots Hawaii, not at a plate boundary Some areas of mantle are hotter than others Hypothesis is that hot spot magma forms at the boundary between the mantle and the outer core Magma rises carrying heat from Earth’s core

Hawaii formed when volcanoes rise above the water. Islands in a straight line because plate is moving over the stationary hot spot. Kauai is the oldest island. As plate move, Kauai moved away from hot spot and became dormant. Islands formed over about 5 million years.

Chapter 11 Types of Volcanoes 11.2

Styles of Eruption Trapped Gases Magma Composition Water vapor and carbon dioxide are trapped in magma by the pressure of surrounding magma and rock As magma nears surface, there is less pressure. Gas escapes easily during quiet eruptions Gas that builds up to high pressures eventually causes explosive eruptions Magma Composition Low in silica means fluid lava, and quiet eruptions Kilauea Lava pours from vents, down the sides of a volcano Trapped gasses can escape easily

Forms of Volcanoes Magma Water Content Silica-rich magma produces explosive eruptions Sometimes forms where plates are moving together Silica-rich magma is thick, and gases get trapped When an explosive eruption occurs, gases expand rapidly, often carrying pieces of lava in the explosion Magma Water Content High water content causes explosive eruptions Forms of Volcanoes Depends on type of explosion and type of lava

Shield Volcano Quiet eruptions Silica-poor lava Flat layers Shield Volcano – broad volcano with gently sloping sides Hawaiian Islands

Cinder Cone Volcano Explosive eruptions Tephra – rock or solidified lava dropped from the air Varies in size Cinder Cone Volcano – steep- sided volcano formed by loosely packed tephra

Composite Volcano Varies in explosions Composite Volcano (Stratovolcano)– volcano formed from cycle of lava and tephra Mostly found where plates come together Mount St. Helens

Volcanoes Igneous Rock Features 11.3

Intrusive Features Igneous rock – rock formed from fast cooling lava at or near Earth’s surface (extrusive igneous rock) and from slow-cooling magma deep inside Earth (intrusive igneous rock) Batholiths Batholiths – largest intrusive igneous rock bodies Hundreds of kilometers wide and long, and several kilometers thick Can be exposed by erosion Granite domes of Yosemite National Park are the remains of a huge batholith that stretches across much of the length of California

Other Features Dikes and Sills Magma can squeeze into cracks in a rock below the surface Dike – magma squeezed into a vertical crack Sill – magma squeezed into a horizontal crack Run a few meters to hundreds of meters long Other Features Volcanic Neck – solid igneous core left after cone erodes away Caldera – large opening made when the top of a volcano collapses after an eruption Igneous Features Exposed Process of erosion wear down rock at the surface exposing features like batholiths, dikes, and sills.