Volcanoes Forces Inside the Earth. What is a volcano? A volcano is a weak spot in the crust where molten material, or magma, comes to the surface. o when.

VolcanoesForces Inside the Earth

What is a volcano?• A volcano is a weak spot in

the crust where molten material, or magma, comes to the surface.owhen magma reaches the

surface, it is called lavaoAfter lava has cooled, it

forms solid rock, building up Earth’s surface

Plate Boundaries

• The Ring of Fire is formed by many volcanoes that rim the Pacific Ocean.• Why do volcanic belts form along Earth’s boundaries?• Huge pieces of crust are diverging or converging, often

fractures in the crust allow magma to reach the surface.

Divergent and Convergent

• form along mid-ocean ridges.

• form along rift valley

• Form where oceanic and continental crust meet or where two oceanic plate collide.– An island arc is formed by

volcanoes along a deep ocean trench

– Can alsoform undercontinents

Hot Spots• An area where material from deep within the

mantle rises and then melts, forming magma.

Properties of MagmaPhysical Properties

• Any characteristic of a substance that can be observed or measured without changing the composition of the substance.

• Density, hardness, melting point, boiling point, whether it is magnetic

Chemical Properties • Any property that

produces a change in the composition of matter.

• Ability to burn, combine or react with other substances

Viscosity of Magma• Viscosity is the resistance of a liquid to flow.– The greater the viscosity, the slower the flow

• ex. honey

– Liquids have different viscosities because of the particles that make them up.

• Viscosity of magma depends on the silica content and temperature– Silica is made of silicon and oxygen• more silica, the higher viscosity, slow moving, cools to

rhyolite• less silica, the lower the viscosity, faster moving, cools to

basalt

Temperature- Ranges from 750° C to 1175° C

pahoehoe• Fast moving• Hot• Low viscosity• Solid mass of wrinkles

aa• Slow moving• Cooler• High viscosity• Rough surface, jagged lava

chunks

Magma Reaches the Surface

• Inside a volcano– Magma chamber- a pocket where the magma collects

under the volcano– Pipe- a long tube in the ground that connects the

magma chamber to the Earth’s surface– Vent- the opening where molten rock and gas leave the

volcano– Lava flow- area covered by lava as it pours out of the

vent– Crater- bowl-shaped area that may form at the top of a

volcano around the central vent

A Volcanic Eruption• What pushes magma to the surface?– The force of the expanding gases push magma from the

magma chamber through the pipe until it flows or explodes out of the vent. • Quiet eruptions

– low-silica, magma bubbles out gently, oozes quietly from the vent• Explosive Eruptions

– high-silica, builds up pressure and explodes– breaks lava into fragments that cool and harden into pieces of

different sizes; from volcanic ash to pebble sized to larger pieces called bombs

– Pyroclastic flow occurs when an explosive eruption hurls out a mixture of hot gases, ash, cinders, and bombs» Obsidian and pumice forms

Mount St Helens

Life Cycle of a Volcano• Geologist use the terms active,

dormant, or extinct–Active or live volcano is erupting or showing

signs of erupting in the near future–Dormant volcano is like a sleeping bear, to be

awakened in the future to become active–Extinct is a dead volcano unlikely to erupt

again

Volcanic eruptions create landforms made of lava, ash, and other materials.

• Shield volcano- form when thin layers of lava pour out of a vent and harden on top of previous layers building a wide, gently sloping mountain.

• Cinder cone volcano- high viscosity lava with ash, cinders, and bombs build up around the vent in a steep, cone-shaped hill or small mountain.

• Composite volcano- tall, cone shaped mountains in which layers of lava alternate with layers of ash formed by lava flow alternating with explosive eruptions of ash, cinder, and bombs.

• Lava plateaus-instead of forming mountains, some eruptions of lava form high level area. – Thin, runny lava travels far

before it cools and solidifies.– This happens again and

again and after millions of years, plateaus form.

Caldera- a huge hole left by a volcanic mountain

Which is Which?

Why would anyone want to live near an active volcano?

• To take advantage of the fertile volcanic soil.– When the volcanic ash breaks down, it releases

potassium, phosphorus, and other substances plants need to grow.• This creates some of the richest soils in the world.

Features formed by magma include volcanic necks, dikes, sill, batholiths, and dome mountains.

• A volcanic neck forms when magma hardens around a volcanic pipe.

• Magma that forces itself into rock layer and hardens becomes a– Dike, slanted though bedrock– Sill, horizontal through bedrock

Batholiths-a mass of rock formed when a large body of magma cools inside the crust.

Dome Mountains- forms when uplift pushes a batholith toward the surface.

Geothermal Activity• “geo” means EARTH• “therme” means HEAT• hot spring- a natural pool of water heated by

a nearby body of magma• geysers- a fountain of water and steam that

erupts from the ground as pressure builds