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Chapter 13 Teacher Notes – Mountains and Volcanoes
Lesson 1 - Mountain Belts
Earth’s two major mountain belts are the circum-Pacific belt and the Eurasian-Melanesian belt.
The circum-Pacific belt forms a ring around the Pacific Ocean (a.k.a. – Ring of Fire) and includes:
The Cascades (North Pacific), The Andes (South America)
Where are the Mountain Belts
The Eurasian-Melanesian belt runs from the Pacific islands through Asia and southern Europe and into
northwestern Africa and includes:
The Alps of Europe and the Himalaya's of Asia
Plate Tectonics and Mountains
Mountains form as a result of collisions between tectonic plates at convergent boundaries.
There are 3 types of boundaries in which these collisions happen & unique characteristics that go with
each:
(1) Collisions between continents and oceanic crust
(2) Collisions between oceanic crust
(3) Collisions between continents
1) Collisions between Continental & Oceanic Crust
When oceanic lithosphere and continental lithosphere collide at convergent plate boundaries the denser
oceanic lithosphere subducts beneath the continental lithosphere.
This produces large-scale deformation which uplifts high mountains.
Also, the subduction of the oceanic lithosphere causes partial melting of the overlying mantle and crust.
This melting produces volcanic mountains.
Examples include the Cascades and Andes.
2.) Collisions between Oceanic Crust & Oceanic Crust
Volcanic mountains commonly form where two plates whose edges consist of oceanic lithosphere collide.
In this collision, the denser, colder oceanic plate subducts beneath the other oceanic plate.
Subduction again leads to volcanism and these eruptions of magma form an arc of volcanic mountains.
Examples include the Philippine Islands Japan, New Zealand and Indonesia.
3.) Collisions between Continents
Mountains can also form when two continents collide.
When the continental lithosphere of both plates collide, subduction is stopped because both plates have
the same densities, the collision continues in an upward motion.
An example of this type of collision is the formation of the Alps in Europe and the Himalaya Mountains in
Asia.
The intense deformation that resulted from the collision uplifted the Himalaya’s, which are growing taller.
Lesson 2 – Types of Mountains
Scientists classify mountains according to the way in which the crust was deformed.
Four Types of Mountains
(1) Folded Mountains, (2) Fault-Block Mountains, (3) Dome Mountains, (4) Volcanic Mountains
1. Folded Mountains
A folded mountain is a mountain that forms when rock layers are squeezed together and uplifted.
The highest mountain ranges in the world (Himalayas) consist of folded mountains that form
when continents collide - Boundary type: Convergent
The same stresses that form folded mountains also uplift plateaus.
Plateaus
Plateaus are large, flat areas of rock, high above sea level and located near mountain ranges.
Plateaus can form where large areas of rock are eroded or when a large portion of flat earth is pushed up
from the earth.
Types of Plateau - Butte
A butte (French) is an isolated hill with steep, often vertical sides and a small, relatively flat top; it is
smaller than mesas, and plateaus.
Types of Plateau-Mesa
A mesa (Spanish – table) a flat-topped mountain or hill.
2. Fault-Block Mountains
Fault-block mountains form when enormous underground pressure forces a whole rock mass to break
away from another.
Boundary Type: Transform or Divergent
On one side of this break the rocks rise; on the other side they sink down.
Some of the most spectacular mountain scenery anywhere are the great rock walls of the
Sierra Nevada which are actually the sides of huge tilted fault blocks.
The Sierra Nevada’s are in fact the broken upper edge of a huge plate that tilts down toward the west.
Grabens
The same type of faulting that forms fault-block mountains also forms long, narrow valleys called grabens.
Grabens develop when steep faults break the crust into blocks and one block slips downward relative to
the surrounding blocks.
3. Dome Mountains
Dome mountains are created when a large amount of magma pushes up from below the Earth’s crust,
but it never actually reaches the surface and erupts.
The pushed up rock (now metamorphic) cools and hardens into a dome shape.
Since the dome is higher than its surroundings, erosion works from the top creating a circular mountain
range.
Example of this is the Adirondack Mountains in N.Y. and Half-Dome in Yosemite.
4. Volcanic Mountains
Volcanic mountains are created when magma from beneath the Earth makes its way to the surface.
When it does get to the surface, the magma erupts as lava, ash, rock and volcanic gases.
This material builds up around the volcanic vent, building up a mountain over time.
Caldera
The word comes from Spanish caldera, meaning "cooking pot."
A Caldera is large, circular depression that forms when the magma chamber below a volcano partially
empties and causes the ground above to sink
Eruptions that discharge large amounts of magma can also cause a caldera to form.
Calderas may later fill with water to form lakes..
Lahar
“Lahar" is an Indonesian word that describes volcanic mudflows or debris flows.
Lahars have the consistency, viscosity and approximately the same density of concrete: fluid when
moving, then solid when stopped.
Location of Volcanic Mountains
Volcanoes can be found at these types of boundaries:
(1) Convergent subduction plate boundaries (Cascades)
(2) Divergent plate boundaries (Mid-Atlantic Ridge)
(3) Hot spots (Hawaii)
Lesson 3 – CH. 13 Volcanoes
The cause of many volcanic eruptions is due to the movement of tectonic plates which is driven by
Earth’s internal heat.
Why Does Rock Stay Solid
Pressure and temperature increase as the depth below the earth's surface increases (heat from the core,
pressure from overlying rocks, etc.).
But, because pressure increases along with temperature, the rocks in the mantle remain solid
For example, a rock's melting temperature on the surface might be 1000 ºC, but 200 km below the
surface under much higher pressure, the melting temperature of the rock might be 1300 ºC.
1) Change in Pressure
So if overlying pressure changes then so does the melting point of the rock.
Take the example of high altitude cooking – just like inside Earth the boiling point goes down with less
pressure.
2) Addition of Fluids
The addition of water and volatile contents promote melting by lowering the melting temperature of rocks.
Thus, a dry rock would have a higher melting point than a rock with water present.
3) Increased Temperature
Lastly increasing the temperature of a rock will also cause melting.
Difference Between Lava and Magma
Magma is liquid rock produced under earth’s surface.
Because magma is lighter then solid rock it flows upward away from denser rock and when it reaches the
surface it is referred to as lava.
How Magma Viscosity Determines the Type of Eruption
How explosive a volcano is depends on how runny or sticky the magma is.
The viscosity, or resistance to flow, of magma affects the force with which a particular volcano will erupt.
Types of Magma
Because oxygen and silicon are by far the two most abundant elements in magma, it is convenient to
describe the different magma types in terms of their silica content (SiO2).
Which has more silica, the granite found on a continent or basalt found in the ocean?
Mafic Magma
Mafic magmas, have relatively low silica and high iron (Fe) and magnesium (Mg) contents.
Mafic volcanoes create a magma that is “Runny like honey”.
Quite Eruptions
Oceanic volcanoes commonly form from mafic based magma.
Because of mafic magma’s low viscosity (thin and runny), magma is hot (above 1700°F) and gases can
easily escape from mafic magma.
Eruptions from oceanic volcanoes, such as those in Hawaii, are referred to as quiet.
Felsic Magma
Felsic magmas, have relatively high silica and low iron (Fe) and magnesium (Mg) contents.
Felsic volcanoes create magma that is “Thicky like Skippy”.
Explosive Eruptions
Felsic lava’s of continental volcanoes, such as Mount St. Helens, tend to be cooler (1400°F), thicker and
stickier.
If magma is thick and sticky (high viscosity), then gases cannot escape as easily.
With felsic based lava pressure builds up until the gases escape violently and explode throwing
pyroclastic material into the air.
Felsic volcanoes are the most dangerous and deadly.
Gas Factor in an Eruption
Silica content is not the only factor, the amount of dissolved gas in the magma provides the driving force
for explosive eruptions.
The viscosity of the magma, however, is the most important factor in determining whether an eruption will
be explosive or non-explosive.
Eruptions and the Cold
You can think of volcanoes in terms of when we get sick.
Typically with a cold your nose is runny (mafic).
However when you have the flu your mucus is thicker (felsic).
Predicting Volcanic Eruptions
1) Earthquakes
One of the most important warning signals of volcanic eruptions is an increase in earthquake activity
around the volcano.
2) Change in Mountain Geology
Also the geology may change due to swelling, subsidence and increased gas emissions.
3) Using Past Eruptions
Predicting the eruption of a particular volcano also requires some knowledge of its previous eruptions.
Just like the gap theory with earthquakes.
Next US Volcano Expected to Erupt
Mount Rainier, (14,410 Feet) the highest volcano in the Cascade Range (Seattle, Washington) and is
potentially the most dangerous volcano.
Lesson #4 Major Volcanic Zones
Like earthquakes, most active volcanoes occur in these 3 major zones:
1) Subduction Zones (convergent boundary)
2) Mid-Ocean Ridges (divergent boundary)
3) Hot Spots
The Ring of Fire
The Ring of Fire is the largest subduction zone on Earth and is an area where large numbers of
earthquakes and volcanic eruptions occur.
The Ring of Fire has 452 volcanoes and is home to over 75% of the world's active and dormant
volcanoes.
Subduction Zones
At the Ring of Fire you will find collisions between oceanic and continental plates and collisions between
oceanic and oceanic plates.
Why Do Volcanoes Occur at Subduction Zones
Volcanoes occur at a subduction zone because the subducting lithosphere brings down moisture and
rock.
The moisture lowers the melting point of the surrounding rock in the area which allows it to melt.
Because liquid rock (magma) is less dense then the solid rock around it, it rises to the surface of Earth.
Why Do Volcanoes Occur at Mid-Ocean Ridges and Divergent Boundaries
Hot mantle rock rises where the plates are moving apart.
This releases pressure on the mantle, which lowers its melting temperature.
Lava erupts through long cracks in the ground, or fissures.
Most divergent volcanoes happen underwater and out of harms way of humans.
Divergent Boundaries Located on Land Example Iceland and Africa
There are two exceptions and they are Iceland and Africa.
Iceland is unique because ½ of Iceland is on the North American plate the other ½ is on the Eurasian
plate allowing us to see how volcanoes work at divergent boundaries.
Iceland is one of the most active volcanic regions in the world, with eruptions occurring on average
roughly every three years (in the 20th century there were 39 volcanic eruptions on and around Iceland).
Africa is another place where we get to witness volcanoes at a divergent boundary.
There are 18 active volcanoes along with two unusual lava lakes.
Example include volcanoes like: Erta Ale and Nyiragongo
Hot Spots
A Hot spot is a volcanically active area of Earth’s surface, commonly far from a tectonic plate boundary.
This occurs because hot material called mantle plumes, rise and reach the lithosphere.
As magma rises to the surface, it breaks through the overlying crust creating a volcano.
Mantle Plumes
Evidence suggests that mantle plumes stay stationary while the lithospheric plate above a mantle plume
continues to drift slowly.
So, the volcano on the surface is eventually carried away from the mantle plume.
The activity of the volcano stops because a hot spot that contains magma no longer feeds the volcano.
However, a new volcano forms where the lithosphere has moved over the mantle plume.
Hot Spot Area’s Around the World
Geologists have identified some 40–50 such hotspots around the globe, with Hawaii, Yellowstone, and
the Galápagos, overlying the most currently active.
Where Hot Spots Occur
Why these hot spots (plumes) occur is still disputed to this day by scientist.
There are other theories out there including:
1) Crack in the Crust
2) Reheated slabs
3) Meteorite damage
1) Cracks in the Earths Crust and Crust Displacement
In theory - Hot-spot volcanoes occur in long chains because they form along cracks in the Earth’s crust.
These crack come from weakness in the lithosphere caused by former ice sheets melting and their
displacement of weight.
2) Reheated Slab
The reheated slab theory states that extra rock, through subduction is reintroduced into the mantle
creating a plume of magma.
3) Meteorite Impact
The meteor impact theory believes that foreign objects crashed into the Earth’s lithosphere damaging
(puncturing) the rock and allowing magma to rise through to the surface.
Lesson #5 Three Types of Volcanoes
Geologists generally group volcanoes into two main kinds and two secondary kinds)
1) Shield volcanoes
2) Stratovolcaones
Secondary volcanoes include lava domes and cinder cones (hornitos).
1) Shield Volcano
Where do find them - These volcanoes typically occur near divergent boundaries and hot-spots
throughout the world.
(Examples include: Kilauea & Erta Ale)
How they work- They are built up slowly by the accretion of thousands of highly fluid lava flows called
mafic (basalt) lava that spread widely over great distances.
These volcanoes commonly erupt “quietly” from vents along fractures (rift zones) that develop on the
flanks of the cone.
Shield Volcano Characteristics
What do they look like- Building a broad profile much like that of a warrior's shield.
Some of the largest volcanoes in the world are shield volcanoes including: Erta Ale and Kilauea 4,091
2) Composite and Stratovolcano Characteristics
Where do we find them- Stratovolcano’s are the most dangerous type of volcanoes and they occur near
or on subduction zones.
(Examples include: Nyiragongo, Mt. Saint Helens, & Mt. Fuji)
How do they work – most are characterized by eruptions of felsic (andesite and dacite) lavas that are
cooler and more viscous than basalt.
The danger exists because this lava is extremely thick, and sometimes it barely flows at all which allows
for a tremendous build up of pressure.
They can erupt with little warning, releasing enormous amounts of material and they don’t always erupt
nicely from their tops as we saw with Mount Saint Helens.
Composite and Stratavolcano Characteristics
What do they look like -They are typically steep-sided, symmetrical cones of large dimension built of
alternating layers of lava flows and may rise as much as 8,000 feet above their bases.
Stratovolcanoes will usually have a central caldera, or crater, at the top, but they will also have a network
of vents.
Another example of a composite - Mt. Shasta, CA (14,179). Lasted erupted 200 years ago & due in 400
years
Secondary Cone / Lava Dome
Where do we find them - They often occur on the flanks of flanks of large stratovolcanoes as is the case
with Mt. Lassen or within the crater of large composite volcanoes, such as Mount St. Helens.
How do they work- Lava domes are considered secondary cones and form from the nearby magmatic
plumbing system.
The domes grow slowly, but can be responsible for highly explosive eruptions.
Secondary Cone / Lava Dome Characteristics
What do they look like- Lava domes can come in many shapes and sizes because they typically do not
have enough gas or pressure to erupt explosively.
Pressure causes the most viscous magmas to move toward the surface, forming steep-sided and bulging
mountains that can be mistaken for a stratovolcano.
Where do we find them – Cinder cones are considered secondary cones because they generally form in
areas of other volcanic activity, including on composite and shield volcanoes.
For example, geologists have identified nearly 100 cinder cones on the flanks of the shield volcano
Mauna Kea, on the island of Hawaii.
3) Cinder Cone
How do they work- Like a Lava Dome, a cinder cone will use the volcanic pluming of a nearby volcano to
fuel itself.
Most cinder cones, are monogenetic, which means that once they finished erupting, they never erupt
again, unlike composite and shield volcanoes.
Cinder Cone Characteristics
What do they look like- The volcano forms when ash, cinders and bombs pile up around the vent to form
a circular or oval cone.
The longer the eruption, the higher the cone and they rarely exceed 800 feet in height and 1,500 feet in
diameter, although some may rise to as high as 2,000 feet or more.