Mountains

#1 A mountain is a large mass of rock that rises a great distance above it’s base. Most mountains result from forces associated with the processes that occur at converging plate boundaries.

#2 Most of the world’s mountains occur in long belts that tend to following converging plate boundaries. The North American Cordillera is a mountain range that runs down the western side of North America from Alaska to Mexico.

#3 Some mountains, such as the Himalayas lie along current plate boundaries.

#4 A continental margin is a boundary between continental crust and oceanic crust.

#5 There are 2 types of continental margins, active continental margins occur along plate boundaries. Passive continental margins do not occur along plate boundaries.

#6 Mountain building takes place near active continental margins.

#7 Passive continental margins are stable areas because they do not occur at plate boundaries. These areas accumulate large amounts of sediment. Some of the sediment comes from the skeletons and shells of marine animals. Much of the sediment is material that has been weathered away from the continent and transported to the continental margin by rivers.

#8 Passive continental margins are related to mountain formation because these margins provide materials that can form mountains. The Andes mountains contain sediments that were deposited on passive continental margins.

#9 Stress is a measure of the amount of force applied over a given area. Stress is applied to rocks at converging plate boundaries. Mountain belts are made of rocks that have been permanently deformed under stress.

#10 There are 3 types of stress in rocks. When compression occurs, rock layers are squeezed together. Compression tends to make rocks thicker and shorter. When tension occurs, rock layers are stretched. Tension makes rock layers thinner and longer. When shear stress occurs, rock layers are pushed in 2 different opposite directions. Shear stress tends to distort the shape of the rocks.

#11 During plate collisions, stress can cause rock layers to crumple into folds. An anticline is an upfold in rock layers. A syncline is a downfold in rock layers. The 2 sides of a fold are called the limbs. The steepness or dip of limbs reflects the intensity of folding. The compass direction of the fold of the rock layers exposed at the surface along the fold is called the strike.

Anticline

Syncline

#12 A fault is a break in the lithosphere along which movement has occurred. A normal fault occurs when the hanging wall moves down with respect to the foot wall. Normal faults occur in areas where tension is pulling the crust apart.

#12 Continued A reverse fault occurs when the hanging wall moves up with respect to the foot wall. Reverse faults are caused by compression.

#12 Continued A thrust fault is a reverse fault in which the fault plane dips 45 degrees or less from the horizontal. A strike-slip fault occurs when rocks on opposite sided of the fault plane move horizontally past each other. The San Andreas fault is a good example of a strike-slip fault.

#13 Joints, like faults are breaks in bedrock, but joints are different in that there is no apparent movement. Joints can provide channels through which water can move through bedrock.

#14 When 2 plates carrying continental crust collide, rocks can fold and crumple into folded mountains. The Alps and the Northern Rockies are examples of folded mountains. Before 2 continents can collide the ocean basin between them must close.

#15 A dome mountain in nearly circular folded mountain. Dome mountains are not found in mountain belts, but are individual, isolated mountains. There are 2 types of dome mountains.

#15 Continued A plutonic dome mountain forms when overlying crustal rocks is forced upward by an igneous intrusion. The igneous intrusion would be younger than the surrounding rock. Tectonic dome mountains result from uplifting forces that arch rock layers upward.

#16 Volcanic activity is a contributor to mountain formation. Tension plays a large role in the formation of fault block mountains

#17 Tensional stress and normal faulting also result in features called horsts and grabens.