Transcript Chapter 8 - SchoolRack

Chapter 8
VOLCANOES
Section 1 - Volcanic Eruptions
There are two types of volcanic eruptions,
Nonexplosive and Explosive
1) NONEXPLOSIVE ERUPTIONS
When most people think about volcanic eruptions
they think of rivers of red-hot lava (lava flows)
Lava flows come from nonexplosive eruptions
These calm outpourings of lava can release a HUGE
amount of molten rock
Some of the LARGEST mountains in the world
grew from repeated lava flows over hundreds of
thousands of years
 2) EXPLOSIVE ERUPTIONS
 During this type of eruption clouds of hot debris and gases
SHOOT out of the volcano
 This often happens at SUPERSONIC speeds
 These eruptions do not produce lava flows
 Instead molten rock is blown into millions of pieces that harden
in the air
 The dust size particles can circle the Earth for years in the upper
atmosphere
 This type of eruption can also blast millions of tons of solid
rock from the volcano itself
 In a matter of minutes, an explosive eruption can demolish
rock formations that took thousands of years to build
 In this case a volcano will actually shrink instead of grow
Cross Section of a Volcano
 All volcanoes share the same basic features
 Magma - hot liquid material (rock) that creates
volcanoes
 Magma collects underground in Magma Chambers
 Some chambers can be up to 160km below the surface
 Vents - holes in the Earth through which magma
rises
 Lava - magma that erupts and flows onto the Earth’s
surface
 Pyroclastic Material - is magma that erupts as
fragments of molten material that solidify in the air
 Volcano - a vent or group of vents combined with the
build up of lava and/or pyroclastic material
Magma
 By comparing the composition of magma from
different types of eruptions, scientists have
determined that the composition of the magma
dictates whether an eruption will be nonexplosive,
explosive, or somewhere in between
 Water
 A volcano is more likely to erupt explosively if the magma
has a high water content
 The more water there is in magma, the greater the pressure
(like a can of soda) and the greater chance that there will be
a violent eruption
 Gases (steam) need more room than liquids (pressure)
Silica - Explosive eruptions are also caused by
magma that contains a large percentage of
silica
Silica rich magma is very thick and has a
tendency to harden inside the volcano
This causes magma and gases to get trapped
inside the volcano until so much pressure builds
up that the volcano goes BOOM !!!!!!
Low silica magma is runny which allows gases
to escape easily, keeping pressure low
What Erupts from a Volcano?
Nonexplosive eruptions produce mostly lava,
while explosive eruptions produce mostly
pyroclastic material
Over many years, a volcano may alternate
between eruptions that produce lava or
pyroclastic material
Eruptions of lava and pyroclastic material may also
occur as separate stages of a single eruption
event
Lava
Lava is magma that flows onto the Earth’s surface
Like magma lava ranges in consistency from thick
to thin
 Blocky Lava is cool, stiff lava that cannot travel far
from the erupting vent
Blocky lava forms jumbled heaps of sharp edged chunks
 Pahoehoe is lava that flows slowly, like wax
dripping from a candle forming a glassy surface
with round wrinkles that look like “ropes”
 Aa is slightly stiffer lava that pours out quickly and
forms a brittle crust that can break and harden
into jagged pieces
 Pillow Lava forms when lava erupts underwater
It forms rounded lumps that are the size and shape of
pillows
 Pyroclastic Material
 Pyroclastic material refers to the rock fragments created by
explosive volcanic eruption and is produced when magma
EXPLODES from a volcano and solidifies in the air. It can
come in a variety of sizes that include:
 Volcanic Blocks - are the largest pieces upwards to the size of a
house
 Volcanic blocks are large pieces of rock (not magma) from the
mountain
 Volcanic Bombs - are large blobs of magma that harden in a
flattened, elongated shape, like a bomb
 Volcanic bombs are greater than 64mm in diameter
 Lapilli - which means “little stones” in Italian, are pebble like
bits of pyroclastic material between 2 and 64mm in diameter
 Volcanic Ash - consist of particles that are less than 2mm in
diameter
Chapter 8 Quiz 1
 1) __________ is the hot liquid material found deep
in the ground, and is the driving force that creates
volcanoes
 2) __________ are holes in the Earth’s crust in
which the above material (#1) flows
 3) __________ includes hot liquid material (#1) and
rock fragments that are BLASTED into the air
 4) __________ are large pieces of rock (from the
mountain) that are blasted into the air, and can be
upwards of the size of a house
 5)___________ is lava that hardens into jagged
pieces
 Bonus) _________ forms when a magma chamber
collapses causing the “roof” above it to collapse
Section 2 - Volcanoes Effects on
Earth
The effects of volcanic eruptions can be seen
both on land (mountains, plateaus, etc.) and
in the air (ash and escaping gases)
Ash and escaping gases can affect global climate
patterns
An Explosive Impact
 Because ash and gases can be ejected high into the
air it can have widespread effects on Earth, including
blocking out the sun for days and over thousands of
miles.
 Ash can also blow down trees and knock down buildings
 Flows - are clouds of hot ash that can flow rapidly
downhill like an avalanche, searing everything in its
path
 Large amounts of ash can sometimes mix with water,
turning the flow into the consistency of wet cement
 These flows can pick up boulders, trees, and buildings causing
MASSIVE destruction
 Fallout - happens when volcanic ash falls back to the
ground
 Buildings have been known to collapse under the weight of
accumulating ash
 Ash can also dam up river valleys, resulting in MASSIVE
floods
 Too much ash can smoother crops causing food shortages
and loss of livestock
 Climatic Fluctuation - are changes in global
temperatures and moisture levels due to the release
of ash and sulfur rich gases that reach our upper
atmosphere
 Temperature shifts can disrupt climates all over the world
causing milder wetter summers and longer harsher winters
resulting in starvation and disease
Different Types of Volcanoes
 There are three basic types of volcanoes:
 1) Shield Volcanoes - are volcanoes built out of layers of
lava from repeated nonexplosive eruptions
 Because lava is runny it spreads out over a wide area, which
over time builds up to form volcanoes with wide bases and
gently sloping sides
 Though their sides are not steep, shield volcanoes can be
enormous and include the largest mountain on Earth, Mauna
Kea in Hawaii
 2) Cinder Cone Volcanoes - are small volcanic cones made
entirely of pyroclastic material from moderately explosive
eruptions
 These volcanoes have narrow bases and steeper slopes
 Cinder cone volcanoes usually erupt for a short period of time
and occur in clusters
 These volcanoes erode quickly because the pyroclastic material
of the volcano is not cemented together by lava
 3) Composite Volcanoes - form from explosive eruptions of
pyroclastic material, followed by quieter outpourings of lava
 These are the most common types of volcanoes and are also known as
stratovolcanoes
 These have broad bases and sides that get steeper toward the summit
Craters and Calderas
Crater - is a funnel shaped pit found at the
top of the central vent in most volcanoes
 A craters funnels shape results from explosions of
material out of the vent, as well as the collapse of
material from the craters rim back into the vent
Caldera - forms when a magma chamber that
supplies material to a volcano empties and its
roof collapses
This causes the ground to sink, leaving a large
circular depression
Lava Plateaus
Most of the lava on Earth’s continents do
NOT come from volcanoes, but ooze from a
series of long cracks called fissures
Runny lava oozing from fissures can spread out
over thousand of square kilometers forming Lava
Plateau
i.e. The Columbia River Plateau formed about 15 million
years ago
Chapter 8 Quiz 2
1) _________ are clouds of hot ash that can
flow rapidly downhill like an avalanche
2) _________ are built out of layers of lava
from repeated nonexplosive eruptions
3)__________ are also referred to as
Statovolcanoes
4) __________ forms when a magma
chamber empties and its roof collapses
Bonus) _________ are places within tectonic
plates where columns of rising magma are
Section 3 - What Causes
Volcanoes
 The Formation of Magma
 The key to understanding why volcanoes erupt is
understanding how magma forms
 All volcanoes begin when magma collects in pockets in the
deep regions of the Earth’s crust and uppermost layers of
the mantle
 The zone of magma formation is between 25 - 160 km below
the surface
 Although the rocks of the mantle are hot and pliable, it is
still not a liquid. Why?
 The weight of the rock above the mantle exerts so much
PRESSURE that the atoms are forced close together
 This increases the melting point (temperature point) at which
the rock will melt
 Mantle rock will not melt (turn into magma) until the
temperature increases or the pressure on it releases
 Because the temperature of the mantle is fairly constant,
pressure release is the main reason why rock melts
Where Volcanoes Form
Most volcanoes are found on tectonic plate
boundaries
These boundaries are where plates either collide
or separate from each other
The plate boundaries around the Pacific Ocean is called
the RING OF FIRE !!!!!!
At these types of boundaries it is easier for
magma to travel upward through the crust
When Tectonic Plates Separate
 As two tectonic plates separate a deep crack, called a
rift, forms
 Mantle material is then able to rise up, decreasing
the pressure on it, and causing it to melt and turn
into magma
 Magma continuously rises up through the rift creating
new crust
 Most divergent boundaries are found under the
oceans where they produce long underwater
mountain chains called mid-ocean ridges
When Tectonic Plates Collide
 The place where two tectonic plates collide is called a
convergent boundary
 The movement of one plate under another is called a
subduction zone
 Convergent boundaries are commonly located where
oceanic plates collide with continental plates
 As the oceanic plate slides under the continental
plate it sinks deeper into the mantle causing it to
melt
 As the magma rises to the surface it cause the silica
rich continental crust to also melt becoming part of
the gooey lava that can cause an explosive eruption
Hot Spots
Not all magma develops along tectonic plate
boundaries
i.e. The Hawaiian Islands are formed over hot
spots
Hot Spots - are places within tectonic plates
that are directly above columns of rising
magma called mantle plumes