Plate Tectonics by Blake Harris - 12/16/12

Have you ever stood on a beach in California wondering why Asia is coming towards you so fast? Probably not, because continents don’t move nearly this quickly. The rate of continental drift is only five centimeters per year. Even if you stood on that beach for your whole life, you wouldn notice anything.

The plate tectonics theory is the theory that Earth’s lithosphere (the top of the mantle and bottom of the crust) is divided up into separate pieces, called plates, that slowly move around on top of the asthenosphere. There are 13 plates, each a different size and shape, and each carrying either continents, seafloor, or both. Earth’s plates are constantly in motion, being pushed apart from each other or towards each other. Plates are moved by convection currents located in the mantle. Evidence that supports the theory of plate tectonics are volcanoes, mountains, trenches and faults. Other evidence that supports this theory are eruptions and earthquakes.   

At all convergent plate boundaries, two plates will move towards each other, causing the two plates to collide. If the two plates are both continental crusts, and since they have the same density, neither one will subduct under the other and they will crumple up to form a mountain range. This collision also will cause an earthquake to occur. An example of this type of convergent boundary is the Indian Plate pushing up against the Eurasian plate, which is forming the Himalayas.
-Feature: Mountains

-Event: Earthquakes

    Convergent Plate Boundary (Continental-Continental)                                        Himalayas in Asia

At convergent plate boundaries involving continental crust and oceanic crust, the denser, heavier oceanic crust, made up of basalt, will subduct under the less dense, lighter continental crust made up of granite. This subduction will form a deep-ocean trench, along with earthquakes. When the oceanic crust subducts into the mantle, it will heat up, melt, and rise up to the continental crust and form a volcano along with eruptions. An example of oceanic crust subducting underneath continental crust would be the Andes Mountains located in South America.

-Features: Volcanoes and Trenches

-Events: Eruptions and Earthquakes

   Convergent Plate Boundary (Oceanic and Continental)                                 Andes Mountains in South America

Convergent plate boundaries consisting of two oceanic crusts, the denser oceanic crust will subduct underneath the less dense crust.This subduction will form a deep-ocean trench, and the melting crust from the mantle will rise up to form a volcanic island arc. The collision of the two plates will also cause an earthquake to occur and will cause eruptions from the volcanic island arc. These island arcs have formed part of Alaska, the Aleutian islands, and they have been formed on the Philippines and Japan.

-Features: Volcanoes, Trench

-Events: Earthquakes, Eruptions

       Convergent Plate Boundary (Oceanic-Oceanic)                                        Aleutian Islands in Alaska

                      Mayon Volcano, Philippines                                                                 Mount Fuji, Japan


Plate motion at divergent boundaries is the opposite of convergent boundaries-- two plates will move apart instead of together. What happens at one of these boundaries is that one plate splits in two, and then those two plates begin to move apart. When one plate begins to be pulled apart, it starts to rift. This will then form a rift basin. When the basin becomes deep and wide enough, an ocean will form there and magma from the upper mantle will erupt from the mid-ocean ridge-- the place where the single plate had split. Magma will continue to erupt from the mid-ocean ridge and push the two plates apart. This type of plate boundary can be found in the middle of the Atlantic Ocean and also in other oceans.
Features: Rift Valleys, Mid-Ocean Ridges

Events: Eruptions

                                Divergent Plate Boundary                                                            Mid-Atlantic Ridge

Two plates at transform plate boundaries move neither towards each other nor away from each other. The two plates move in opposite directions alongside each other. The line where the two plates are separated is called the fault. The two plates moving against each other along the fault will cause earthquakes. One place where this is happening, located in North America, is the San Andreas fault in California.

Features: Faults

Events: Earthquakes

                         Transform Plate Boundary                                                      San Andreas Fault in California


As I mentioned in the first paragraph, earthquakes and volcanoes are both evidence for the theory of plate tectonics. Earthquakes are caused by two plates grinding against each other, like in transform and convergent plate boundaries. Two plates can only make these vibrations (the earthquake) if they are both moving, therefore proving the theory of plate motion. Volcanoes are also evidence. They are evidence of subduction zones. When a plate subducts, it enters the mantle and melts because of the heat. The now melted plate rises to the surface, and the lava then comes out of the crust through a volcano.
Both of these pieces of evidence can be found around the Ring of Fire. The Ring of Fire is the “ring” of volcanoes and earthquakes located around the Pacific Ocean. There is such a large number of these around the ocean because of the type of plate boundary-- Convergent Plate Boundary consisting of an oceanic crust and continental crust or two oceanic crusts. At each of these boundaries, subduction occurs. When the denser plate subducts, it heats up as it enters the mantle. The plate then melts and rises up towards the crust. The rising magma will form volcanoes, and since there are so many plates subducting around the Pacific, volcanoes are in abundance.
Earthquakes are also caused by these boundaries. The collision of the two plates, as you might imagine, would cause quite a vibration. Also, when the plate begins to subduct, the two plates grinding together have a similar effect. Again, since collisions and subduction happen all around the coast of the Pacific, earthquakes occur all around the Ring of Fire.

So, if you live to see the day when Asia and North America join, you’ll know why it happened.

Seafloor Spreading by Alexander Li

The theory of seafloor spreading is the thought that oceans are changing sizes and shapes due to mid-ocean rifts, thus changes the geography of the Earth. In order to put together the theory of seafloor spreading scientist had to know about three crucial geographic features: mid-ocean ridges, deep-ocean trenches, and the mantle.
 Mid-ocean ridges are the place where the oceanic crust is expanding. It is where oceanic crust is diverging allowing magma to flow up from the mantle to form into new oceanic crust. They are large underwater mountain ranges.
 A deep-ocean trench is a place where oceanic crust is subducting under continental crust due to the fact that oceanic crust is denser. Deep-ocean trenches are being pushed by the mid-ocean ridges.
 The mantle might seem a little random, but it is there because you need to know about the convection currents and how they push and pull the continental plates.
 All of those features fit together into the theory of seafloor spreading. When the convection currents in the mantle create mid-ocean ridges, the oceanic crust is pulled apart creating an opening in the crust to the mantle allowing magma to rise forming new oceanic crust. On either side of the mid-ocean ridge there is oceanic crust that is either subducting under continental crust at a deep-ocean trench, or pushing continental crust away from the mid-ocean ridge.


Click here to see animation for seafloor spreading

(http://panda3.phys.unm.edu/nmcpp/gold/phys161/lec/convection1.gif)

Red dots - Areas with volcanic activity in the last one million years
Yellow lines and Black lines and arrows - Fault or rift
Red lines - actively spreading rifts and transform faults
Blue lines - subduction zones
arrows on red lines - spreading rate cm per year
(http://en.wikipedia.org/wiki/Plate_tectonics)

There are several pieces of evidence behind the theory of seafloor spreading. They are: the igneous rock that makes up the oceanic crust; ages of the rock that makes up the oceanic crust; the magnetite pattern in the ridges.

One of the pieces of evidence is the igneous rock that forms along the mid-ocean rift. Scientists were puzzled why and how igneous rock formed under the ocean. The answer is the mid-ocean rifts allowing magma flowing from the mantle to rise up and form igneous rocks.

(http://serc.carleton.edu/images/research_education/cyberinfrastructure/ridge/pillowlava.v2.jpg)

 Another piece of evidence is ages of the oceanic crust. Scientists have found out what age of the sea floor is. They have fully mapped out all the oceans on Earth. If you look at the map below you will see the ranges of the ages of the oceanic crust. The younger crust would be found closer to the rifts because that is where the new crust is being created and the oldest crust would be father away from the rifts because the older crust is being pushed away by the newly formed crust.

(http://0.tqn.com/d/geology/1/0/V/J/seafloorage.gif)

 A third piece of evidence are the magnetic stripes that shows when the Earth's magnetic poles were reversed. The magnetic strips form when the magma rises. There is a magnetic mineral called magnetite in the magma that aligns itself with the magnetic poles when the magma is not solid yet. On the oceanic crust there are 2 different variations of the magnetite that point in different directions. Scientists think that the poles reversed periodically during Earths history. Since the strips form symmetrically across the rift, it shows how the oceanic crust is pushed evenly in both directions.

(http://en.wikipedia.org/wiki/Magnetic_striping#Magnetic_striping)

Link to animation of magnetic stripes

 Subduction at deep-ocean trenches determine whether the ocean grows or shrinks. If the oceanic crust subducts faster than the mid-ocean ridge expands, then the ocean would most likely shrink. If the trenches don't pull much under or are not there at all, then the ocean will most likely expand. When the oceanic crust subducts, the oceanic crust melts. Occasionally the melted material rises, because it is less dense, and forms a volcanic chain along the trench.

(http://www.vulkaner.no/v/vulkinfo/tomtech/image010.jpg)

Click to see an animation for subduction of oceanic crust at a deep-ocean trench

 Currently, the mid-ocean ridges and deep-ocean trenches are hard at work changing the geography of the Earth. The Pacific Ocean is shrinking due to the amount of trenches surrounding it. Also the East Pacific Rise is much smaller than all the trenches combined. to compensate for the loss of ocean in the pacific, the Mid-Atlantic ridge is currently expanding the Atlantic because of the absence of trenches. The Indian Ocean is also growing larger too.

2- Mid-Ocean Ridges
3- Deep-Ocean Trenches
(http://www.geothermal-energy.org/pliki/Image/geo/What_is_geothermal_en_html_2f5bdb15.jpg)

 In class we made models of the mid-ocean ridges and deep-ocean trenches. The slits A and C represent the area where the paper/oceanic crust subducts. Slit B represents the mid-ocean ridge where the new crust is being formed. The paper that has slit A, B, and C (the black line below) in is the border between the lithosphere and the asthenosphere. The paper that goes through slits A, B, and C represents the oceanic crust. The different colored markings on the paper that goes through slots A, B, and C represent the magnetic stripes

Interior of the Earth by Emilija Iannace 12/2/12

The Earth is not just a hollow sphere covered in rocks as you might have once imagined.  It turns out that the inside of the Earth is made of hot and dense solid and liquid materials.  The interior of the Earth consists of four main layers.  From the surface, the layers progress from the Crust, Mantle, Outer Core, and Inner core. As you go deeper into the Earth the heat, pressure, and density increases.  Each layer is hotter than the next so more pressure and density is added on from the layer above.




Of the Earth’s four layers, the mantle has the most diverse material.  The mantle consists of layers called the Lithosphere and the Asthenosphere.  The Lithosphere is a solid layer of brittle rock made up of the very upper mantle and the crust.  The Asthenosphere is in the mantle right below the Lithosphere.  Its rock is solid but able to flow, like tar, because it is a little bit lower in the mantle and is hotter.



The crust is the Earth’s outer layer of land and ocean floor, and it is the thinnest, coolest layer, averaging 32 km thick.  The crust consists of  granite continental crust and basaltic oceanic crust. Continental crust is thicker than oceanic crust, but the granite is less dense, while the oceanic crust is thinner, but the basalt is more vast and dense.



As we now know, the Earth’s lithosphere and asthenosphere are the two different layers in the mantle.  The lithosphere is a lot thinner than the asthenosphere, and is made of the crust and the top of the mantle, so the materials inside are rock.  The asthenosphere is thicker and softer (hotter), so the materials inside slowly flow.  Both layers have rock materials and are above the mantle.



The Earth’s mantle is the next layer below the crust, and it contains the lithosphere and the asthenosphere.  The mantle plays a big part in the Earth’s shifting plates because it creates convection currents in the asthenosphere from the core’s heat.  If the convection currents flow in opposite direction, the heat pries apart the crust and causes rifting and earthquakes.  If the currents flow towards each other the plates collide and build mountains.




The outer core is thicker than the inner core, and it is full of molten metal, while the inner core is full of solid metal.  The inner core is almost twice as hot as the outer core.  They are both made up of nickel and iron, and together, they create the center of the Earth.

The Earth’s outer core is responsible for creating Earth’s magnetic field.  The molten nickel and iron of the outer core rotates, and the movement creates magnetic and electric charges.  Imagine that the Earth has a giant bar magnet inside of it.  The magnetic field is connected from the North end of the magnet to the South end.  The size of the field changes, and sometimes the field reverses (North Pole becomes South Pole).  The Earth’s magnetic field extends thousands of kilometers  into space, creating a sort of magnetic bubble called the magnetosphere.

 

In class we looked at half a hard-boiled egg and half an orange that could be used to describe the Earth and its layers. Both were strong representations of the Earth’s interior, but some parts also had weaknesses.  The hard-boiled egg had a thick, solid shell like the Earth’s crust.  The egg white made an accurate mantle, and the yolk was a good imitation of the whole core.  The only problem with the hard-boiled egg model was that it had an oval shape, unlike the round Earth.  Also, the whole core was solid, instead of the outer core being liquid with a solid inner core.  

The orange half had all of the layers, with an accurate crust, mantle and core.  The orange’s juicy flesh part showed a liquid core and a solid inner core.  The weaknesses of the orange half were that the outer core was too big and the inner core was too small.