Download Chapter 14 Volcanoes

Plate Tectonics and
Volcanoes
Earth Science
Chapter 18
Volcanoes
• Volcanoes
– Hills or
mountains made
from hardened
magma
Volcanoes
• 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
• Magma/lava is made
of:
– Silica
– Water vapor
– Magnesium
– Iron
Volcanoes
• 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)
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)
Volcanoes
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
• 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