Making Mountains

• Fold and thrust mountains
• Upwarp mountains
• Fault block mountains
• Volcanoes

Introduction to Fold and Thrust Mountains

By far, most mountains on the continents are of this type.  They are produced by forces of compression and almost always at a plate boundary (the Rocky Mountains are an exception).  This means that converging plates are responsible for this type of mountain.

When forces of compression act on rock, the rock may break, creating a reverse fault.

Here is a good page dealing with faults

However, if the force is applied over a long time period the rock may actually fold and bend rather than break.

What decides if a rock will fault or fold?
 

• Temperature:  Intrusive rock becomes very hot when compressed.  The hotter it is, the more like it will soften and bend/fold.  The cooler it is, the more brittle it is likely to be (more likely to break/fracture ... producing a fault).

• Pressure/Confinement:  Confined rocks at great pressure are more likely to fold and bend.  This can happen deep in the earth or at a plate boundary where plates are converging.
   
• Rock Type:  Rocks can be made of many different combinations of elements.  Certain elements tend to form rocks that are brittle and other elements tend to form rocks that are more bendable.  Sandstone is a rock type that is very brittle and would more likely break/fracture.  On the other hand, rock salt is far more likely to bend or fold.

• Time:  Rocks might have the stress applied suddenly and they can't bend quickly enough so they break instead.  If the stress is applied more gradually, the rock can adjust to the change and is more likely to fold.

When Plates Collide ... Mountains are made!

Fold and Thrust Mountains

We already know that when Continental Plates and Oceanic Plates meet, the oceanic plate is subducted under the continental plate. The tremendous forces of compression creates faults, folding, and uplift ... subducted crust causes melting ... creating volcanoes ... basically, all "heck" breaks loose.

This describes the Andes in South America, the Cascades in the Northwest US and the (inactive) Sierra Nevada and Coast Range of California.

Most of the mountains formed in this collision are volcanic in nature (from the melting below) but there is also major uplift of the continental crust as the sea floor is driven under it.  In fact, as the sea floor is being driven down, the edge of the continental crust act like a "wedge" which scrapes off the lighter sea floor sediments ramming into it.  This creates a thick deposit of sea floor sediments called an "accretionary wedge".  It should be clear that this material is simply recycled continental rocks which was washed to the sea by erosion ... but now they have been transformed to sedimentary (marine) rocks.
 

The Sierra Nevada Mountains are highly eroded (volcanic) mountain roots which resulted when the Pacific Plate once was subducted under the area (it no longer active).  The Coastal Range of California was an accretionary wedge forced up as the plates scrapped against each other and sediments accumulated.

Colliding Continental Plates:

When continents collide it gets even more dramatic.  The Himalayas, Alps, and the Appalachians are examples.

The ancient Greeks were puzzled as to why there were marine fossils on summits of mountains.  The collision of continental plates first start out by putting the "big squeeze" on sections of oceanic crust.  This produces a thick accretionary wedge (on each continental margin) prior to contact.  This material is ultimately found on the summit of a mountain peak.  What happens when the continental plates meet?   The edges of the continents could fold upward to form mountains. An example of this is the Appalachian Mountain range which was formed when the African Plate collided with the North American Plate in the formation of Pangaea (about 300 million years ago).  The Himalayan Mountains formed as a result of the collision of the Indo Australian Plate with the Eurasian Plate about 50 million years ago.
   

Isostasy

(animation)

Finally, as long as we are discussing crustal uplift ... we need to talk about the concept of isostasy.  This is the idea that the crust is "floating" on the mantle (similar to the way an iceberg will float in the ocean).  Icebergs have a lower density than the surrounding salty seawater so they float ... but always so that 9% of the ice volume is above sea level (which means that 91% is hidden in the water).  What happens if some of the ice (at the surface) melts?  The entire iceberg will move upward so 9% will again be above the surface.  The same can be said about a boat floating in the water.  If a passenger jumps off the boat, the boat responds by moving up slightly.   It is believed that the crust in mountainous regions acts the same way.  The concept that the crust will adjust itself in this way is known as isostasy.

Now imagine that erosion strips a great deal of material from a mountainous region.  The entire land mass will respond by uplifting.  It is believed that a similar reaction is occurring presently in  regions of northern Canada (Hudson Bay) because huge volumes of mass were "recently" removed when the ice sheet retreated (over 300 meters of uplift has resulted).
 

Upwarp Mountains

Most of the time, mountains are formed at the edge of colliding plates ... but not always.  A section of a plate may experience horizontal tectonic forces of compression which make the surface warp upwards.  When this happens it is exposed to increased elevation and is subject to greater erosion.  The Bighorn mountains in Wyoming and the Black Hills (Mount Rushmore) of South Dakota (both associated with the Rocky Mountains) are upwarp mountains.

The Badlands in South Dakota is an area of sediments that is now exposed to the surface.  It was once material that was weathered and eroded from the nearby Black Hills (an upwarped mountain range).
 
How is it possible to form a mountain far from an existing plate boundary?  When I first studied mountains, geologists were clueless because this just didn't fit into the model of plate tectonics.  Now geologists may have an answer.  It is believed that a section of oceanic plate is, indeed, involved in the process.  Suppose a section of oceanic plate was subducted under the North American plate.  If this section of oceanic plate slid under the North American plate (like sliding one piece of paper under another) instead of being forced deep into the mantle, you can account for the forces of compression necessary to make these mountains.  Apparently this is what happened to form sections of the Rockies some 60 million years ago.

(animation)

Fault block mountains

Click here to see animations of faults

On occasion, crust can be pulled apart ... creating forces of tension.  This may be caused by material below the surface welling upward, creating tension stress on the rocks above. A normal fault occurs when the rock splits due to these forces.

Permission from http://www.gpc.edu/~janderso/physical/geo101.htm click here

This can produce a long chain of mountains, the most famous are the Grand Tetons in Wyoming.

A normal fault can produce some geologic features such as a graben.  The most famous are Death Valley in California, the Dead Sea, and the African Rift Valley.
 

Most of Nevada has "strings" of mountain chains which seem to go forever.  This is known as the "Basin & Range" province which are all produced by forces of tension.

Ok, so how do you account for these forces of tension in the crust?  It is believed that about 65 million years ago a section of subducted oceanic crust rested under the continental plate and began to sink into the mantle.  Hot material from the mantle rose up into the void that was left behind.  As this material moved up, it created forces of tension on the continental crust above.

(animation)

The last section in this unit deals with the last kind of mountain - Volcanoes!

©Jim Mihal 2004, 2006 - all rights reserved