Chapter 10 Volcanoes and other Igneous Activity

Section 10.1 Volcanoes and Plate Tectonics

Origin of Magma Magma forms in the crust and upper mantle when solid rock partially melts. The formation of magma depends on several factors, including heat, pressure, and water content. Decompression melting – occurs when rock rises and melts due to reduced pressure. Water content – “wet” rock melts faster than “dry” rock

Volcanoes and Plate Boundaries Most volcanoes form along divergent and convergent plate boundaries. Some volcanoes form far from plate boundaries above “hot spots” in the crust. Divergent Boundary Volcanism – as plates pull apart magma rises to fill the gap in rift valleys. Most are found at mid-ocean ridges, but some are found on the continents. The Great Rift Valley in Africa is an example

Divergent Boundary Volcanism: Africa’s Great Rift Valley and Mt Divergent Boundary Volcanism: Africa’s Great Rift Valley and Mt. Kilimanjaro in Tanzania

Divergent Boundary Volcanism Mid-Atlantic Ridge

Volcanoes and Plate Boundaries - Convergent Convergent Boundary Volcanism – occurs where plates collide and one plate subducts (goes under) another plate. There are 2 types. 1. Continental crust collides with oceanic crust – the oceanic crust is more dense and subducts underneath the continent. This creates an ocean trench and a subduction zone. As the oceanic crust melts back into the mantle, magma rises creating inland volcanoes on the continent. The Cascades of California, Oregon, and Washington are and example.

Convergent Plate Volcanism – Subduction Zones

Convergent Boundary Volcanism Mt. St. Helens of the Cascades

Convergent Plate Volcanism: Pacific Ring of Fire Pacific Ring of Fire -the area on Earth that has the most earthquakes and volcanoes. It is an area of subducting plate boundaries, where the Pacific Plate is moving under the neighboring continental and oceanic plates. The Cascade Mountain Range of the Western United States is a part of the Ring of Fire.

Convergent Boundary Volcanism 2. Oceanic crust collides with another oceanic crust. One plate will be denser than the other and subduct beneath. This creates a very deep ocean trench, and when the plate melts back into the mantle magma raises upward and creates an island arc of volcanoes. The Aleutian Islands of Alaska are an example.

Convergent Boundary Volcanism: Island Arcs – Aleutian Islands

The Deepest Place in Earth’s Crust Convergent Boundary Volcanism: Island Arcs- Marianas Islands and the Marianas Trench The Deepest Place in Earth’s Crust

Intraplate Volcanism: Hot Spots Hot Spot – weak area of the Earth’s crust within a plate that allows magma to come to the surface in the form of a mantle plume. Examples in the United States include the Hawaiian Island chain and Yellowstone National Park.

Hot Spots: Hawaiian Islands

Hot Spots: Yellowstone

Section 10.2 The Nature of Volcanic Eruptions The primary factors that determine whether a volcano erupts explosively or quietly include characteristics of the magma and the amount of dissolved gases in the magma. Viscosity – the substance’s resistance to flow; whether the magma is thick and sticky or thin and runny. The more silica in magma the greater the viscosity – thicker.

Factors Affecting Eruptions: Viscosity Granitic Lavas are very viscous (thick) because of their high silica content. They trap gases inside. This makes them erupt violently. Mostly found on the continents. Basaltic Lava are less viscous (thin) because of low silica content. They allow gas to be released easily. This allows them to erupt quietly. Mostly found in the oceans.

Volcanic Material Depending on the type of eruption, volcanoes may produce lava flows or eject pyroclastic materials, or both. All volcanic eruptions also emit large amounts of gases.

Volcanic Materials: Lava Flows Lava Flows – Basaltic lavas flow faster than Granitic lavas Paehoehoe – faster moving basaltic lava that wrinkles and looks like twisted ropes Aa – Slower moving basaltic lava that forms a surface of rough, jagged blocks.

Volcanic Materials: Gases Volcanoes release tremendous amounts of gases into the atmosphere. These gases include water vapor, carbon dioxide, nitrogen, sulfur, chlorine, hydrogen, and argon.

Volcanic Materials: Pyroclastic Materials Particles produced in volcanic eruptions are called pyroclastic materials. Ash – fine dust and ash carried by the wind (less than 2mm) Lapilli – cinders; small particles that range in size from small beads to walnuts. (2-64mm) Blocks – Larger than 64mm and made of hardened lava. They can be as large as houses. Bombs – Glowing lava that is ejected during the eruption. The outside crusts over in the air and they splatter when they make impact.

Pyroclastic Materials Volcanic Ash Volcanic Bombs Volcanic Blocks

Volcanoes Vent – opening through which lava flows out. Volcano – repeated eruptions eventually build up a mountain. Crater – steep-walled depression at the top of the volcano, which contains the vent.

Types of Volcanoes Shield Volcano – made of alternating basaltic lava flows. They are broad, gently sloping, tall, slightly domed structures. Ex. Hawaiian Islands

Types of Volcanoes: Cinder Cinder Cones – made of loose pyroclastic material, such as ash and lapilli. They are relatively small compared to other volcanoes.

Types of Volcanoes: Composite Composite Cones – also called stratovolcanoes. Most dangerous volcanoes on Earth. Made of both layers of lava and pyroclastic material. Most are found in the Pacific Ring of Fire. Generate explosive eruptions due to trapped gases. Mt. Fuji in Japan >>>>>>>

Composite Volcanoes: Mt. Vesuvius and Pompeii

Composite Volcanoes: Cascade Range on the West Coast of the United States

Volcanic Landforms: Calderas Calderas – depression in a volcanic mountain caused by a collapse of the crater. Crater Lake in Oregon is a caldera caused by the eruption of Mt. Mazama. Wizard Island is the cone forming in the lake.

Volcanic Landforms: Volcanic Necks Volcanic Necks – a landform made of magma that hardened inside a volcano and was later exposed by weathering and erosion. Ship Rock, New Mexico and Devils Tower, Wyoming are examples

Volcanic Hazards Lava Flows Pyroclastic Flows – superheated volcanic particles and gases moving hundreds of miles per hour down the slopes of a volcano. Lahars – mudflows that happen when water- soaked volcanic ash and rock slide rapidly downhill. Ice and snow melted by an eruption can trigger a lahar.

Section 10.3 Intrusive Igneous Activity Plutons – the structures that result from the cooling and hardening of magma beneath Earth’s surface. Types of Plutons: Sills Laccoliths Dikes

Plutons and Batholiths Sill – a pluton that forms when magma flows between parallel layers of sedimentary rock. Laccolith – also forms between parallel layers of sedimentary rock. Laccoliths are lens-shaped which pushes up the overlying rocks. They can make dome mountains. Dikes – a pluton that forms when magma moves into fractures that cut across rock layers. Batholiths – larger than a pluton, a large body of intrusive igneous rock that has a surface exposure of more than 100 square kilometers.

Intrusive Igneous Rock Features

Intrusive Igneous Rock Features Sill Dike

Intrusive Igneous Rock Features Laccoliths Batholith

Live Volcano Cams Mt. St. Helens Mt. Rainier Mt. Redoubt Hawaiian Volcano Iceland Volcano