The Forever Moving Land

The Forever Moving Land


     The land below us is always in motion.  Plate tectonics studies these

restless effects to give us a better understanding of the Earth and its past.

New molten rocks are poured out in the form of magma from the mid-ocean ridges.

The rock is recycled and re-entered back into the earth in deep ocean trenches

through convection current.  The convection current in the mantle drives plates

around either against or away from each other.  These collisions give rise to

earthquakes, volcanoes, mountains, and continental drift.  The crashing and

spreading of the plates forms the landscape of the Earth as we see it today.

The positions of the land masses today is a result of continental drift.  During

the Earth's existance, the magnetic fields have never been stable.  Solidified

magma containing magnetic imprints reveal periods of time when the Earth's

magnetic fields have actually been reversed.

     Approximately 4.55 billion years ago, the Earth was just a ball of

molten material.  Since then, parts of the Earth have cooled forming the solid

crust-mantle.  This process has been occurring for roughly about 3.8 billion

years.  The mantle is about 2900 km. thick, which lies above a layer of molten

magma that still exists today.  The immense heat from the magma (approximately

2700(C) causes convection in the mantle (Figure 1).  Convection is caused by

non-uniform temperature in a fluid and density differences.  This continuous

convection is the cause of plate movement.  Each complete cycle, called a

convection cell, drives the plate in the direction of the cell.  How does a

'solid' mantle move?  The mantle may be solid but, as with most solids, it will

deform if long term stress is applied;  "...like Silly Putty which seeps into

the rug when left unattended, mantle material flows when subjected to small

long-term stresses."1

     Presently, there are more than fourteen plates in the Earth's crust

(Figure 22).  Upwelling hot magma flows out from mid-ocean ridges and then cools

down when exposed to the cooled environment outside;  the layer of cooled magma

forms the lithosphere.  When magma flows out from the ridges, the crust is

fractured and a new ocean floor is built spreading perpendicularly away from the

ridge.  Because of this constant upwelling, the ocean is relatively shallow in

these areas.  Sea floor spreading and continental drift are the products of this

continual upwelling.  The cooled magma will, in time, sink back down into the

Earth in the deep ocean trenches.  The mantle sinking down produces subduction

zones or Benioff zones.  The deepest part of the ocean resides in these areas.

There are three types of boundaries where plates meet: divergent boundaries --

the upwelling of magma;  convergent boundaries where the plates collide

producing mountains, volcanoes, and earthquakes;  and transform boundaries --

lateral movement.  Transform plates are caused by fracture zones.  When a rift

opens from the upwelling of magma it causes a crack in the crust.  As new magma

rises to the surface, the crack increases caused by the pressure, resulting in a

horizontal faulting.  The fractured plate pieces travel in the same direction as

the original plate was traveling -- away from the ocean ridge.

     During the early 1900's, a theory of a 'super-continent' was developed

by Alfred Wegener.  He was ridiculed for his ideas that continental drift

produced the present positions of the continents from a single 'super-continent'

called Pangea.  This theory is widely accepted today, however.  There was

abundant evidence for Wegener to believe in the existance of Pangea.  The shape

of the continents could be pieced together like a giant jigsaw puzzle suggesting

that the continents were once 'glued' together.  The fossils found on the

continents were not distinct to that particular land, but were also found in

lands that were separated by thousands of kilometers of water.  Fossils

indicated that identical species existed in different continents.  Geological

structures also d
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