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Mountain Building Chapter 11

11.1 WHERE MOUNTAINS FORM

What is a Mountain? • Large mass of rock • Rises far above its base

A. Mountain Belts • Most form in long belts that follow convergent plate boundaries • Some are old boundaries, some are current boundaries • Appalachians – OLD boundary • Himalayas – Current boundary

Appalachians

Himalayas

B. Continental Margins • Boundary between continental and ocean crusts • Active Margins – Along plate boundaries

• Passive Margins – Not at plate boundaries

Atlantic Continental Margin

•Passive •Creates Mountain Building Materials

Active Continental Margin •Mountain Building Happens

•Subducting Boundaries •Usually Volcanically Active •Earthquake Prone

How are these margins related? • Passive margins contain a lot of sediment • When that passive margin becomes active, those sediments will become part of the new mountains

11.2

HOW MOUNTAINS FORM

STRESS!!!! • Stress within the earth causes rock to break or fold

A. Types of Stress • 1. Compression – Rock layers are being squeezed inward – Makes layers thicker and shorter

• 2. Tension – Layers are stretched – Makes rocks thinner and longer

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• 3. Shear Stress – Rocks are being pushed in opposite directions – Distorts the rock along the plane

Where does this happen? • Along plate boundaries • Usually all occur together • Different rocks respond differently to the three types of stress

B. Folds • Along continental margins • Stress causes rocks to warp and crumple • Over time, erosion exposes the rock

Anticlines • An upfold in the rock strata

Synclines • A downfold in rock strata

Limbs • The sides of an anticline or syncline

C. Faults • Fault plane – the plane along which movement in earth’s crust occurs

• Above the Fault Plane: Foot Wall • Below the Fault Plane: Hanging Wall

1. Normal Fault • Hanging wall moves DOWN • Tension is pulling crust apart

2. Reverse Fault • Hanging wall moves UP • Compression pushing crust together

3. Thrust Fault • A type of reverse fault • The major difference: the angle of the fault plane – 45 degrees or less from horizontal

4. Strike-Slip Fault • Rocks on opposite sides of the fault move past each other

5. Joints • A break in the bedrock – no movement has occurred • Usually in the form of a plane • Gives a channel in which fluids (molten material, soil, water) can move

11.3

TYPES OF MOUNTAINS

A. Folded Mountains • Two colliding continental plates • Rocks don’t break, they crumple • Ocean between plates must disappear first – Due to subduction – Continues until crusts meet

Himalayas are Folded

B. Dome mountains • A type of folded mountain • Nearly circular • Usually solitary

Plutonic Dome • Made by plutonic intrusion • Pushes up older rock • Core of mountain is younger

Tectonic Dome • Made by uplifted rock layers • Rocks at core are older – No new material – Layers erode, exposing older rock

C. Volcanic Mountains • Different types of volcanoes form in different areas (remember chapter 9)

D. Fault-Block Mountains • Formed by Compression or Tension • Blocks of crust are dropped (tension) or lifted (compression) • Form Normal or Reverse faults

Grabens and Horst • Horst- formed by compressional forces • Graben – formed by tension forces