Transcript Unit 4 Chapter 13

Unit 4 Chapter 13
Volcanism
There are approximately 485 active volcanoes in the world.
Volcanic eruptions are a major source of some of the
most dramatic changes to the Earth’s surface. Some
eruptions are more powerful than an atomic bomb.
By studying temperature within the earth, scientists
can learn more about the volcanic eruptions.
Formation of Magma
Magma is molten rock under the earth’s
surface
It can form under 3 conditions:
1.Temperature of the rock rises above the melting point
of the material.
2.If enough pressure is removed from the rock, the
melting point will decrease and the rock will melt.
3.Addition of fluids such as water may decrease the
melting point of some rock.
Volcanism
Volcanism is anything that includes the movement
of magma. The melting of the area around the
initial movement can melt further adding more
material to the magma.
Lava is magma that comes to the surface.
Volcano – an opening or landform in the earth
through which molten rock, gases and ash erupt.
1. Magma chamber 2. Country rock 3. Conduit (pipe) 4. Base 5. Sill 6.
Branch pipe
7. Layers of ash emitted by the volcano 8. Flank 9. Layers of lava emitted
by the volcano
10. Throat 11. Parasitic cone 12. Lava flow 13. Vent 14. Crater 15. Ash
cloud
Major Volcanic Zones
Most volcanoes are in the area of convergent and divergent zones. A
major zone is the Pacific Ocean. The Ring of Fire encircles this area.
Subduction Zones
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they occur at subduction boundaries thick lava (magma) very
gassy, therefore very, very explosive with a lot of tephra
(pyroclastics)
they form steep sided volcanoes with a lot of activity
Most famous volcanoes
Mt St Helen's & Mt Vesuvius
The Aleutian Islands, island arcs, by the North Pacific Ocean,
Mid Ocean Ridges
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They are long narrow fractures in oceans or on land with the
largest amount of magma
They contain basaltic lava with only few gases, smooth easily
flowing lava
They are found in spreading centers (division boundaries)
Mid Atlantic Ridge
Hot Spots
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It is a volcano not at a plate boundary
They produce smooth basaltic lava - non gassy - very hot
they form large volcanic rises under water until it is big enough
to rise above the surface
Hot spots do not move, the plate moves over the hot spot
Ex. Hawaii
Intrusive Activity
Since magma is less dense than rock will push upward and
intrude into the overlaying rock. This can cause it to change
(Metamorphic), melt or even crack the surrounding area.
Sometimes the magma will cool or solidify without erupting. This
will harden within the volcano. After some time, the
surrounding sedimentary rock will erode away leaving the
igneous intrusion called a pluton.
Section 2 Volcanic Eruptions
Volcanoes are windows into Earth’s interior. Lava can provide a look into what is
inside the earth. There are two types of magma/lava, Mafic and Felsic.
Rhyolitic (Felsic)
a. high in silica
b. Most viscous (slow moving)
c. Light colored
d. Usually associated with explosions
e. Lowest melting point
f. by Continental Hot Spots (Yellowstone Caldera)
Basaltic Magma (Mafic)
a. Low in silica
b. Fast moving (Least viscous)
c. Dark colored
d. Highest melting point
e. Rarely explosive
f. by Rifts, oceanic hot spots (Hawaii)
Types of Eruptions
Viscosity
This is the magma/lava’s resistance to flow. Magma high in
silica resists flow, low in silica flow very fast.
Magma contains many gases – H2O, CO2, SO2 , H2 , H2S
(Hydrogen Sulfide)
Very gassy - explosive eruption usually Felsic
No gas - mild non-explosive eruption usually Mafic
Quiet Eruptions
Oceanic eruptions are among the quietest eruptions. That is
because they are made from mafic magma which has a low
viscosity. Not explosive.
Lava Flows
Aa - rough, jagged surfaces
Pahoehoe - rope like lava
Blocky lava
– cools quickly on top while lava flows underneath
causing the lava on top to chunk and break. Most viscous.
Explosive Eruptions
Pyroclastic materials
It is any explosion that happens during volcanic activity. It can
be explosive and it can be mild. It contains super heated
gases, ash and other objects that are ejected from the opening.
Pyroclastic flow:
Is a dense, superheated cloud that travels down hill. You can
not out run this movement.
Types of Pyroclastic Material
Pyroclastic material is classified by size.
The solid fragments are also called tephra.
1. Ash - less than 2mm
2. Blocks & bombs - more than 64mm
Blocks always solid
Bombs become solid in flight
3. Lapilli’s - anything between 2mm & 64mm
4. Lahars is fast moving mud flow from melting snow and ice
mixing
with loose gravel.
Types of Volcanoes
There are 3 main types of volcanic cones that can be produced by
a volcano.
Shield
– Broad base
– Cooled lava layers because of low viscosity; less explosive
– Mauna Loa, Hawaii – 4170ft above sea level, base is 5000ft
below sea level.
Cinder Cone
A cinder cone happens when molten lava spewed from a vent
piled up.
It is the smallest of the volcanoes, formed by larger ones.
It has a sloped angle that can be as much as 40o
Composite
A composite volcano is made from alternating layers from previous
eruptions piling up around a vent.
They have very violent eruptions
They can lay dormant for a long time until it needs to relieve pressure
again.
Mount St. Helen’s is called a stratovolcano because of its size.
Other unique volcanic features:
Lava Plateau (Dome)
This is not a normal dome mountain. This happens when lava
comes up from a crack in the ground. It can spread on the surface
making a layer of basalt on top. This can cover large areas.
Columnar jointing
Are unique six sided columns that are formed when
basaltic lava cools & cracks, you know, the internal arrangement
of its atoms. You can find this anywhere, Iceland, New York,
California, etc.
Devils Tower, Wyoming
Calderas
A caldera forms from an eruption. The magma beneath a volcano
is released than the top of a volcano collapses in. A large crater
is formed which can later fill with water.
Mount Mazama in Oregon filled with water after its collapse and is
now called Crater Lake.
Predicting Volcanic Eruptions
Scientists study the eruptions to see if there are patterns to help predict
eruptions.
Earthquake Activity
Warning signs of an eruption. Earthquakes usually precede an eruption.
Patterns in Activity
Before an eruption there may be an upward movement of magma,
causing a swelling of the outer area. Scientists look for the history of a
volcano to help them try to predict when it may erupt again. A
dormant volcano can erupt again.
Other Volcano Information
Extraterrestrial Volcanoes
Moon
– Evidence of volcanoes are the dark spots which is mostly basalt
– Evidence is also there because we can see volcanic formations
Mars
– Has a number of shield volcanoes
– Olympus Mons – largest known volcano in the solar system
– 1999 Mars Global Surveyor saw lava flows only 20-60 million years old.
Venus
– Has about 1600 large volcanoes
– Scientist feel that the volcanic activity is causing the heavy thick
atmosphere
Io
– Most volcanically active place in the solar system
– Very active today
Some of the most powerful ones in our
history:
1. Krakatoa Indonesia 1883
Most violent eruption in historic times
Loudest natural sound ever heard (1900 miles)
blew away 1/2 an island
broke windows 90 miles away
36,000 people died
2. Vesuvius 79 AD
Pompeii was buried under 30 feet of ash everyone died
3. Mt Popocatepetl (Mt. Popo) - current