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Geologic Processes Related to Mountaineering

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Geologic Processes Related to Mountaineering

A perfect example of a compressional mountain range. K2 - June 2004. Image reproduced with the permission of EscaladeQuebec

Mountain Building

Image courtesy of the USGS.

The Cycle of All Mountain Ranges

When a mountain range is young, it will be high, steep and jagged. As it ages it will be subjected to weathering processes and will be slowly worn away. Eventually it will be buried under sediments, and only the metamorphic rock that was generated will be left below the sediment. Mountains form in linear belts because they result from the interactions between plate boundaries.

Three Main Types of Rock

Igneous - cooled from lava. either volcanic (extrusive) or plutonic (intrusive). Volcanic rocks form basalt, andesite, rhyolite,dacite and obsidian Plutonic rocks include gabbro, diorite and granite, which can be found in the Rocky Mountains in Colorado and Wyoming. Granite is often confused with granodiotire and diorite, which are found in the Sierra Nevada of California.

Sedimentary - Particles deposited by water or wind in layers or beds. Sediments are classified by grain size and change from loose particles into cemented rocks.

Metamorphic - changed from one of the above by heat and/or pressure. Six general types in increasing order of heat and pressure - Slate (harder, shinier shale), Schist (flaky/friable rock), Gneiss (looks like granite but is hard and banded), Amphibolite (hard like gneiss, but no banding), Marble (heated limestone, hard and resistant or crumbly and hard to clime because of the large calcite crystals and sugary texture). Greenstone encompasses the remaining types including metamorphosed basalt (Serpentine) which is a rare, shiny green/black rock that is very slippery, highly fractured and is hazardous to climbers though it is rare. Another greenstone is migmatite, which is a rock that has been metamorphosed to the point of turning back into magma and forming an igneous rock, cycling back through the cycle.

Of all the minerals on the planet, those of feldspar, quartz, olivine, pyroxene, amphibole, biotite and calcite are the most abundant. All but calcite are silicates and are quite resistant to weathering. Calcite is strong in arid climates, but with the introduction of water is dissolves quickly. Feldspar and quartz are the most resistant and compose most granites and sandstones, and hence are the most reliable and most enduring rocks that climbers can climb reliably and safely.

Deformation

When a rock is subjected to stress, a number of changes in the rock occur. Jointing, the creation of a fracture without offset is formed with little pressure or heat (brittle deformation).

Folds, where the rock is bent but not broken occur at greater depths in the earth where they are heated and pressurized (ductile deformation). Up folds are anticlines, and down folds are synclines. Display a huge range in scale from inches to hundreds of meters Huge for climbers. Best place to consistently place protection. Igneous rocks crack in the cooling history of the rock. Basalts develop columns and granites produce several sets of joints including exfoliation joints caused by released pressure. Sedimentary and metamorphic rocks form joints when subjected to stress. Small scale joints can cut through a sedimentary bedding, producing rocks that break easily and are risky to climbers.

Joints - Brittle change in which cracks form in rocks. Huge for climbers. Best place to consistently place protection. Igneous rocks crack in the cooling history of the rock. Basalts develop columns and granites produce several sets of joints including exfoliation joints caused by released pressure. Sedimentary and metamorphic rocks form joints when subjected to stress. Small scale joints can cut through a sedimentary bedding, producing rocks that break easily and are risky to climbers.

Faults - Break in rock due to tectonic motion. Some can uplift an entire mountain range (Wasatch fault in Utah). Some hazards can arise due to the breccia that can accumulate in a fault zone. Mountaineers often see reverse faults, thrust faults and normal faults in mountain formations. Reverse and thrust faults form when continents collide (found in the Himalayas and the Alps). The Himalayas are still being thrust the faults in the range creating numerous earthquakes each year. Normal faults pull rocks apart and can create fault block mountains.

MOUNTAIN BUILDING EVENTS

VOLCANIC MOUNTAINS

Image courtesy of the USGS.

Examples of volcanic mountain ranges are the Andes and the High Cascades. There are three basic types of volcanoes. Shield, Stratovolcanoes and calderas. Shield volcanoes erupt basaltic lava and have gently sloping sides. Minimal climbing opportunities are created on shield volcanoes. Lofty stratovolcanoes are the most intriguing to climbers (examples include Mt. Hood, Mt. Rainier and Mt. Fuji). Stratovolcanoes are also called 'composite cone' volcanoes and they erupt ash, cinders and lava. They are found along subduction zones. Calderas are hard to identify because of their explosive nature, after which nothing more than a flat plains section forms with slight hills surrounding it. The eruptive products, welded tuffs, often erode into cliffs which are ideal for climbers (an example would be the yellow rocks of Oregon's Leslie Gulch).

Volcanic mountains can also form when they are eroded away and the remaining cooled magma chamber is left. Sharp, steep, isolated peaks such as Ship Rock in New Mexico and Devil's Tower in Wyoming are formed.

Devils Tower, an igneous intrusive body exposed by erosion. Devils Tower National Monument, Wyoming. Image courtesy of the USGS.

COMPRESSIONAL MOUNTAIN RANGES

These occur when two continental places meet and buckle upwards because they are relatively light compared to the lithosphere and asthenosphere. Rather than subducting, the crust tends to buckle and be pushed upward or sideways. A crustal root, a downward bulge is also formed in a continental collision, causing high grade metamorphism to occur.

Image courtesy of the USGS.

A Continental Collision Example: The Himalayas

Image courtesy of the USGS.

Image courtesy of the USGS.

Image courtesy of the USGS.

FAULT BLOCK (EXTENSIONAL) MOUNTAIN RANGES

In areas where the earth's crust is stretched apart and the brittle crust is stretched far enough, it breaks into large blocks along fault lines. One end is forced up, and the other down, forming a block shaped peak. One face will be nearly vertical , and the other will be a gentle slope. This formation can be seen in the Basin and Range including the Wasatch Range and the Sierra Nevada. Through erosion, streams and glaciation have carved out valleys and peaks in the blocks forming deep cut valleys and lakes. Fault block ranges are rare, and are usually not very extensive, however the highest peak in the continuous United States, Mt. Whitney, is a fault block mountain.

SUSPECT TERRANE

A recently discovered process in which small crustal fragments from island arcs or other sources collide with and merge onto continents. This process of accretion leads to further mountain building. This process can be seen along the western coast of the United States.

HOT SPOTS

A hot spot is an isolated volcano or section of magmatic activity that is not associated with a plate boundary and is rather independent and constant in its location. The Hawaiian islands are the product of an oceanic hot spot, and the plate movement over the hot spot can be traced by the chain of islands that have been weathered below sea level.

Image courtesy of the USGS.

Image courtesy of the USGS.

One of the best known hot spots exists within the North American Plate and is located in the Yellowstone National Park region. There are numerous calderas at this location and the ash from the explosions has been seen in sediment deposits as far away as Iowa and Northern Mexico. There are numerous hot pools, springs and geysers at the park because of the magmatic activity trapped below the surface by a zone of pressurized steam and water.

Weathering of Rocks

Pit weathering on the sandstone are what makes this climb possible. Peter Wigginton at the Red River Gorge, Kentucky. Copyright © 2005 Andrew Walters.

Pit weathering, Triassic sandstone. Wupatki National Monument, Coconino County, Arizona. August 1951. Image courtesy of USGS.

Here again, the same water that was dripping in our faces during the climb was responsible for weathering the rock and making the holds were were using. Peter Wigginton on Creature Feature (5.9)at the Red River Gorge, Kentucky. Copyright © 2005 Andrew Walters.

Types of Weathering

Physical – Mechanical breakage. Works together with chemical weathering. Most active in polar and desert regions.

Chemical - Decomposition due to interaction with water the "universal solvent". Dominates in wet climates as a result.

Biological – Action of organisms which often combines physical and chemical weathering processes.

Some relevant examples of physical weathering for mountaineering include:

Jointing - the release of pressure caused by sedimentary layering in which rocks expand when erosion removes the overlying sediments and the rock develops joints. If the rock being unloaded is an igneous pluton, it will crack in parallel layers (exfoliation). Examples of exfoliation include Half-Dome, Stone Mountain, and Enchanted Rock.

Joints in granite cut by veins of quartz, feldspar and biotite, opposite Ten Mile Creek, Las Animas canyon. San Juan County, Colorado. October 10, 1900. Image courtesy of the USGS.

Frost/Root/Salt Wedging - When water enters a jointed crack, and then freezes, it expands pushing the crack wider and wider. If this process is repeated, the water penetrates further and further and can eventually result in disintegration of the rock since it exposes more surfaces to weathering. This process is prevalent in mountain environments. If the rock is broken, the debris will roll down the slope it is on and accumulate into a talus pile of boulders at the base of the slope, forming at the angle of repose (the greatest angle a substance can maintain without slipping downhill, generally between 30 and 40 degrees). Similar processes occur when roots penetrate cracks forcing them apart, or when salt enters with water and then forms crystals, forcing the crack wider as crystals grow.

Root wedging seen in vegetation growing in crack on the "Governor", granite boulder on crest of Naushon Island. Elizabeth Islands, Massachusetts. Image courtesy of the USGS.

Frost-shattered joints and fractures in rock in Wilkins Mountains. Antarctic Peninsula. Antarctica. January 8, 1978. Image courtesy of the USGS.

Blocky frost heaved boulders of Shepard Formation. Boulder Pass area. Glacier National Park. Flathead County, Montana. August 16, 1981. Image courtesy of the USGS.

Snow and Ice

Formation - snow forms in the atmosphere as water condenses at temperatures below freezing. It forms around the center of a foreign particle such as dust. The freezing usually forms hexagonal shapes though columns, needles and plates also occur. Shape is dependent on air temp and the amount of water vapor available. Crystals in the air near freezing will form a flake (aggregate of other crystals), but if it contains water droplets near freezing, they will freeze and become rounded pellets called graupel (soft hail). It also follows that the higher the temp the snow forms in, the denser it will be (heavy and wet). Wind also makes snow denser by breaking up falling crystals into fragments that pack together into fine grained snow.

Surface Variations of Snow Cover and the Mountaineering Dangers they Present

Rime - A dull white, dense layer that forms at ground level. It is formed by water droplets on trees and the group freezing. Because wind lowers the temperature rime will build up in the direction of the wind. No significant danger posed, though it can break and trap feet if it is expansive.

Hoarfrost - This too forms on solid objects at ground level, but instead of a frozen droplets, crystallized shapes such as blades, cups and scrolls form. It is the result of sublimation (the process of converting water vapor to ice without turning to a liquid). If hoarfrost is buried, it can be an avalanche danger, though on the surface it poses no problems and produces incredible skiing conditions.

Powder snow - Not only light, fluffy, new fallen snow. It is new snow that has lost some of its cohesion because of the temperature changes between pits and peaks of the crystals, leading to a loss of even cohesion. Good skiing, but can also produce loose-snow avalanches. Hard to walk through, rope cuts it, axes don't hold, crampons clog and anchors need reinforcing.

Corn snow - After the top layers of snow melt in the spring, the water often freezes again creating coarse, rounded pellets on the top layer of snow. They form from the repeated cycle of melting and freezing that occurs in the spring. In its frozen state it is easy to travel through, but when it melts, it can lead to loose-snow avalanches.

Rotten snow - Another spring condition in which soft, wet layers form below firmer layers. Can lead to slab avalanches that go all the way to the ground. Continental climates produce more rotten snow than marine climates.

Melt water crust - A solid layer or crust of snow that melts and refreezes. It is usually easy to break through which makes travel and skiing difficult. If rain interacts with the surface, dendritic finger will form, like the veins of a river, and can reduce the possibility of an avalanche by stabilizing the snow. A good walking surface, but at time it can be slippery and dangerous.

Wind slab - As the wind blows up layers of snow, they fall back down, compacting together and forming a harder crust. If heat is combined with the wind, the process is exacerbated. Travel is usually easy on wind slab, but fractures can occur in the layer which can create unwanted stress that could trigger an avalanche.

Firnspiegel - Clear ice layer found in spring and summer that is often described for its sheen as "glacier fire". Occurs when the sun melts the layer below the top of the snow, while the top layer freezes. It is easy to break through and not hazardous.

Verglas - Thin layer of clear ice formed on rocks. Usually only at high elevation in spring and summer when a freeze/thaw cycle is occurring. Forms when rain freezes upon contact (freezing rain). Forms a highly slippery surface that is hard to see, much like black ice on a roadway.

Sun cups - Ridges in the snow formed by uneven melting times. Melting times are effected by the fact that the ridges evaporate, but the water that is trapped in the hollows promotes rapid melt, so the hollows melt at a faster rate. Wind destroys the cups, and the cups need dry weather to form in general. They can cause someone to catch an edge while skiing/snow boarding and they can make uphill travel uneven and difficult. Downhill, the hollows form natural steps.

Nieve penitentes - Formed from sun cups. Pillars are created when sun cups intersect. At low altitude they are not an issue, but at higher elevations, especially in the Andes, they can be several feet tall, impeding travel significantly. Ropes can easily catch on the pillars.

Drain channels - In the spring, drainage pattern in the snow begin to form, though this is not seen on the surface. Rather the flow of water goes down until it reaches a permeable layer or the ground. The dirt that is brought higher accelerates melting times. Can contribute to avalanche danger if underlying layers integrity is compromised.

Sastrugi - Small ripples formed from the wind on a dry snow top layer. In high elevation settings, the effect is significant and creates large ripples. A field of hard, several foot high ripples can make travel difficult since ground and rock can be exposed between ridges.

Cornices - Deposits of snow on the lee side of a ridge, formed by the wind. They overhang the ridge, hiding where the ground ends and they can break, possibly over large vertical drops. Falling cornices are dangerous and can create avalanches. Older fractured cornices can often be covered by newer cornices. (Look at the ridge of the glaciated mountain below to see a cornice).

Glacier Formation - When snow doesn't melt, it is carried over to the next winter and is built upon. When enough snow builds up, a downhill movement will begin and a glacier will form. Crystals turn to ice and turn to glacial ice through the process of firnification. Forms when airspace between grains is sealed off, making the ice airtight and denser.

Crevasses - Fractures in a glacier that are usually 80-100 ft deep. The colder the glacier, the more like glass it acts, and the deeper the crevasses can go. The more temperate, the shallower the crevasses will be. When a glacier moves quickly, extensive fracturing occurs, leading to isolated sections of glacial ice called seracs.

Crevasse near camp 18 in Wilkins Mountains. Geologist visible in crevasse. Antarctica Peninsula.Antarctica. January 2, 1978. Image courtesy of the USGS.

Ice Avalanches - Caused by glacier movement, temperature and serac locations. Common in warm environments when water can flow underneath a glacier, accelerating it. Other reports relate the greatest frequency of ice avalanches with cold morning conditions when the ice is most brittle. Regardless, they can happen easily at any highly fractured zone.

Loose-Snow: Can be very wet and heavy, can trigger a slab avalanche or serac falls. Occur when new snow builds up and exceeds the angle of repose. As some snow falls, it dislodges more and more. If a large grain is set in motion by a broken bond (caused by sun and heat) it can start a loose-snow avalanche. Humans are also often the cause of these avalanches.

Slab: Usually larger and deep. Harder to anticipate due to their dependence on the stability of buried layers. In a slab avalanche the general formation of snow is a bed layer sandwiching a weak layer and being covered with a slab layer. Weak layers include buried hoarfrost, and depth hoar since they lack significant shear strength. Most dangerous if the next storm after a layer of hoarfrost is deposited begins with cool, calm conditions. Buried graupel is another classic weak layer. If the overlying layer, the slab has enough cohesion, it may fracture causing large sections to slide down the slope. Brittle wind blown surfaces are common slab components as are needle-shaped crystals that interlock easily. Bed layers are commonly crust, old snow, glacial ice, grass or bedrock.

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Copyright © 2005 Earlham College. Revised April 14, 2005 . Send corrections or comments to Andrew Walters