Plate Motion

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Plate Motion Types

Divergent (pulling apart from each other)

Convergent (pushing together)

Conservative (can slide past each other)

There is no gap opened or closed in this motion, so surface area is conserved

The plate boundaries are described as divergent, convergent or conservative

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Hot spot tracks

Hawaii sits in the middle of the Pacific ocean, near the centre of the Pacific tectonic plate

A string of volcanic islands including Kauai, Oaha, Molokai, Maui and Hawaii known as Big Island

Hawaii is currently volcanicly active since Jan 1983, most voluminous in over 500 years.

By the end of 2016 lava flows covered 144 square km adding 179 hectares of new land to the SE shore, 4.4 cubic km of lava.

Over 10 km in height from base 6km below sea level making it taller than Mt Everest

Aer the flows have cooled and solidified, they are colonised by plants and weathering commences

Eruptions build up the volcano, weathering knocks it back down again

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Hot spot tracks continued

Over time volcanoes like Hawaii sink, the weight of the volcano causes a big dip in the lithospehere underneath it

This causes subsidence

The lithosphere deforms like a sagging bookshelf and the underlying athenosphere deforms underneath it, moving out of the way

The ocean floor has a chain of seamounts (submarine peaks) following on a line from the Hawaii island chain. The Hawaii islands are only a small visible part of the submarine mountain chain

These are extinct volcanoes which have sunk back beneath the waves

This is a very long line tracking along the pacific sea floor up the aleutian trench, over 6000 km long

From Hawaii radiometric dating shows an increase in age all the way along the Hawaiian Ridge and Emporer Seamounts from SE to NW

A dog leg occurred along the ridge dated to 47 Ma

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Hot spots continued

Island seamount tracks like Hawaii are called hot spot tracks, they sit on extremely hot spots of mantle running 2000 km down in the earth.

They are called mantle plumes, much hotter than the surrounding areas they are less dense which makes them bouyant and causes the hot material to rise

The plume of hot mantle produces a hot spot , it partially melts the mantle and the magma gets to the surface and errupts as in Hawaii

The mantle plume is stationary central to the centre of the Earth, the lithosphere is not fixed and moves across this fixed point. The volcanoes can be dated from errupted magma and shows a track of when that section of the lithosphere was above the plume.

The direction of the volcanoes getting older shows the direction of motion of the plate

The chain of volcanoes produced by a hot spot records plate motion

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Hot spots continued

Speed of plate motion can be calculated using the precise location of the volcano and the age

The trend of volcanoes aging to the NW shows that the plate is moving in this direction

speed = distance/time

the seamount dated 47 Ma is approx 4000 km away from Hawii (Big Island)

4000 km is 4000 000 m

the plate has moved 4000 000 m in 47 000 000 y

speed = 0.085 m/y

multiply by 100 to show speed as 8.5cm/y

The pacific plate is the fastest moving of the plates

Prior to 47 Ma the pacific plate was moving in a due N direction

Three plumes forming three seamounts in the Pacific ocean support the plumes being stationary relative to each other.

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Absolute and relative plate motion

Analogy using trains, the trains movement is relative to the absolute train station, the two trains move relative to each other

One train moving out of the station while the other is station are described as being in relative motion as the distance between the two trains is changing

A fixed reference is needed for absolute motion, the train moved away from the station in absolute motion the stationary train is not in absolute motion.

To determine the movement of the plates, fixed references such as the mantle plume (volcanoe island chains are needed) this tells us about absolute plate motion in the past

Hot spot tracks are a record of absolute plate motion in the past

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Relative and absolute motion

Two plates on a collision course are the South American and Nazca plates, they are moving towards each other, an example of a convergent plate boundary

The Nazca plate is moving away from the the Pacific plate, this is a divergent plate boundary

Even though the Phillipine and Pacific plates are moving in the same direction the Pacific plate is moving quicker and this is a convergent boundary as they will collide.

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How do we know plates move?

Earthquakes are evidence

Help us locate boundaries

vectors determine the direction and type of boundary

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Measuring plate motions today

GPS stallites in space are used to look at precise locations on points on the earths surface

GPS studies have allowed us to investigate modern plate motions and more about plates that do not have the hot spot tracks on them.

Eastern Papua New Guinea is very active in plate tectnonics

There are many more plates than the 7 major ones

The vectors have all been calculated using GPS

Plate tectonics means that there is always change on the planets surface