Mountain Orogeny

Three types of plate boundary

ORIGIN OF MOUNTAINS

• Orogeny = process of mountain building, takes tens of millions of years; usually produces long linear structures, known as orogenic belts

Two main processes:1) Deformation: continental collisions, resulting in folding and thrust-faulting2) Volcanic Activity

Other processes: Metamorphism, intrusions: batholiths, etc.

TYPES OF MOUNTAINS(according to their origin)

• Fault-block: tension, normal faultingex. Sierra Nevada, Wasatch, Grant Tetons

• Folded: compression, reverse faultingex. Appalachians, Alps, Himalayas, Urals, Atlas, Andes

• Volcanic: Shield and compositeex. Cascades, Mid-Ocean Ridges, Oceanic Hot Spots* Dome Mts: similar to volcanic, Adirondacks, Black Hills

• Complex: mixture of most of the aboveex. Rockies, Alps, Himalayas

Organization of Mountains

• Every mountain is part of a 

Mountain Range (ie. Green Mountains, Great Smoky, Blue Ridge, the Cumberland, White Mountains)

• groups of ranges make up a 

Mountain System (ie. Northern Appalachian Mts.)

• groups of systems make up 

Mountain Belts (ie. Appalachian Belt)

HANGING WALL

1. Fault-block mountainslarge areas widely broken up by faults

•Force: TENSION•Footwall moves up relative to hanging wall

Normal fault

Tilted fault-block range: Sierra Nevada from east,Steep side of block fault; Ansel Adams photo

Tilted Fault-blockSierra Nevada from westSide, low angle

Yosemite valley the result Of glaciation on low-anglerelief

Central cores consists of

intrusive igneous rocks

(granite).

Half Dome is a core (batholith)

that was exposed by erosion,

Batholith

Wasatch RangeFrom Salt Lake City

Typically fault-Block system

Grand Tetons: another fault-block system

Alternating normal faults lead to a characteristic pattern called a“horst and graben” system. An area under tension will often havemultiple mountain ranges as a result.

Horst and graben

Horst and Graben Landscapes

Figure 12.14

•tilted fault-block mountains in Nevada

•result of a horst and graben system

•Nevada is under tension because of rising magma which is unzipping the system, all the way fromBaja California

Sierra Nevada and Wasatch Ranges part of this system

Basin and Range province:

Reverse faults can also form Fault Block Mts.

•Force: COMPRESSION•Hanging wall moves up relative to footwall•Two types: -low angle -high angle

Individual layers can move 100’s of kilometersAlps are a great example

Flatirons(Boulder, CO)Classic example of high-angle reverse faults-> Form “Sawtooth Mtns”due to differential erosion

Seal rock

White Cloud peak

SAWTOOTH RANGE,IDAHO

Alice Lake

•Thrust (reverse) faults main cause of folded mountains

Folded mountains

• Where rock does not fault it folds,

either symmetrically or asymmetrically.

upfolds: anticlines

downfolds: synclines

Classic folded terrain: well-developed anticline

Appalachian Mountains of the US

Atlas Mountains, Northern Africa

Zagros Crush Zone (Iran/Iraq)

AlternatingAnticlines andSynclines

Volcanic mountains

•Shield• Gradual slope, very tall to ocean floor, slow

flowing eruptions, composed of layers of lava

•Composite (Strato-)• Explosive, made of pyroclastic material and lava.

steep

•Cinder cone   • Very steep slopes, made of pyroclastic material,

Types of Volcanic Material

1. Pyroclastic material: rock fragments ejected from volcano

2. Ash: less than 2 mm in diameter

3. Dust: less than .25 mm diameter

4. Bombs: spinning cooling large blocks of material, cool to circular shape

5. Blocks: very large, as big as houses

Mafic Lava

• Dark colored (when hardened)• Rich in Mg (magnesium) and Fe (iron)• Forms oceanic crust.• Mafic lava has a low viscosity and flows easily.• Seen in slow erupting Shield Volcanoes and hardening into

Basalt (extrusive) or Gabbro (instrusive)                • Dominant at Mid-Ocean ridges, Oceanic Hot Spots (Shield

Volcanoes of Hawaii), Island Arcs and can be found at Rift Valleys, Continental Hot Spots (Yellowstone). 

Shield volcanoes

-Compressive forces-Mafic lava

At hot spots•gentle-sloping•basaltic lava flows

Mauna Kea

Shield volcanoHot SpotBasalt

Mauna Loa inBackground

Kilaeua is Behind MaunaLoa

Felsic Lava

• lighter colored (when hardened), rich in Si (silica). Forms continental crust.

• Felsic lava has a high viscosity and DOES NOT FLOW EASILY.

• Seen in continental Composite or Stratovolcanoes and result in EXPLOSIVE eruptions.  Ex. Cascade Mts. and Mt. St. Helens.  Can harden into granite (intrustive) and less likely rhyolite (extrusive).

• Dominant at Oceanic: Continental Convergent plate boundaries/subduction zones and can be found at Continental Hot Spots (Yellowstone). 

Composite (Strato-) volcanoes

-andesitic composition-felsic lava-steep cones, explosive

Encountered at subduction zones

Mt Rainier:

example of composite volcano

Guagua Pichincha, EcuadorQuito in foregroundComposite volcanoes explosive

Why do shield and composite volcanoes differ in composition?

Mafic magmas rise along fractures through the basaltic layer. Due to the absence of granitic crustal layer, magmas are not changed in composition and they form basaltic volcanoes.

Mountainous belts have thick roots of Felsic rise slowly or intermittently along fractures in the crust; during passage through the granite layer, magmas are commonly modified or changed in composition and erupt on the surface to form volcanoes constructed of granitic rocks.

Cinder Cones

• Made of only pyroclastic rocks

• Build cone-shaped hill• Most erupt only once• Low level eruptions

• Paricutin, Mexico

Volcanism at Mid-Ocean Ridges• Majority of Earth’s volcanism

• Hydrothermal vents

• Chimney-like structures “Black Smokers”

• Sulfur-bearing minerals or Sulfides

• Incredibly diverse ecosystems, chemosynthesis

Importance of Volcanism

• Eruptions can affect climate

ex. Mt. Tambora eruption 1816 The Year Without Summer

• Origin of life on earth

Some theories suggest life began at mid-ocean ridges in chemosynthetic environments

•continental-continental collision•tend to have a little of everything: volcanoes,folds, thrust faults, normalfaults

Complex Mountains

ALPS

HIMALAYAS

View of Everest andKhumbu ice fall from Kala Patar, Nepal Himalayas

Mountain orogeny summary

• Orogeny = mountain building event• Plate tectonics used to explain mountain building• Plate collisions- 3 types: • Forces: tension, compression, shear• Mountain types: faulted, folded, volcanic, complex• Examples of each type• Types of volcanoes• Types of Lava• Importance of volcanism