Volcanoes

1. Volcanoes Chapter 7

2. Volcanoes • A volcano is a weak spot in the crust where molten material, or magma, comes to the surface • Magma is the molten mixture of rock-forming substances, gases, and water from the mantle • Once it has reached the surface= lava • Solid rock forms once lava cools

3. Volcanoes and Plate Boundaries • Volcanic belts form along the boundaries or Earth’s plates • Ring of Fire- formed by many volcanoes that rim the Pacific Ocean • Diverging boundaries: • Volcanoes form along mid-ocean ridges, rift valley’s, Great Rift Valley in East Africa • Converging boundaries: • Volcanoes often form where two oceanic plates collide or where an oceanic plate collides with a continental plate. In both situations, an oceanic plate sinks through a trench. Rock above the plate melts to form magma, which then erupts to the surface as lava. • Island arc- echoes curve of deep-ocean trench (Japan, New Zealand, Indonesia, etc.)

4. Volcanoes at Converging Boundaries

5. Hot Spot Volcanoes • Hot Spot is an area where material from deep within the mantle rises and then melts, forming magma • A volcano forms above a hot spot when magma erupts through the crust and reaches the surface.

6. Properties of Magma- Physical and Chemical • Element- a substance that cannot be broken down into other substances • Compound- a substance made of 2 or more elements that have been chemically combined • Physical property- any characteristic of a substance that can be observed or measured without changing the composition of the substance • Chemical property- any property that produces a change in the composition of matter • Each substance has a particular set of physical and chemical properties • These properties can be used to identify a substance or to predict how it will behave

7. Viscosity • Viscosity- the resistance of a liquid to flowing • Some liquids flow more easily than others • Greater the viscosity- the slower it flows • (ex. Honey) • Why? • The movement of the particles that make up each type of liquid affects viscosity. The greater friction among the liquid’s particles, the higher the viscosity.

8. Viscosity of Magma • The viscosity of magma depends on silica content and temperature • The compound silica is made up of oxygen and silicon; it is the most abundant material in Earth’s crust • More silica- higher viscosity, light-colored and does not flow very far • Cools to form Rhyolite rock • Less silica- lower viscosity, dark-colored and flows readily • Cools to form Basalt rock

9. Viscosity of Magma • Viscosity increases as temperature decreases • Temperature differences produce two different types of lava: • Pahoehoe- fast-moving, hot lava with low viscosity; solid mass of wrinkles, billows, etc. • Aa- cooler and slower-moving; has a higher viscosity; forms a rough surface of jagged lava chunks

10. Volcanic Eruptions • Lava begins as magma, which usually forms in the asthenosphere • Liquid magma is less dense than the solid material around it therefore magma flows upward into any cracks in the rock above • Inside a volcano: • Beneath a volcano, magma collects in a packet called the magma chamber • Magma moves upward through a pipe, a long tube in the ground that connects that magma chamber to Earth’s surface • Vent- an opening where molten rock and gas leave the volcano • Lava flow- the area covered by lava as it pours out of a vent • Crater- bowl-shaped area that may form at the top around the central vent

11. A Volcanic Eruption • Dissolved gases are trapped in magma and are under tremendous pressure • As magma rises toward the surface, the pressure of the surrounding rock on the magma decreases; the dissolved gases expand and form bubbles • The size of the bubbles greatly increases and exert an enormous force • 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

12. Volcanic Eruption

13. Kinds of Eruptions • Geologists classify volcanic eruptions as quiet or explosive • Quiet eruptions: • If magma is low in silica. Has low viscosity and flows easily. Gas bubble out quietly. Can produce both pahoehoe and aa

14. Kinds of Eruptions • Explosive Eruptions: • If magma is high in silica. Has high viscosity, and is thick and sticky. Dissolved gases cannot escape from the thick magma and build up pressure until they explode. • Breaks lava into fragments that quickly cool and harden into pieces of different sizes. Smallest= volcanic ash. Pebble-sized= cinders. Larger pieces= bombs (size of baseball to a car) • Pyroclastic flow- occurs when an explosive eruption hurls out a mixture of hot gases, ash, cinders, and bombs

15. Volcanic Eruptions • Within the last 150 years, major volcanic eruptions have greatly affected the land and people around them.

16. Stages of Volcanic Activity • Geologists often use the terms active, dormant, or extinct to describe a volcano’s stage of activity • Active or live- one that is erupting or has shown signs that it may erupt in the near future • Dormant or sleeping- one that is expected to awaken in the future and become active • Extinct or dead- unlikely to erupt again • Monitoring volcanoes: • Geologists use instruments to detect changes in and around a volcano • Monitor: Gas exchange, tilting, temperature, and small earthquakes

17. Volcanic Landforms from Lava and Ash • Volcanic eruptions create landforms made of lava, ash, and other materials. These landforms include shield volcanoes, cinder cone volcanoes, composite volcanoes, and lava plateaus. • Shield volcanoes- thin layers of lava pour out of a vent and harden on top of previous layers; forms a wide, gently sloping mountain • Cinder-cone volcanoes- a steep, cone-shaped hill or small mountain that is built up by ash, cinders, and bombs

18. Volcanic Landforms from Lava and Ash • Composite volcanoes- when lava flows alternate between explosive eruptions and quiet eruptions of ash, cinder, and bombs. Tall, cone-shaped mountains in which layers of lava alternate with layers of ash. • Lava plateaus- high, level areas of many layers of thin, runny lava that erupt from long cracks in the ground,

19. Volcanic Landforms from Lava and Ash • Calderas- the huge hole left by the collapse of a volcanic mountain. The hole is filled with pieces of the volcano that have fallen inward, as well as some lava and ash. • People often settle close to volcanoes to take advantage of the fertile volcanic soil. Some volcanic soils are among the richest in the world.

20. Volcanic Landforms from Magma • Magma beneath Earth’s surface cools and hardens into rock; over time, weathering exposes the layers of hardened magma • Volcanic necks- forms when magma hardens in a volcano’s pipe (looks like a giant tooth) • Dike- magma that forces itself across rock layers and hardens; can be seen slanting through bedrock • Sill- when magma squeezes between horizontal layers of rock

21. Volcanic Landforms from Magma • Batholiths- a mass of rock formed when a large body of magma cools inside the crust; often form the core of many mountain ranges • Dome mountains- smaller bodies of hardened magma; forms when uplift pushes a batholith or smaller body toward the surface. The hardened magma forces the layers of rock to bend upward into a dome shape

22. Geothermal Activity • In geothermal activity, magma a few kilometers beneath Earth’s surface heats underground water. • Hot Springs- forms when groundwater is heated by a nearby body of magma or by hot rock deep underground. Water collects in a natural pool • Geyers- a fountain of water and steam that erupts from the ground; pressure builds until the mixture suddenly sprays • Geothermal energy- an energy source that people uses