Chapter 6 Volcanoes Introduction to Volcanoes Bill Nye

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.

• 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