Chapter

11 Mountain Building Where Mountains Form:

Mountain Belts: Most of the world’s mountains occur in long belts that tend to follow convergent plate boundaries.

North American �Cordillera�extends from�Alaska�to �Panama�

l By studying the ages of rocks and how

mountains formed, they were able to determine that the Appalachians formed at plate boundary that existed millions of years ago!

Continental Margins l  A continental margin is a boundary between

continental crust and oceanic crust. l  Active margins occur along plate boundaries. l  Passive margins do not occur along plate

boundaries. l  Example of Passive: Underwater continental

margin along the east coast of North America that marks the boundary between the North American continent and oceanic crust.

l  Both are part of the North American Plate!!

l Mountain Building takes place near active continental margins!

l Example: West Coast of South America - The Nazca Plate carrying the oceanic crust is subducting beneath the South American Plate which is carrying the continental crust.

l This area is very mountainous and prone to earthquakes and volcanic activity!

Sunrise warms the icy southern end of the Andes during a rare break in the weather in Chile's Torres del Paine National Park. The Andes mountains, which span the entire western coast of South America, formed when the Nazca plate subducted under the South American plate.

Types of Stress

Folds

Rock Deformation

u Anticlines •  Anticlines are most commonly formed by the

upfolding, or arching, of rock layers. u Synclines

u Monoclines •  Monoclines are large step-like folds in otherwise

horizontal sedimentary strata.

•  Synclines are linear downfolds in sedimentary strata.

•  Synclines are often found in association with anticlines.

Anticlines and Synclines

Monoclines

Faults

Rock Deformation

u Normal Faults •  Normal faults occur when the hanging wall block

moves down relative to the footwall block. u Reverse Faults and Thrust Faults

•  Reverse faults are faults in which the hanging wall block moves up relative to the footwall block.

•  Thrust faults are reverse faults with dips less than 45o.

Folds

Rock Deformation

u Joints •  Joints are fractures along which no appreciable

movement has occurred.

Strike-Slip Fault •  Strike-slip faults are faults in which the

movement is horizontal and parallel to the trend, or strike, of the fault surface.

Four Types of Faults

Joints

Folded Mountains

Types of Mountains

u Folded Mountains •  Mountains that are formed primarily by folding

are called folded mountains.

Mountains are classified by the dominant processes that have formed them. •  Orogenesis is the collection of processes that

result in the forming of mountains.

Folded Mountains

Fault-Block Mountains

Types of Mountains

Large-scale normal faults are associated with structures called fault-block mountains. •  Fault-block mountains are formed as large

blocks of crust are uplifted and tilted along normal faults.

•  Grabens are formed by the downward displacement of fault-bounded blocks.

•  Horsts are elongated, uplifted blocks of crust bounded by faults.

Fault-Block Mountains

Domes and Basins

Types of Mountains

When upwarping produces a circular or elongated structure, the feature is called a dome. •  Uplifted mountains are circular or elongated

structures formed by uplifting of the underlying basement rock.

Domed Mountains