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Plate Tect, volcanoes and Mtn bldging


Matthew Lewis

on 1 February 2012

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Transcript of Plate Tect, volcanoes and Mtn bldging

Plate Tectonics, Volcanoes, and Mountain building
In the early days of geology scientists started to realize that earth did not look the same today as it did in the past:
example of evidence: Fish fossils found high in mountain ranges
Fossil from Wyoming
One scientist noticed the similarity between the coastlines of South America and Africa on opposite sides of the atlantic ocean and wondered if they were once joined and had drifted apart.
Alfred Wegner
He searched for evidence to back up his idea of "continental drift"
Wegners evidence for "continental drift"
1. - Fossils of several extinct species were found on different landmasses.
Matching fossils
2. - Rocks of the same age and type were found in mountain belts seperated by oceans
Matching Rocks
3. - Geologic records showed that areas currently in the tropics once had glaciers covering their surface
Ancient Climates
Despite Wegners evidence, his theory was ultimately rejected because he had no way of explaining how the continents moved
The evidence that started to convince scientists that large pieces of the earth were moving came from studyies of the ocean floor.
Features found on the ocean floor-
- underwater mountain ranges

- deepest places of the ocean
Mid-Ocean Ridge
occured in narrow bands
usually have a rift valley in their center
activity along mid-ocean ridges at the bottom of the ocean lead to the idea of sea floor spreading
What is Sea-Floor Spreading?
Sea-floor spreading is the idea that volcanic action along mid-ocean ridges creates new oceanic crust that pushes old oceanic crust away from the ridge.
When dense oceanic crust runs into continental crust with a lower density the oceanic crust sinks under the continental.

This process is called
Evidence for Sea-floor Spreading
Sediment depth and Age of Rock-
The farther you go out from mid-ocean ridges the depth of the sediment on the bottom of the ocean increases and age of the rock increases
Paleomagnetism of Ocean Rock
When magma with magnetic elements solidifies, the magnetic elements will freeze into place preserving the polarity of earths magnetic field.
The earths magnetic pole alternates over time, instead of all compass needles pointing north they will point to the south
This alternating of earths magnetic field produces magnetic strips in the ocean floor
Earthquake patterns
In regions where subduction was occurring the depth of earthquakes produced a pattern
Theory of Plate Tectonics
Earths lithosphere is made of plates that move slowly relative to each other. This motion is driven by convection in the mantle
The earths surface is broken up into about 12 major plates and some other minor plates
The locations of the plates was deduced from the distribution of earthquakes worldwide
Plates move in three different ways in relation to each other

: Towards each other
: Away from each other
: Sliding past one another
The motion of earths plates influences weather, climate, life, ocean currents and many other things around the world
Types of Plate Boundaries:
Oceanic - Continental Convergence:
Type of Motion:
What happens?
Dense oceanic crust subducts under the continental plate and melts into the mantle
Landforms created:
Trenches form where oceanic plate subducts under continental crust
Volcanic mountain ranges form inland as melted oceanic plate return to the surface
Examples from nature:
Continental Crust
Oceanic crust
Volcanic Mountains
Continental-Continental Convergence
Type of Motion:
What Happens?
Continental crusts having the same density push each other up no subduction takes place
Landforms created:
Mountain ranges
Example from nature: Himalayan Mountains
Oceanic-Oceanic Convergence
Type of Movement:
What Happens?
One of the oceanic plates subducts below the other and melts into the mantle
Landforms Created:
Volcanic island arcs, Ocean Trenches
Example from Nature: Southeast Asia
Types of Divergent boundaries
Oceanic - Oceanic Divergence
Type of motion
What Happens?
Magma upwells from the mantle and erupts on the surface creating new oceanic crust, this new crust pushes the older crust away
Landforms created:
Mid-Ocean Ridges
Example from nature: Mid-Atlantic Ridge
Continental- Continental Divergence
Type of Motion:
What Happens?
Upwelling Magma from the Mantle stretches and reaches up through the crust creating volcanoes.

Magma continues up over the millenia shoving the continental crust apart
Landforms created:
Rift Valleys, eventually ocean ridges can form
Example from nature: East African Rift Valley
Transform Boundaries
Type of Motion:
What Happens?
Crustal plates slides past each other distorting landforms and causing earthquakes
Landforms Created
None - existing landforms may be offset
Example from nature: San Andreas Fault
What causes Plate motion?
Plate Motion is caused by Thermal Convection in the mantle
As cold oceanic crust subducts into the mantle it pulls the attached crust along with it
This is called "Slab Pull"
The combination of Slab Pull and convection in the earth's mantle drive plate motion
Magma is created in three ways:
1. Raising the temperature
Can be caused by:
Friction between plates
Heat from the interior of the earth
Hot magma that rises towards the surface
2. Decreasing Pressure -
As rock rises in the earth, pressure on the rock decreases causing the rock to melt
3. Increasing the water content -
rock with a high water content melts at lower temperatures than rock with a low water content
Most volcanoes occur along plate boundaries
example - Ring of fire
Volcanoes can occur at other places besides the boundaries of plates though, this is called
Intraplate Volcanism
Intraplate Volcanism
is caused when the mantle develops a hotter than normal place called a
Hot Spot
An example of Intraplate volcanism are the Hawaiian Islands
Factors affecting volcanic eruptions:
Viscosity - the lower the temp the higher the viscosity (slower it moves), some lavas are naturally more viscous than others
Dissolved Gases - the amount of dissolved gas in the magma affects the type of eruption. Higher contents lead to more explosive eruptions
The composition of the magma affects the type of eruption -
Basaltic Magmas -
are the least viscous type of magma with the lowest amount of dissolved gases

Eruptions are usually quiet and relatively gentle

example Hawaiian Islands
Granitic Magmas -
have a high viscosity and dissolved gas content
eruptions tend to be explosive
example SP Crater in Arizona
Types of Volcanic Material -
Lava flows
Basaltic lavas travel quickly (10-300) meters per hour

Granitic lavas often move too slowly to be seen
Gases -
often are emitted continuously by the volcano, scientists measure the gases coming out of vents to possibly help predict an eruption
Pyroclastic material-
Magma that is ejected into the air, that solidifies before it reaches the ground
Types of Volcanoes
Shield Volcano -
Formed through the accumulation of basaltic lava flows
How are they formed?
What do they look like?
They have a broad slightly domed shape to them
Example -
Mauna Loa in Hawaii
Cinder Cones-
How are they formed?
Cinder cones form from ejected pyroclastic material
Usually form quickly in the space of a couple months to a couple of years
What do they look like?
Cinder cones are steep sided, usually at an angle of at least 30-40 degrees
Example - Paricutin Volcano, Mexico
Composite Cones -
How do they form?
What do they look like?
Form from a combination of lava flows and pyroclastic eruptions
can generate large explosive eruptions
Steep sided like cinder cones but can grow much taller over time
Example - Mount Fuji, Japan
Other Volcanic Features
formed when the magma chamber of a volcano empties and collapses leaving behind a crater
Volcanic Necks -
form when the magma hardens in the pipe of the volcano. The surrounding mountain is eroded away leaving behind the neck
Magma that hardens inside the earth, usually inside tubes associated with the volcano
A large body of intrusive igneous rock that has a surface exposure of more than 100 square kilometers
Mountain Building
In the Earth's crust rocks deform due to stress
This is usually associated with plate boundaries
The amount of deformation is influenced by
Rock Type
Different types of stress
These stresses cause different types of Folds and Faults
What are Folds?
Folds form when rock layers bend due to stress
Types of Folds
1. Anticlines and Synclines
Most common types of folds, form through compression
2. Monoclines
Caused by a fault in the bedrock below
Faults are formed when rock breaks due to stress
Parts of a fault
Normal Fault
form when the hanging wall moves down in relation to the foot wall
Reverse Fault
Forms when the hanging wall moves up in relation to the footwall
Thrust Fault -
similar to reverse faults
A low angle fault associated with folding of rock
Strike-Slip Faults
occur when the fault moves parallel or horizontal to the surface of the fault
Folded Mountains
Fault-block Mountains
Form in areas where folding and reverse faults are common
Formed by compressional forces
Associated with convergent boundaries
Examples: Appalachian Mountains, Himalayan Mountains
Form in areas of large scale normal faults
Formed by Tensional Forces
Associated with divergent boundaries
Examples: Grand Teton Mountain Range, Sierra Nevada Mountains
What are Faults?
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