The Formation of Tectonic Plates and the Continents of the Earth

  Soon after the molten, hot, liquid round mass we now call earth was formed (4.5 billion years ago), it began to cool.  During this cooling  process lighter materials floated above a heavier liquid mass.  This separation of materials caused layers to form.  The cooling lighter layers formed a thin skin or crust around the outside of the earth.  This lighter (or more buoyant) crust floated on top of a thick liquid mantle (see Figure 1).
 

Figure 1.  This diagram shows a cross-section of the earth.  The major tectonic plates of the Earth are shown in tan.  Arrows indicate the direction of plate movement.  The continents are shown in color (Re-drawn from Strahler and Strahler 1987).

    The Earth's crust is about four miles thick under the ocean and may be up to forty miles thick under some mountain ranges.  The Earth's mantle lies directly under  the crust.  It is a layer of rock about 1800 miles thick.  Heat and pressure at this great depth keep the material in a very thick liquid state.  This thick (or viscous) liquid flows in sluggish currents as you might imagine thick, cooling lava to flow.  Beneath the mantle is the Earth's outer core. It is hotter and under more pressure than the mantle so it has more fluid-like properties.  This "liquid rock" is comprised mainly of silicon (what sand is made of) and iron and nickel and it is about 1400 miles thick.   The center of the earth is a solid ball 1540 miles in diameter comprised mainly of iron and nickel.  Intense pressure at such great depth keeps the inner core from turning liquid despite a temperature of 3700 degrees Celsius.
 

    As cooling continued, the crustal materials began to move on the sluggish currents.  This movement eventually caused the crust to break apart forming tectonic plates.  Most scientist think there are about eighteen tectonic plates on the Earth.  These plates are are about 60 miles thick (the distance from Fort Collins  to Denver) and are constantly moving, but at a very slow pace.  Currents of heat (convective currents) rising from deep in the core and up through the mantle, cause the plates to move.  As the plates drifted around the globe, they collided one-by-one, until they became grouped together in one large mass.   This all was occurring about 500 million years ago.  About 200 million years ago the plate movements brought  the pieces together into one single land mass called Pangaea (see Figure 2).  Pangaea is Greek for "all lands", which was comprised of all the continents:  South America, Africa, India, Antarctica, Australia, Asia, and Africa.  About 20 millions years Pangaea broke into two continents, Gondwanaland and Laurasia.

Figure 2.  Map showing the giant continent Pangaea (comprised of Gondwanaland and Laurasia) as it might have appeared 250 million years ago.  Note the approximate location of the land that is now the state of Colorado!  (Re-drawn and modified from McKnight 1994).

    Geologists refer to active margins when discussing mountain building.  These active margins are where tectonic plates come together.  Volcanoes and earthquakes are much more frequent around the boundaries of tectonic plates.  It is also at these boundaries that plates collide and scrape against one another.  It is the scraping of plates against one another that causes earthquakes.  As humans, we become very aware of these on-going processes when we hear about (or get caught in!) natural phenomenon like earthquakes and volcanoes (visit the USGS Cascades Volcano Observatory, United States Geological Survey, Volcanoes and Natural Hazards, and the  USGS National Earthquake Information Center homepage links).  When plates collide they may either slide over one another or collide head on (like a car wreck).  These head on collisions cause buckling and folding in the plate.  This is the fundamental physical process that facilitates mountain formation.
 

Plate Collision and Mountain Formation

    The process of mountain building is called Orogenesis.  When plates collide head-on (plate-to-plate collision), tremendous force is generated.  When a plate that is moving hits a plate that is stationary or is moving towards it, energy is transferred inland and the pressure generated causes uplift.  When one plate rides over another (subduction), the top plate may buckle and crack where it bends upward.  This uplift causes materials from deep within the Earth push upwards generating mountains (see Figure 3).
 

Figure 3.  Schematic illustration of subducting plates.  The gray plate and yellow plate collide and one rides over the other.  The pressure from the yellow plate (which is deep beneath the Earth's surface) causes buckling and uplift of the crust (Re-drawn from Holdaway, 1997).

    One possible definition of mountain is any part of a land mass which juts up conspicuously above its surroundings.  The word mountain is a relative term.  Are the Rockies more of a mountain range than the Appalachian Mountains?  What some might consider mountains, others might think are more like foothills.  There is a "mountain" in Florida that is only 120 feet tall!  Our tallest peak in Colorado, Mt. Elbert, is 14,433 ft high.  The Himalaya mountains in Asia, contain the highest mountain peak in the World, Mt. Everest.  Mt Everest is 29, 029 ft high!  If you lived in the Himalayas, the 14,000 foot  peaks that we see along the Front Range of Colorado would seem like foothills.  As Albert Einstein would say, "Everything is relative".
 

    Mountain building occurs along these mobile belts that slide around the earth atop the fluid outer core. The "newest" mountains are the ones on the rims of continents.  Older mountain ranges tend to be on the interior of continents.  Which do you think is older?  The Cascades of California, The Rocky Mountains or the Appalachian Mountains.  Why?
 
    Most mountains occur in rows called ranges not as independent peaks in the middle of nowhere.  Parallel ranges and intervening plateaus form chains such as the Andes and the North American Cascades and Rocky Mountains.    Why do mountains occur where they do?  Look at the relief map on the Colorado Front Range homepage.   Look at the mountains, and then come back here to answer this question. 
 
     The Himalayas are now the fastest growing of the mountain chains.  What might be the relationship between plate tectonics and the rapid rate of growth of the Himalayas?

    Which of the mountain building processes above (plate-to-plate collision or subduction) formed the mountains of western North America?
 
    Another type plate movement that causes mountain formation is sea-floor spreading.  This  means the plates are pulling apart.  Molten rock fills in the gap, in effect creating new crust.  This process creates undersea mountains.

    You may wonder how this actually works.  Remember, when plates subduct, one plate is forced under another.  When this happens the plate remelts and enters the mantle.  It is this remelted mantle rock that emerges in the gap that makes new crust.  The is a perpetual process (meaning that it never stops).

    What is the relationship of earthquake zones to fault lines in the earth?

    Let's now bring this discussion back home to Colorado and talk about the landscape that we live in.