The Continental Margin is the boundary between continental crust and oceanic crust.

• Active Continental Margins occur along plate boundaries - such as the Andes Mts. of South America.
• Passive Continental Margins do not occur along plate boundaries. These margins are stable areas of shallow water where the major activity is the buildup of sediment - such as the Atlantic coast off of North America which has built up to 100's of miles long and up to 10 km thick!
• How are passive continental margins related to mountain building? Some mountains of today were actually passive margins of the past.
• In other words - oceans advanced over the continents; deposited sediment; receded, leaving the deposit to form a large mountain

Collision between Ocean and Continental Plate; or Two Continental Plates

• Subduction boundaries can form either from one ocean plate subducting under another ocean plate or by an ocean plate subducting under a continental plate
• It has been found that when two ocean plates collide (one subducts), a volcanic chain seems to occur on the overriding plate
• The Himalayan Mountains in India/China are a perfect example of a collision boundary
• The Mountains are growing about 5 cm higher each year. Mt. Everest is part of this range
• Note: When an ocean plate collides with a continental plate, the ocean plate subducts because it is heavier and denser.

• A Fault is a break or crack in Earth's crust along which movement occurs
• The surface that separates the two moving pieces is the fault plane
• A normal fault occurs when the rocks on one side of the fault plane drop down with respect to the rocks on the other side. Typically where tension is pulling the crust apart. The valleys that often form may produce a large, super-deep lakes (like Lake Baikal in Russia - deepest in the world!)
• A reverse fault occurs when one side of the fault plane is driven up over the other side. Where tension is pushing the crust together.
• Thin-skinned thrusting as discussed in Chapter 13 is often the result of the reverse faulting.
• A strike-slip fault occurs when the rocks on opposite sides of the fault plane move horizontally past each other. The San Andreas fault is a perfect example of this.

Folds

• During plate collisions, the rock layers along the continental margins are crumpled into folds
• An Anticline is an upfold in the rock layers
• A Syncline is a downfold in the rock layers
• The sides of the fold are called Limbs
• The steepness, or Dip, of the limbs reflects the intensity of folding
• The Strike is the compass direction of the fold as it lies exposed on the surface of the ground

Uplifting

• Not all mountain building exists from folding
• In some areas the layers are raised to higher levels with little deformation at all
• Fossils are often signs of uplift - fossils of prehistoric oceans are found high in mountainous areas, showing that the area has been uplifted over time
• Raised beaches can be evident in areas that show that the layers have been uplifted
• Simply making periodic measurements can result in finding areas of uplift….each year the area rises 2 inches, etc.

Tilting

• All sedimentary rock forms in level layers.
• Finding layers that have tilted, shows signs of mountain building
• Both folding and uplifting can cause the tilting of rock layers
• Fault-Block Mountains are examples of large blocks of rock that have been uplifted or tilted
• Many parts of the Rocky Mts. are created from the process of fault-blocking

Overturning

• In some areas of the world, rock areas are so severely tilted that they have overturned
• Ripple marks are features that form on the floor of a quiet body of water - once hardened into rock, they help show signs of overturning (as shown on the board)
• Mud cracks form in fresh wet clay ground. If you find the cracks open downward, this shows that the rock layer has been overturned
• Shells with curved surfaces also show if areas have been overturned. If the curved side of the clam is down, the area has been overturned (as shown on the board)

Dome Mountains

• Nearly circular mountains that form two different ways:
• Plutonic dome mountains are formed when overlying crustal rocks are pushed upward by the intrusion of an igneous mass, such as a laccolith. Many parts of the Rocky Mts. are created by plutonic activity.
• Tectonic dome mountains are the result of the uplifting forces that arched the rock layers upward. The Black Hills of South Dakota are examples of tectonic dome mountains.