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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
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