Chapter 11

Mountain Building

Rock Deformation

It is theorized that all continents were once mountainous masses and grow by adding mountains to their edges.

If this is so, then how do mountains grow in the middle of continents?

Factors Affecting Deformation

Every rock has a point at which it will bend and/or break.Deformation is the term that refers to all changes in the original shape and/or size of a rock body. Most deformation occurs at plate margins.

Stress is the force per unit area acting on a solid. Under great stress, rocks tend to deform usually by:FoldingFaultingFlowingFracturing

The change in shape or volume of a body of rock as a result of stress is called strain.

Rocks can be bent into folds if stress is applied gradually and not going beyond the breaking point.

We already know that rocks under stress have elastic properties and will go back to their original shape and size when the force is removed. (elastic rebound)

Once the elastic limit or strength of a rock is surpassed, it either flows or fractures.

The factors that influence the strength of a rock and how it will deform include:TemperatureConfining pressureRock typeTime

Temperature and Pressure

Rocks deform permanently two ways:Brittle deformationDuctile deformation

Rocks near the surface, where temperatures and confining pressures are low, usually behave like brittle solids and fracture once their strength is exceeded.

This type of deformation is called brittle failure or brittle deformation.

Examples: china, bones, pencils, glass

At depth, where temperatures and confining pressures are high, rocks show ductile behavior.

Ductile deformation is a type of solid-state flow that produces a change is size and shape of an object without fracturing the object.

Examples: modeling clay, bee’s wax, caramel candy, and most metals

Question ????????????

Placing a penny on a railroad track and having a train run over it, causing the penny to flatten out is an example of?

ductile deformation

Rock Type

The mineral composition and texture of a rock also affect how it will deform. Granite and basalt contain minerals with strong internal molecular bonds will fracture.Sedimentary rocks that are weakly cemented or metamorphic rocks that are foliated are more likely to deform by ductile flow.

Rocks that exhibit ductile flow may include:SoftHalite (rock salt)GypsumShaleIntermediate strengthLimestoneSchistMarble

Time

Natural stress applied over a long period of time will fold rocks.

Forces that are unable to deform rock when first applied may cause rock to flow if the force is maintained over a long period of time.

Types of Stresses

Three types of stresses rocks undergo include:Tensional forces – rocks being pulled in opposite directionsCompressional force – rocks are squeezed and shortenedShear stress – when a body of rock is distorted

Stresses

Folds

During mountain building, flat lying rocks (both sedimentary and igneous) are bent in wave-like ripples called folds.

There are three types of folds:Anticline – an upward fold (arch)Syncline – a downward fold (trough)Monocline – closely associated with faults,

monoclines are large step–like folds in otherwise horizontal layers of sedimentary strata.

Prominent features of the Colorado Plateau which coves Colorado, New Mexico, Utah, and Arizona.

Anticline and Syncline Process

Circular Anticline

Syncline

Joints – fractures in rock strata where no movement has taken place

So faults are……………

Three types of faults

Normal – caused by tensional stressReverse (or thrust) – caused by compressional stressStrike-slip – commonly caused by shear stress – San Andreas Fault

Normal

Reverse or Thrust

Strike-Slip

What type of fault? Strike-slip

What type of fault? Normal

What type of fault?Reverse or

Thrust

Thrust fault

Types of Mountains

The collective processes that produce a mountain belt are called orogenesis.The dominant processes that have formed them classify mountains.

Folded Mountains

Mountains formed primarily by folding are called folded mountains. Thrust faults are also important in the formation of folded mountains. These mountains are called fold and thrust belts. Examples: Appalachian Mountains, the northern Rocky Mountains, and the Alps of Europe

Fault-Block Mountains

Large-scale normal faults are associated with structures called fault-block mountains. These mountains form when large blocks of crust are uplifted and tilted along normal faults. Examples: Teton Range of Wyoming and the Sierra Nevada Range of California.

Normal faulting occurs when tensional stresses cause the crust to be stretched or extended. As the crust is stretched, a block called a graben, which is bounded by normal faults, drops down. Graben is German for ditch or trench. Grabens produce an elongated valley bordered by relatively uplifted structures called horsts. Examples: Basin and Range Province of Nevada, Utah, and California.

Rift or Grabben

Graben or Riftexample is: Death Valley

250 ft. below sea level

Dead Sea is 1200 ft below sea level

Domes and Basins

These mountains are produced by broad upwarping in the basement rock deforming the overlying sedimentary strata. A dome is when upwarping produces a circular or elongated structure or dome.

How are these domes uncovered? Erosion strips away the highest

portion of the sedimentary beds exposing older igneous and metamorphic rocks in the center.

The oldest rocks form the core of these mountains. Example: Black Hills

Downwarped structures having a circular shape are called basins. These structures have gently sloping bed similar to saucers. Basins are thought to be formed by the accumulation of sediment, whose weight caused the crust to subside.Example: Basins of Michigan & Illinois In the case of a basin the oldest rocks are found in the center with the youngest rocks on the flanks.

Mountain Formation

Just how old are mountains? Appalachians are 100s of million of

years oldHimalayas are about 45 million

years old… Just a youngster!

Mountain Building at Convergent Boundaries

Most mountain building occurs at convergent plate boundaries. Colliding plates provide the compressional forces that fold, fault, and metamorphose the thick layers of sediment deposited on the edges of landmasses.

Ocean-Ocean Convergence

Converging oceanic plates result in subduction that creates magma leading to the growth of a volcanic island arc once it grows above sea level. Examples: Aleutian Islands in Alaska, Japan, Ocean-ocean convergence mainly produces volcanic mountains.

Ocean-Continental Convergence

Example: West coast of South America – the Andes MountainsThis produces a continental volcanic arc. During subduction, sediment scraped from the subducting plate is stuck against the landward side of the trench. This accumulation of sediment and metamorphic rock is called an accretionary wedge.

Ocean-continent convergence produces mountains in two roughly parallel belts. The continental volcanic arc develops on the continental block and the accretion wedge is on the seaward belt. The types of mountains formed by ocean-continent convergence are volcanic mountains and folded mountains.

Continent-Continent Convergence

A convergent boundary between two plates carrying continental crust, a collision between the continental fragments will result and form folded mountains.  Example: Himalayan Mountains and the Tibetan Plateau, European continent collided with Asia producing the Ural Mountains in Russia.

Mountain Building at Divergent Boundaries

These mountains are usually formed on the ocean floorExample: Mid-ocean ridges that extend 65,000 kilometers.The mountains that form along ocean ridges at divergent plate boundaries are fault-block type mountains.

Non-Boundary Mountains

Example: Hawaiian IslandsContinental AccretionAccretion is when small crustal fragments collide and merge with continental margins.

Terrane – is any coastal fragment that has a geological history distinct from that of the adjoining terranes. Some may be no larger than volcanic islands, while other are immense, such as the entire Indian subcontinent. Some may have been microcontinents similar to present day Madagascar. Many others were island arcs like Japan and the Philippines.

All these materials add to the continent width and thickness displacing other fragments further inland.

Mountains of Accretion

Example: Mountains in western North America and western Canada contain rock, fossils, and structures different from the surrounding area. These materials have been accreted (added to) the western margin of North America.

Principle of Isostasy

This is a gradual up and down motion of materials that make up the interior of continents away from continental margins. Earth’s crust floats on top of denser more flexible rocks in the mantle. The concept of a floating crust in gravitational balance is isostasy.

Mountain belts float on top of more dense crustal roots that extend into the mantle. The denser mantle supports the mountains from below.

If more material were added to the top of the mountains the combined block would sink until a new isostatic balance was reached. However, the top of the combined block would be higher than before and the bottom would be lower. This process of establishing a new level of gravitational equilibrium is called isostatic adjustment.

Isostatic Adjustment of Mountains

Continental ice sheets provided evidence of crustal subsidence followed by rebound. The weight of 3 kilometers of ice depressed Earth’s crust by hundreds of meters. In 8000 years since the last ice sheets melted, uplift of as much as 330 meters has occurred in Canada’s Hudson Bay region.

Most mountain building causes the crust to shorten and thicken. Because of isostasy, deformed and thickened crust will undergo regional uplift both during mountain building and for a long period afterward.As the crust rises (rebounds) the processes of erosion increases and the deformed rock layers are carved into mountains.

Erosion removes material from the summit reducing the load causing the crust to rise. This will continue until the mountain block reaches its “normal” thickness.