Volcanoes

Target #16- I can summarize the conditions needed for magma to form

• Despite the high temperature in the mantle, most of the layer remains solid due to extreme pressure–Magma: liquid rock produced under

Earth’s surface

Target 16- cont.

• Melting Point: the temperature at which a substance will change from a solid to a liquid

• Magma can form in three ways– Temperature rises above the

melting point of rock– Pressure decreases lowering

the melting point of rock– Addition of fluids, like water,

can decrease the melting point

Target 17- I can define volcanismTarget #18- I can differentiate between magma

and lava

• Volcanism: any activity that includes the movement of magma and gases toward or onto Earth’s surface–Magma rises because it is less dense

than the surrounding rock• magma that flows onto Earth’s surface is

called lava• Flows from an opening, or vent

Target #19- I can differentiate between mafic and felsic magma

• Mafic: magma that is rich in magnesium and iron– Dark in color– Makes up oceanic

and continental crust

• Felsic: magma that is rich in feldspar and silica– Light in color–Makes up

continental crust only

• When magma erupts from a volcano and then cools & hardens, it forms an igneous rock

Target #20- I can describe viscosity

• Viscosity: resistance to flow–Affects the force with which a particular volcano will erupt

Target #21- I can describe quiet eruptions

• Quiet Eruptions–Form from mafic magma–Low viscosity–Occur with oceanic volcanoes–Magma “oozes” out of the vent

Target #22- I can describe the lava rocks produced from quiet eruptions

• When mafic lava cools rapidly, a crust forms on the surface of the flow

• If the lava continues to flow after the crust forms, the crust wrinkles– Pahoehoe: a volcanic

rock that forms from hot fluid lava• As it cools it forms a

smooth, ropy texture• Means “ropy” in

Hawaiian

Target #22- cont.

• If the crust deforms rapidly, or grows too thick to form wrinkles, the surface breaks into jagged chucks– Aa: a jagged volcanic rock that forms from

lava that• has the same composition as pahoehoe

– Blocky Lava: a volcanic lava that has a high silica content, and is highly viscous

Target #23- I can describe explosive eruptions

• Explosive Eruptions–The felsic lavas of continental volcanoes tend to be cooler and stickier than the fluid lavas produced by oceanic volcanoes• Pyroclastic material: fragments of rock that form during a volcanic eruption

Bellwork

•What are the differences between an explosive eruption and a quiet eruption?

Types of Volcanoes

Target #24- I can identify where volcanoes are located

• Volcanoes are usually located at the edge of plate boundaries– Ex: A major zone of

active volcanoes encircles the Pacific Ocean• Known as the Pacific

Ring of fire

– Subduction zones• Continental-oceanic

convergent boundaries• Oceanic-Oceanic

convergent boundaries

Target #24 cont.

• The largest amount of magma is found at divergent boundaries along mid-ocean ridges–The magma erupts to form

underwater volcanoes• These volcanoes produce pillow lava

Target #25- I can differentiate between shield, cinder, and composite

volcanoes• Shield Volcanoes:

volcanic cones that are broad at the base and have gently sloping sides– Covers a wide area– Generally forms from

quiet eruptions– Layers of hot, mafic

lava flow out around the vent, harden, and slowly build up to form the cone

– Ex: Hawaiian Islands

Target #25- cont.

• Cinder Cones: a volcano that has a very steep slope– Not usually tall- max

height is not more than a few hundred meters

– Form from explosive eruptions and are made of pyroclastic material

Target #25- cont.

• Composite Volcanoes: made of alternating layers of hardened lava and pyroclastic material– Quiet eruptions deposits

lava and an explosive eruption follows depositing ash

– Also known as stratovolcanoes

– Develop to form large volcanic mountains

Target #26- I can describe a caldera

• Caldera: a large, circular depression that forms when the magma chamber below a volcano partially empties and causes the ground above to sink– Calderas may fill with

water to form lakes• Ex: Mount Mazama in

Oregon (known as Crater Lake)

Target #27- I can describe how a hot spot forms an island chain

• Hot Spot: an area of volcanic activity within the interior of a tectonic plate– Magma rises from the

mantle in the middle of a plate, and not at a plate boundary

– The rising magma creates a volcano, producing an island

– As the plate shifts over the magma, a new volcano and new island is formed

– Continues creating a chain of islands• Ex: Hawaiian Islands