Folding and Mountain Building

Here are some more variations you might see:

• A plunging fold means that the fold axis is not horizontal. There is an angle to the fold axis with respect to the earth.

• A symmetrical fold means that the limbs fold away from the hinge at the same angle. If this doesn't happen, it is asymmetrical.

• You can also get a really wacky fold called an isoclinal fold, where the limbs have gotten so bent that they are parallel to one another. This would be the geological equivalent of a hairpin turn.

• If a fold falls over due to pressure on one side more than the other, it is an overfold. An overfold that is extremely folded over is called an overturned fold.

• The most extreme overturned fold is a recumbent fold, where the limbs are hairpinned back to back and the entire thing has fallen over onto its side.

• Chevron folding is zigzag folding where none of the limbs are curved.

Folds affect the topography of the land. Once weathering and erosion happen, the rock types less resistant to it will be eroded away, leaving behind more resistant layers. Now you'll get ridges where layers have resisted erosion and valleys where layers have given into erosion.

FOLDING AND MOUNTAIN BUILDING

Mountains are built out of deformations of the earth's crust. Most mountains are a mixed bag of folding but you'll see certain patterns in mountain building that are created out of specific deformation trends. Three types of mountains can then be identified:

• Fault block mountains – these are made by different patterns of faults. Reverse faults or thrust faults will push up more brittle rocks into mountain ranges. Some known fault block mountains are the Sierra Nevada Mountains, the Tetons, and the Harz mountains located in Germany.

• Fold and thrust mountains – these are due to large compressional areas of stress so great that crusts from two tectonic plates collide. Folding and faulting happen together to make the Himalayas, the Appalachians, parts of the Alps, and the

Rocky Mountains.

• Volcanic mountains – you know all about these. These are not deformational.

Magma simply builds these mountains up by the bottom up. The Hawaiian

Islands, Iceland, the Cascade mountains, and many others are volcanic mountains.