Rock-strewn peaks are generally the result of weathering. The rocks may be the result of chemical weathering, biological weathering, salt weathering and physical weathering if not a combination of them. What are these types of weathering?
Physical weathering can be the result of erosion by the wind blowing sand, sand-blasting the terrain until only the hardest rocks remain. In these cases, rock-strewn peaks are of the hardest remaining stone, usually igneous rock that is more resistant to physical weathering, as sedimentary rock is peeled away by the wind.
Water is a major cause of physical weathering. Both igneous rock and sedimentary rock are carved away by the constant flow of water, but igneous is more likely to remain in place. Wave action combines repeated stresses to the stone while chemical effects break it down. This leaves rock strewn peaks jutting above the waterline, with the shore rendered smooth by constant abrasion.
Another form of physical weathering is due water is the slow destruction caused by repeating freezing and thawing. Water ice is one of the substances on earth that actually expands instead of contracts upon freezing. Water will seep into soil and most types of stone. When microscopic water droplets repeatedly freeze and thaw, larger cracks can form. Water pools in these cracks after the next storm or simply filling the space as water seeps through the rock above. When it freezes, it expands. This can cause portions of the rock to eventually break off, since the ice applies constant pressure to the rock around it until the mechanical pressures are relieved, typically by mechanical failure and portions of stone falling to the ground below.
Physical weathering can be caused by another form of ice, glaciers. During the Ice Ages, glaciers up to two miles tall covered North America and Europe. The heavy weight of the ice pushed down the land while the force of its relentless weight cut grooves into the ground. Boulders the size of houses could be carried miles from their original home. Smaller mountains were often scoured by the glaciers sweeping over it. When the last Ice Age ended about 12,000 years ago, some of the smaller mountains that had been trapped under glaciers became rock-strewn peaks covered with everything from fine gravel to large boulders left there by the retreating ice.
Physical weathering is greatest at high altitudes, high elevations and areas of high relief (where one rock overhangs the ground, increasing the odds of pressure leaving to it shearing off). Physical weathering is common in frozen regions, temperate areas prone to thermal cycling (freezing and thawing), mountains and deserts.
Biological weathering is that weathering caused by living organisms. The most common cause of biological weathering is the growth of grasses, bushes and trees into small niches of soil in the rock. While the seedling may be sheltered in a small crack, it eventually sends burrowing roots into the ground. As the roots dig into the soil, they break up the mechanical bonds that hold the rock in place. Eventually, the roots will break up the rock into soil, assuming the rock itself doesn’t break apart.
Biological weathering contributes to chemical and water related weathering. The roots provide a conduit for water to seep into the rock, where it may later freeze and expand or react with the surrounding rock.
Chemical weathering occurs when chemical reactions break down the rock. Water will dissolve rock salt, eroding natural salt deposits into nothing. Likewise, water seeping through limestone formations reacts with it chemically and wears it away. Gypsum dissolves and weathers away on exposure to water. Rainwater sometimes carries carbonic acid, which eats into rock. Oxygen can react with many materials, altering its chemical structure and converting it into a material more easily worn away. When olivine reacts with carbonic acid or water, part of its mass ends up washed away while the iron or hematite remains behind. Feldspars react with carbonic acid to turn into the more erodible clay.
Chemical weathering occurs more quickly in warm, wet climates. Hot dry climates lack the water for needed hydrolysis.
Salt weathering occurs when saline solutions like ocean water or salty groundwater (such as when farmland is over-watered) fills small cracks in the rock before evaporating. When the salty water evaporates, salt crystals remain. If they are heated up, such as due to hot climates or volcanic activity, they expand. This creates pressure against the surrounding rock and can break it up. This effect is greatest with calcium chloride, magnesium sulfate and sodium sulfate. Salt weathering is most common on the coast and in desert environments.