Chapter 6: Volcanoes

Introduction to Volcanoes Bill Nye Episode 74: Volcanoes 10 Most Active Volcanoes What is a Volcano?

Where Are Volcanoes Found on Earth’s Surface? Have you ever witnessed a volcanic eruption? Molten material rises high into the atmosphere. The ground is covered in volcanic ash. Magma is the molten mixture of rock-forming substances, water and gas from the mantle. Lava is magma that reaches the surface.

Volcanoes and Plate Boundaries Volcanoes form a regular pattern on Earth. Volcanic belts form along the boundaries of Earth’s plates. They occur where the Earth DIVERGES. They occur where the Earth CONVERGES The Ring of Fire is a major belt of volcanoes in the Pacific Ocean. North America South America Philippines Japan

Diverging Boundaries Volcanoes form along mid-ocean ridges. Underwater Lava pours out of the rift valley Builds new mountains Can be found on land. Great Rift Valley

Converging Boundaries One plate subducts under the other The more dense plate sinks into the mantle and creates a deep-ocean trench Water in the sinking plate leaves the crust, causing the rock to melt Can form chains of islands called island arcs Japan Caribbean Aleutians New Zeland Also occur where ocean plate is below a continental plate Andes Mountains Plate collisions produce volcanoes Mount St. Helens Mount Rainier

Hot Spots Some volcanoes are results of “hot spots” in the mantle. This is a place deep within the Earth where the mantle rises through the crust and melts to form magma. A volcano is formed from this when the magma erupts through the crust and reaches the surface. Hot spots stay in the same place for millions of years while plates move over them.

6.2 Volcanic Eruptions What are pipes and vents used for in your home, school and other buildings?

What Happens When a Volcano Erupts? Think about magma. Magma is less dense than the material around it, so when it reaches the surface a volcano can form.

Inside a volcano Magma Chamber: a pocket where magma collects beneath the surface Pipe: tube that extends from Earth’s crust up through the top of a volcano Vent: an opening where rock and gas leave a volcano Central and side Lava Flow: the spread of lava as it pours out a vent Crater: a bowl-shaped area that may form at the top of a volcano

A Volcanic Eruption During an eruption magma rises to the surface, pressure on mamga decreases. As pressure falls the gas bubbles are released from the magma When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent. The type of explosion depends on the silica content and whether the magma is thin and runny or thick and sticky.

Magma is hot and low in silica. Gases in magma bubble out gently Quiet Magma is hot and low in silica. Gases in magma bubble out gently Lava oozes quietly from the vent and can flow for many kilometers. Forms rock: Pahoehoe Fast moving and thinner Solid mass of rope-like coils Aa Cooler and thicker Rough surface with jagged chunks Formed Hawaiian Islands 750-1150 degrees Celsius Explosive Magma is high in silica. Trapped gases build up pressure until they explode. Erupting gases and steam push the magma out with incredible force. Like plugging the cork on a bottle Forms volcanic ash, cinders and bombs

Volcano Hazards Both types of eruptions can cause damage far from a crater’s rim. During an explosive eruption a volcano can put out gases, hot rock and ash that flow down the side of a volcano. This is called a PYROCLASTIC FLOW. Landslides can also form. Tsunamis are also a hazard that can be caused if a volcano that is an island arc collapses.

Stages of Volcanic Activity Activity may last 10 to 10 million years! Geologists often use the terms active, dormant, or extinct to describe a volcano’s stage of activity. Dormant: sleeping, may wake in the future to become active Extinct: dead, unlikely to erupt ever again Hot Spot Volcanoes may become extinct after they drift away from the hot spot! We are sometimes able to predict when an explosion will occur Changes in land around a volcano gives us these clues. Tiltmeters detect changes in surface elevation Small earthquakes can be monitored Gases escaping from a volcano can be monitored Rising temperatures in underground water may signal magma is near the surface

6.3 Volcanic Landforms Do all volcanoes look the same?

What Landforms Do Lava and Ash Create? Volcanic eruptions create landforms made of lava, ash and other materials. Shield volcanoes Cinder cone volcanoes Composite volcanoes Lava plateaus Caldera Huge holes left by the collapse of a volcano

Cinder Cone Volcanoes Forms from high silica volcanoes Ash cinders and bombs build up around the vent in a steep, cone-shaped hill or small mountain Paricutin in Mexico

Composite Volcanoes Varying amounts of silica content in eruptions Tall cone-shaped mountain where layers of lava alternate with layers of ash Mount Fuji in Japan Mount St. Helens in Washington More than 4800 meters high!

Shield Volcanoes Wide and gently sloping Formed from thin layers of lava hardening on previous layers Typically formed from hot spot volcanoes on the ocean floor. Hawaii Mauna Loa is over 9000 meters above the ocean floor!

Lava Plateaus Formed when lava flows along cracks in an area. Moves a larger area and then cools and hardens Repeated floods of lava over millions of years The Columbia Plateau in Washington State, Oregon and Idaho

What Landform does Magma Create? Volcanic Necks Forms when magma hardens in an old volcano’s pipe and the rock around the pipe wore away Dikes and Sills Dikes form when magma forces itself across rock layers Sills form when magma squeezes between horizontal rock Dome Mountains Formed when uplift pushes layers of rock to move upward into a dome Black Hills Mountains in South Dakota Batholiths A mass of rock created when magma cools inside the crust Form the core of many mountain ranges Water and ice allow the batholith to look like a mountain