Rey writer

Plate Tectonics is the driving force of geologic structures that shape the world that we live in. The

first indication that continents might have been linked physically took place as soon as there was

mapping. It was known by the sixteenth and seventeenth century that the continents fitted together like

pieces of a puzzle. This and other proofs listed below lead to the concept of continental drift, that the

large continents had once been linked together. The scale movements of continents across the across

the surface of the planet was mind boggling.

By the close of the nineteenth century the geologist Edward Suess postulated that the southern

continents were joined together. This was from evidence seen by fossil animal distribution. Ancient life

forms have ranges just as modern animals. Tigers are found in India and Siberia, not in North America.

These ancient animals couldn’t get from South America to Africa, from Antarctica to Australia to India.

There were rock layers both sedimentary and igneous that were continuous on all of the five continents.

He christened this large continent Gondwanaland.

In 1915 Alfred Wegener wrote a book on continental drift. He proposed another super continent

called Pangaea. This continent included all of the modern continents. His mechanism for the movement

of the continents was that the continents moved through ocean crust, drifting until they joined then

breaking apart again. Wegener and others argued for their evidence pointing out rock similarities

type, similarities in rock trends and ages using all of the information gathered from Steno's laws and

fossil assemblages. They also showed that certain distant continents had similar plants and animals that

entered a different evolutionary path after the continents separated. An example of this is the fresh

water fish the Arowana found in both South America and Australia. Another example of this is the large

amount of marsupial

fossils found in South

America and Australia.

Placental mammals

were the dominant

mammalian life forms

in Eurasia, North

America and Africa.

Only in South America

and Australia were

marsupial fossils

dominated. It wasn't

until 3 million years ago

North and South

America joined

allowing placental

mammals to gain

dominance in South America. Despite this evidence Wegener and his followers mechanism for

continental movement was badly flawed. They proposed that the tidal forces from the Sun and Moon

like the tides in the ocean. This force is much too weak to move a continent so the theory was rejected.

Fossil plants and animal distribution on southern continents

In 1928 a geologist Arthur Holmes proposed the convection currents split the continents and that

these currents were in the mantel. Remember the knowledge of a mantel was already known at this

time. Ironically this was a mechanism had been proposed by Benjamin Franklin in 1782. Many

geologists argued against this and Holmes bowed to their criticism.

It wasn't until WWII that there was evidence to

show that Holmes (and Franklin) was correct. A

marine geologist, pioneer of marine seismology,

deep ocean photography by the name of Doc Ewing

found that the sea floor was composed of young

basalt, not old granite (we knew absolute age

dating this time) as originally thought. The other

finding was that there was mapping of the

undersea Mid-Atlantic Ridge. In the center of this

ridge was a rift valley. These were mapped by

Marie Tharp and Bruce Heezen. It was found that

the rift was and active feature with volcanic activity

as well as seismic activity. Harry Hess and Robert

Dietz proposed that the Earth's crust separates

along the rifts and forms new oceanic crust.

At this point in time many thought the Earth was continuously expanding. It took a massive

earthquake (1964 Alaskan Earthquake) to show the destruction of the ocean floor. Here a large portion

of the seafloor moved beneath the Alaskan crust. In 1965 J. Wilson proposed the theory of plate

tectonics. That seafloor was recycled down deep trenches and at these regions there were earthquakes

and volcanoes. This movement explained the rock folds and faults. It also explained many of the

minerals. It was also shown that the rates and directions of movements of crustal rocks were

mathematically consistent. Wilson the proposed plate boundaries. There are three of them, divergence,

convergence and transform.

Divergence, found in the Mid-Continent Rifts, created the new seafloor, calling it seafloor

spreading. Mantel temperature difference drives this. A plume of heat or possible actual upwelling

mantel moves upward to the plastic asthenosphere. Here convection currents developed. The

asthenosphere drags against the lithosphere splitting it. New material from the asthenosphere moves

upward and forms a new ocean crust material. This happens in the rift valleys of the Mid-Continental

Rifts that bisect the ocean floor.

Rift valleys form in the center of continents forming with volcanoes and earthquakes as the continents

tear apart. This can be seen in the Great Rift Valley of East Africa creating parallel valleys. The ocean

plate if produced by basalt (iron rich and silica poor igneous rocks). The minerals found here are olivine

and pryoxine and some feldspar. When rifts are on the continent the upwelling magma first melts

continental crust so you find quartz, mica, and feldspars. An example of a continetal rift valley is the rift

valley found in Iceland. Here new Atlantic Plate is being made as North America moves away from the

Eurasin Plate. Iceland sites on top of the Mid-Atlantic Rift. Not all rifts splits continents. Instead some fail

leaving zones of weakness. An example of this is our own New Madrid Seismic Zone as well as the

Wabash Seismic Zone. These rifts failed hundreds of millions of years ago.

There are continental rifts that you see today that have been successful. Here the rift valley continues

until new seafloor made up of basalt

forms. These flood forming a linear sea

as seen in the Gulf of California (Sea of

Cortez) and the Red Sea. At the Gulf of

California, Baja California splits off of

the North American Plate forming new

seafloor. The Red Sea is the breaking off

of the Arabian Plate from the African

Plate.

The final phase of divergence is the

expanding ocean. The upwellings have

created underwater mountain chains.

The mountain chains are split by the rift

valley where active volcanoes and

magma create new ocean floors.

Convergent boundaries are scattered

over the globe. Here plates collide and

ocean crust is recycled back into the earth for two of these boundaries. There are three convergent

plate boundaries: 1. Ocean-Ocean, 2. Ocean-Continent, 3. Continent-Continent. These convergent zones

are marked by forms of metamorphic rocks and specific minerals. They are also marked by deformation

of rock layers with folds and faults as well as volcanoes and earthquakes.

Ocean-Ocean Convergent plate boundaries has one plate descending beneath another. This is called

subduction. This event takes place along in narrow deep-sea trench. The trenches are the deepest

portion of the ocean. The some descend as deep as 11 km, an example and the deepest the Marianas

Trench of western Pacific. At this an older portion of the oceanic lithosphere sinks into the

asthenosphere and then recycled

into the mantle as the

asthenosphere convection currents

drags it down. The water from the

subducting plate is squeezed out of

the hydrated minerals and rises

into the asthenosphere. There

the water drops the melting

temperature of the asthenosphere

and this molten material creates a chain of volcanoes, an island arc. There are many examples of this,

Indonesia, Philippians, Aleutians, and Japan. Here you can find volcanoes and major earthquakes. Some

of these earthquakes are small and some huge like the magnitude 9 in 2004 at Sumatra and the recent

2011 magnitude 9 at Japan. The minerals here have undergone both mixing with ocean crust and

fractionation and are intermediate in nature.

Ocean-Continent Convergent plate boundaries have subduction again. Here the heavy iron rich ocean

lithosphere sinks beneath the silicone rich continental crust. The continental margin crumples and

deforms and uplifts rocks into mountain chains that parallels to the deep sea trench. As the oceanic

lithosphere moves beneath the continental crust lighter material is scrapped off onto the continental

crust expanding the continent.

Again the water from the hydrated

minerals melts the asthenosphere.

This material rises up and melts

continental crust. Such a boundary

can be seen at the Cascade Range.

It can also be seen in South

America were the Nazca plate dips

beneath the South American Plate

creating the Andes. This is the

location of the largest earthquake in recorded history, magnitude 9.5 in Chili in 1960 and a magnitude

9.2 in Alaska in 1964.

Continent-Continent Convergence takes place when two continents collide. This is how Pangaea and

Gondwanaland were assembled. It happens when one oceanic crust being subducted under a

continental crust has a continent on it. As the subducting plate sinks the continental crust collides and

the two continental lithospheres join. This collision doubles the thickness of the crust at this area. The

Indian and the Eurasian plates have

done just this raising up the Tibetan

Plateau and the highest mountain

range that exist today, the

Himalayan Mountains. Other

example of this is the Alps, and the

Pyrenees Mountains. In the past the

continuous collisions from 440 to

approximately 300 million years ago

formed the Appalachian Mountains on our own continent.

The last boundary is the Transform-Fault Boundary. These boundaries are normally associated with

divergent plate boundaries. They offset the Mid-Oceanic Ridge and allow for differences in magmatic

upwellings. In certain areas this boundary can be found on continents. One such area is the San Andreas

Fault in California and the Anotoli Fault in Turkey. Both have been sites of major earthquakes and

thousands of deaths. Notice there are no volcanoes associated with this plate boundary.

Plates are normally bounded by multiple boundaries. Our plate has the Mid-Atlantic Oceanic

Ridge (divergent plate boundary) and two (or more) transform-fault plate boundary. One of these

transform-fault plate boundaries is between the North American Plate and the Carrabin Plate. This was

the location of the Haitian earthquake in 2010. The other is the transform-fault boundary between the

Pacific Plate and the North American Plate as represented by the San Andreas Fault. We also have a

transform-fault plate boundary in Canada. Our plate overrides the Pacific Plate at the Aleutian Islands

and continues to a portion of Japan (our plate is more then North America). Our plate also boarders

three different convergent plate boundaries, in the Pacific Northwest our plate override’s the Jaun de

Fuca Plate. In the country of Mexico the North American Plate overrides the Cocos Plate. We have a

junction with Eurasia that has not been identified (probably a transform-fault).

The proof of the divergence and seafloor spreading was recorded on the floor of the ocean

itself. Geologist modified the magnetometers developed to detect submarines from the magnetic fields

from their steel hulls. These modified magnetometers were sensitive the magnetic field on theocean

floor. They discovered a magnetic anomaly. The magnetic field alternated between high andlow values

in narrow parallel bands that were symmetrical with the mid-ocean ridge. The reason for the magnetic

field is the mineral magnetite. It is charged and floats free in magma until that magma cools. Once the

magma cools the magnetite will point to magnetic north. The anomaly was an increased field

alternating with a decrease in the field. The

increase in the magnetic fields is oriented

to where the magnetite points to our

magnetic north. The decrease in the field is

when the magnetite points in the opposite

direction. This reversal shows dramatic

change in the geodynamo so that the field

reversed. This stripping on the seafloor

parallels the mid-oceanic ridge. The

stripping where the field is normal or

reversed are called magnetic chroms.

When combined with absolute age dating

this was proof spreading centers.

Deep-sea drilling combined

with absolute age dating gives us the

seafloor isochroms. The youngest rock

is found at the spreading centers. The

oldest rock is found adjacent to

specific continents. The oldest rock is

180 million years old. The continents

are much much older. In our own state

the highest point, Taum Sauk

Mountain, is 1.28 billion years old. This

means that the seafloor is new and now gives credence to Wagner’s Pangaea. We can reconstruct the

plate movement by using the transform-fault plate boundaries we can deduce the movement of the

plates. The isochroms will place the continents at different points of time. Because of subduction the

original seafloor was destroyed by subduction returning to the older rock

information we can rejoin the jigsaw puzzle. Using old mountain belts such as

the Appalachians of North America and the Caledonian Mountains in

Iceland and Scotland we join North America to Europe. The fossils

magnetism of continental fragment record it ancient orientation and

magnetic latitude. This can be augmented by paleoclimate information

from the fossils and minerals. Look at the on pages 70 to 71 to see the

growth of the world as we know it and our potential future position. To

get a picture of the time it will take to get to future world here is a simple

exercise .Assignment Using the fact that the plates in Iceland are moving

apart (creating new land for Iceland) at 25mm per year how long will it take for

Iceland to grow 1 km.

Further proof of this theory of plate tectonics came with GPS (global positioning system). GPS stations

set up over continents record the changes in positions of the continents. While this gives us an accurate

measurement of the speed of the boundaries for this brief period of time it doesn't give us changes of

plate movement over time. For that do the exercise. Here absolute time calculated by isotope decay and

position curtsy GPS can be combined to give changes in plate movement over time.

The movements of these plates are powered by our geodynamo. Convection currents upwell and to

work on the plates. There are two thoughts of for the convection currents. one proposes one current

circulating between the upper and lower mantel. The other theory is that of two convection currents.

the convection current of the lower mantel and creates a convection current in the upper mantel. The

felling at the moment is that the spreading centers are controlled in their movement by the descent of

the plate at the subduction zones. This is called slab pull ridge push causing a slow upwelling at the

spreading centers along the fracture represented by the rift valley.

There is another upwelling from the mantel. This is the mantel plume. A narrow slender cylinder of fast

rising mantel material. These areas are often far from the spreading centers. An example of mantel

plumes can be seen in the Hawaiian Islands, the Galapagos Islands and Yellowstone.