Mechanical weathering is the set of weathering processes that break apart rocks into particles (sediment) through physical processes.
The most common form of mechanical weathering is the freeze-thaw cycle. Water seeps into holes and cracks in rocks. The water freezes and expands, making the holes larger. Then more water seeps in and freezes. Eventually, the freeze-thaw cycle can cause rocks to split apart.
Abrasion is another form of mechanical weathering; it’s the process of sediment particles rubbing against each other. This occurs mainly in rivers and at the beach.
Alluvium is sediment that has carried by and deposited from running water. Like this example from Kansas, alluvium tends to be clean and sorted.
Alluvium is young sediment—freshly eroded rock particles that have come off the hillside and been carried by streams. Alluvium is pounded and ground into finer and finer grains (by abrasion) each time it moves downstream.
The process can take thousands of years. The feldspar and quartz minerals in alluvium weather slowly into surface minerals: clays and dissolved silica. Most of that material eventually (in a million years or so) ends up in the sea, to be slowly buried and turned into new rock.
Blocks are boulders formed through the process of mechanical weathering. Solid rock, like this granitic outcrop on Mount San Jacinto in southern California, fractures into blocks by forces of mechanical weathering. Every day, water seeps into cracks in the granite.
Every night the cracks expand as the water freezes. Then, the next day, water trickles further into the expanded crack. The daily cycle of temperature also affects the different minerals in the rock, which expand and contract at different rates and cause the grains to loosen. Between these forces, the work of tree roots and earthquakes, mountains are steadily dismantled into blocks that tumble down the slopes.
As blocks work their way loose and form steep deposits of the talus, their edges begin to wear down and they officially become boulders. When erosion wears them down smaller than 256 millimeters across, they become classified as cobbles.
Roccia Dell’Orso, “Bear Rock,” is a large outcrop on Sardinia with deep tafoni, or large weathering cavities, sculpting it.
Tafoni are largely rounded pits that are formed through a physical process called cavernous weathering, which starts when water brings dissolved minerals to the rock surface. When the water dries, the minerals form crystals that force small particles to flake off the rock.
Tafoni are most common along the coast, where seawater brings salt to the rock surface. The word comes from Sicily, where spectacular honeycomb structures form in the coastal granites. Honeycomb weathering is a name for cavernous weathering that produces small, closely spaced pits called alveoli.
Notice that the surface layer of rock is harder than the interior. This hardened crust is essential to make tafoni; otherwise, the whole rock surface would erode more or less evenly.
Colluvium is sediment that has moved downhill to the bottom of the slope as a result of soil creep and rain. These forces, caused by gravity, yield unsorted sediment of all particle sizes, ranging from boulders to clay. There is relatively little abrasion to round the particles.
Sometimes rocks weather by peeling off in sheets rather than eroding grain by grain. This process is called exfoliation.
Exfoliation can occur in thin layers on individual boulders, or it can take place in thick slabs as it does here, at Enchanted Rock in Texas.
The great white granite domes and cliffs of the High Sierra, like Half Dome, owe their appearance to exfoliation. These rocks were emplaced as molten bodies, or plutons, deep underground, raising the Sierra Nevada range.
The usual explanation is that erosion then unroofed the plutons and took away the pressure of the overlying rock. As a result, the solid rock acquired fine cracks through pressure-release jointing.
Mechanical weathering opened up the joints further and loosened these slabs. New theories about this process have been suggested, but are not yet widely accepted.
The mechanical action of frost, arising from the expansion of water as it freezes, has lifted the pebbles above the soil here. Frost heave is a common problem for roads: water fills cracks in asphalt and lifts sections of road surface during the winter. This often leads to the creation of potholes.
Grus is a residue formed by weathering of granitic rocks. Mineral grains are gently teased apart by physical processes to form clean gravel.
Grus (“groos”) is crumbled granite that forms by physical weathering. It’s caused by hot-and-cold cycling of the daily temperatures, repeated thousands of times, especially on a rock that is already weakened from chemical weathering by groundwater.
The quartz and feldspar that make up this white granite separate into clean individual grains, without any clay or fine sediment. It has the same makeup and consistency of the finely crushed granite you would spread on a path.
Granite is not always safe for rock climbing because a thin layer of grus can make it slippery. This pile of grus has accumulated along a roadcut near King City, California, where the basement granite of the Salinian block is exposed to dry, hot summer days and cool, dry nights.
Sandstone at San Francisco’s Baker Beach has many closely spaced, small alveoli (cavernous weathering pits) due to the action of salt crystallization.
Rock flour or glacial flour is raw rock ground by glaciers to the smallest possible size. Glaciers are huge sheets of ice that move very slowly over the land, carrying along boulders and other rocky residue.
Glaciers grind their rocky beds exceeding small, and the smallest particles are the consistency of flour. Rock flour is quickly altered to become clay. Here two streams in Denali National Park merge, one full of glacial rock flour and the other pristine.
The rapid weathering of rock flour, coupled with the intensity of glacial erosion, is a significant geochemical effect of widespread glaciation. In the long term, over geologic time, the added calcium from eroded continental rocks helps pull carbon dioxide from the air and reinforces global cooling.
Saltwater, splashed into the air by breaking waves, causes widespread honeycomb weathering and other erosive effects near the world’s seacoasts.
Talus or Scree
Talus, or scree, is the loose rock created by physical weathering. It typically lies on a steep mountainside or at the base of a cliff. This example is near Höfn, Iceland.
Mechanical weathering breaks down exposed bedrock into steep piles and talus slopes like this before the minerals in the rock can alter into clay minerals. That transformation occurs after the talus is washed and tumbled downhill, turning to alluvium and eventually into soil.
Talus slopes are dangerous terrain. A small disturbance, such as your misstep, can trigger a rock slide that may injure or even kill you as you go downhill with it. Additionally, there is no geological information to be gained from walking on scree.
The Wind can wear away rocks in a process like sandblasting where conditions are right. The results are called ventifacts.
Only very windy, gritty places meet the conditions needed for wind abrasion. Examples of such places are glacial and periglacial places like Antarctica and sandy deserts like the Sahara.
High winds can lift sand particles as large as a millimeter or so, bouncing them along the ground in a process called saltation. A few thousand grains might hit pebbles like these over the course of a single sandstorm. Signs of wind abrasion include a fine polish, fluting (grooves and striations), and flattened faces that may intersect in sharp but not jagged edges.
Where winds come persistently from two different directions, wind abrasion can carve several faces into stones. Wind abrasion can carve softer rocks into hoodoo rocks and, at the largest scale, landforms called yardangs.
CITE THIS ARTICLE
Alden, Andrew. “Mechanical Weathering Through Physical Processes.” ThoughtCo, Feb. 16, 2021, thoughtco.com/mechanical-or-physical-weathering-4122976.