Download Chapter 12: Volcanoes - Ms. Banjavcic`s Science

Chapter 12:
Volcanoes!
Volcanoes and Earth's Moving
Plates
A volcano is an opening in
Earth that erupts gases, ash and
lava.
 Volcanic mountains form
when layers of lava, ash and
other material build up
around these openings.

Kilauea (kee low AY ah),
located in Hawaii, is the world's
most active volcano.
 The most recent series of
eruptions began in 1983 and
still continues.

How do volcanoes form??
Magma is forced upward
because it is less dense than the
rock around it.
 Ex: Air bubbles rise to surface
in shampoo/syrup bottles.
 After many thousands or even
millions of years, magma reaches
Earth's surfaces and flows out
through an opening called a vent.
 As lava flows out, it cools
quickly and becomes solid,
forming layers of igneous rock
around the vent.
 The steep walled depression
around a volcano's vent is called
a crater.

Where do volcanoes occur??
Divergent Plate Boundaries: the
mid-Atlantic ridge is an area
where the plates are moving
apart.
 When plates separate, they
form long, deep cracks called
rifts.
 Lava flows from these rifts and
is cooled quickly by sea water.
 Sometimes, the volcanoes and
rift eruptions rise above sea
level, forming new islands
such as Iceland.
 In 1963, the new island
Surtsey was formed during a
volcanic eruption.

Convergent Plate Boundaries: places
where the Earth's plates move
together...especially form volcanoes where
an oceanic plate slides below a continental
plate.
 Magma forms when the plate sliding
below another plate gets deep enough
and hot enough to melt partially. The
magma is then forced upward to the
surface.
 Volcanoes that form at convergent plate
boundaries erupt more violently than
other volcanoes do.
 Hot spots: form when areas between the
Earth's mantle and core are unusually hot
and rock at these areas is forced toward the
crust where it melts partially.
 Magma breaks through the crust to form
several volcanoes.
 Not at a plate boundary.
 Formed Hawaiian islands


What would happen to the big island of Hawaii if the Pacific plate stopped moving?
Types of Volcanoes
What controls eruptions?
 Trapped gases like water vapor and carbon
dioxide build up in volcanoes.
 If the gas escapes easily, it is a quiet eruption.
 If gas and water vapor build up, eruptions can
be explosive.
 At what plate boundary could water vapor
build up?
 Composition of Magma
 Quiet eruptions: magma that is low in silica and
are basaltic.
 This type of lava pours from volcanic vents
and runs down the sides of a volcano (pg.
338)
 Explosive eruptions: magma that is high in
silica and is andesitic.
 This type of magma is thick and pressure
builds up an explosion occurs...often carrying
pieces of lava.

Forms of Volcanoes
Shield Volcano: broad, gently sloping
volcano formed by quiet eruptions of
basaltic lava.
 The basaltic lava flows over Earth's
surface covering large areas with
this deposits of basaltic igneous
rocks when it cools.
 Basaltic lava can also flow onto
Earth's surface through large cracks
(not volcanoes) called fissures and
creates flood basalts.
 Accounts for the greatest volume of
erupted volcanic material.
 Much of the new seafloor that
originates at the mid-ocean ridge
forms as underwater flood basalts.

Cinder Cone Volcano: steepsided, loosely packed volcano
formed when tephra falls to the
ground.
 Tephra: bits of rock or solidified
lava dropped from the air after
an explosive eruption.
 Vary in sizes from volcanic
ash, cinders, large rocks and
blocks.

Composite Volcano: volcano built
by alternating explosive and quiet
eruptions that produce layers of
tephra and lava.
 Found mostly where Earth's
plates come together and one
plate sinks below the other.
(What plate boundary is this?)

Igneous Rock Features

Intrusive Features
 Most of the magma never reaches Earth's surface...as magma cools
underground it produces underground rock bodies that could become
exposed later at the surface by erosion.
 Batholiths: large intrusive rock body that forms when magma being
forced upward toward Earth's crust cools slowly and solidifies
underground.
 Dikes: magma that is forced into a crack that cuts across rock layers and
hardens.
 Sills: magma that is forced into a crack parallel to rock layers and
hardens.

Other features
 Volcanic neck: solid igneous
core of a volcano left behind
after the softer cone has
been eroded.
 When a volcano stops
erupting, the magma
hardens inside the vent.
 Erosion, usually by water
and wind, begins to wear
away at the volcano.
 Caldera: large circularshaped opening formed
when the top of a volcano
collapses.
 Crater Lake in OR is a
caldera that filled with
water and is now a lake.
Effects of Eruptions
When volcanoes erupt, they often
have direct, dramatic effects on the
lives of people and their property.
 Lava flows destroy everything in
their path.
 Falling volcanic ash can collapse
buildings, block roads and can
cause lung disease in people and
animals.
 Sometimes, volcanic ash and
debris rush down the side of a
volcano: pyroclastic flow.
 Temperature inside a pyroclastic
flow can be high enough to ignite
wood.
 When sulfurous gases from
volcanoes mix with water vapor in
the atmosphere, acid rain forms.
