Plate Tectonics and Volcanoes

Plate Tectonics and Volcanoes

Earth Science Chapter 18

Volcanoes

• Volcanoes–Hills or

mountains made from hardened magma

• Magma– Molten rock that is below the surface of the

earth– Located in the mantle– Forms when rocks melt due to subduction

• Lava– Molten rock that has reached the surface of

the earth– Forms when volcanoes erupt

Volcanoes

Volcanoes

• Magma/lava is made of:– Silica– Water vapor– Magnesium– Iron

• 3 Types of magma/lava–Felsic (Rhyolitic)

• Made of 60% or more silica– Intermediate (Andesitic)

• Made of 50-60% silica–Mafic (Basaltic)

• Made of 50% or less silica

Volcanoes

• Properties of Felsic (Rhyolitic) magma– Light in color– High silica (SiO2) content– Contains a lot of water vapor– Does not flow easily– Highly viscous (thick)– Often solidifies before reaching the earth’s

surface– High amount of gases (bubbles stuck in thick

liquid)

Volcanoes

• Properties of Mafic (Basaltic) magma– Dark in color– High in iron– Low silica content– Flows easily– Does not contain a lot of water vapor– Low amount of gases (bubbles escape easily)

Volcanoes

Mafic Lava

Volcanoes• 2 Manners of Eruptions

1. Pyroclastic• Highly explosive• Felsic lava (trapped gases build up

pressure)• Lots of poisonous gases and tephra, little

magma erupted• Pyroclastic flow – hot stream of gases and

tephra that flows down the volcano

Volcanoes• Types of Tephra:

– Bombs• Extremely large (> 64 mm) chunks erupted

from volcano• Ejected as liquid, harden as they fall

– Blocks• Erupted as solid pieces (> 64 mm)

– Lapilli• Smaller than bombs ( up to 64 mm)

– Ash• Fine grained/very small chunks (< 2 mm)

2. Non-Pyroclastic– Non-explosive– Mafic lava (very little trapped gases)– Lava erupts more continuously, but flows out

quietly– Mid-Atlantic Ridge is best example

Volcanoes

Volcanoes

• 3 Types of Volcanoes– Shield cone

• Broad or wide• Mafic lava• Mellow eruptions• Hawaii (caused

by a hot spot)

Shield Cone

Volcanoes– Cinder cone

• Narrow• Tall (not as tall as

composites)• Felsic lava• Pyroclastic eruptions

–Lots of tephra and gases, not much lava

• Paricutin, Mexico

Cinder Cone

Volcanoes– Composite Cone

• Alternating layer of lava and cinders (ash)

• Alternates pyroclastic and non-pyroclastic eruptions

• Tall, snow-capped peaks

• Mount St. Helen’s

Composite Cone

Volcanoes and Boundaries– Subduction Boundaries

• Pyroclastic eruptions–Felsic lava–Slabs are pushed down into the mantle–Pressure and temp rise – water turns to

steam–Hot fluids melt mantle rock & magma

migrates upward.–Cinder and composite cones have

pyroclastic eruptions

Ring of Fire

Volcanoes and Boundaries

• Subduction Volcanoes– Ocean/Ocean

• Chain of volcanoes called an island arc– Ocean/Continent

• Volcanic arc• Continental crust has higher silica content• Magma varies in composition (what it’s

made of)

Volcanoes and Boundaries

• Subduction Volcanoes

Volcanoes and Boundaries

• Divergent Boundary– Non-pyroclastic

• Mafic lava• Magma produced during sea floor

spreading• Magma rises to fill in rift where plates have

separated• Sometimes called basaltic (most of the sea

floor is made of basalt)

Volcanoes and Boundaries

• Big Idea:–Plate motions provide the

mechanism by which mantle rocks melt to generate magma.

Plutonic Structures

• There is much more magma under the surface of the Earth than we see with volcanoes.

• That magma forms other Igneous structures (plutons) that we can identify.

• It solidifies in the Earth’s crust, hardening in other rocks.

Plutonic Structures• Types:• Batholiths

– Hardened magma that forms the cores of many mountain ranges

– Largest type of intrusion

– Sometimes exposed at the surface due to erosion

Plutonic Structures

• Stock– Hardened magma

exposed at the surface due to erosion

– Small batholith

Plutonic Structures• Dike

– Magma cools inside a fracture• A fracture is a

break in the crust with no movement

– Cuts vertically across the layers of rock

Plutonic Structures

• Sill– Magma

goes into rock layers horizontally and hardens

Plutonic Structures

• Laccolith– Goes into layers

horizontally, but the stiff magma is unable to spread to form a sill

– Instead it pushes the land up to form a dome

Plutonic Structures

• Volcanic Neck– Plug of hardened

magma left in the vent

– Cone is completely eroded