Weathering and erosion

Rocks seem hard and strong.

But slowly, rocks crack and crumble into smaller pieces. Then the pieces get carried away. Over a very long time, this changes how the land looks.

Rocks look as if they last forever, but they are slowly being broken down and moved. Geologists use two words for this. The British Geological Survey (BGS) explains that weathering is the breaking or wearing down of rocks while they stay in place. Erosion is the moving of the broken pieces somewhere else.

The pieces can be as big as boulders or as tiny as clay. Wherever they are finally dropped, they pile up as sediment. Weathering and erosion work very slowly, but given thousands or millions of years, they can wear down mountains and carve out deep canyons.

BGS distinguishes two processes that are often confused. Weathering is the decay of rock in situ, wearing it down or breaking it while it stays in place; it can be physical, chemical or biological. Erosion involves moving the fragments, which can range from boulders to fine clay, by gravity, wind, rain, rivers, oceans and glaciers. Deposition, the third step, lays the sediment down when the carrying agent loses energy.

All three belong to the rock cycle: BGS describes how weathering and erosion break rock into sediment, which is deposited, buried, compacted and cemented into sedimentary rock. On the timescales of geology, these slow processes can wear down whole mountain ranges, as on the Appalachians, or cut deep canyons such as the Grand Canyon.

Water gets into cracks in rocks.

When it freezes, the ice pushes the crack wider. This happens again and again. At last, the rock splits apart.

Physical weathering breaks rock without changing what it is made of. One of the strongest tools is ice. Water seeps into a crack and freezes. Water grows bigger when it turns to ice, so the ice pushes the crack wider. When it melts, more water can get in, and the next freeze pushes the crack even wider. This is called freeze-thaw.

Wind can blast sand grains against rock, slowly wearing it away like sandpaper. Rain and waves wear rock away too, over long periods of time, and changes in temperature can weaken rock as well.

Physical (mechanical) weathering disaggregates rock without altering its minerals. BGS lists temperature changes and exposure to wind, rain and waves. In freeze-thaw, water that soaks into joints and fissures expands as it freezes, prying the cracks apart; repeated cycles enlarge them until blocks break off, as in the limestone of the Yorkshire Pennines.

Wind armed with sand acts like sandpaper, abrading whatever surface it meets and blowing the loosened grains away; BGS points to Brimham Rocks in North Yorkshire, whose horizontally bedded sedimentary rocks show wind erosion. Rain and waves wear rock down slowly over long periods.

Rainwater can slowly change rock, a tiny bit at a time.

Plants and animals break rocks too. Roots grow into cracks. Rabbits dig into them. Even our shoes wear rocks down!

Chemical weathering changes the minerals a rock is made of. BGS explains that rainwater picks up carbon dioxide from the air, which makes it slightly acidic. When this rainwater touches some kinds of rock, it reacts with their minerals and slowly breaks them down. Rocks such as limestone can slowly dissolve, which is how many caves begin.

Living things cause biological weathering. A plant growing in a crack pushes it wider as its roots grow. A rabbit digging a burrow in a crack can make it bigger until the rock splits. BGS points out that even you can weather rock: boots walking over the same patch of rock again and again slowly wear it down.

Chemical weathering alters a rock's minerals. Carbon dioxide dissolved from the air makes rainwater slightly acidic, and this weak acid reacts with minerals in rock (BGS). BGS notes that soluble rocks such as limestone, chalk and gypsum dissolve when attacked by acidic rainfall or groundwater; the landscape this creates is called karst, explored on the caves and karst page.

Biological weathering is driven by living things: roots that widen cracks as they grow, burrowing animals such as rabbits enlarging fissures, and the feet of people and animals wearing paths into rock (BGS).

Broken bits of rock do not stay still.

They slide downhill. Rain washes them into rivers. Rivers, waves, wind and ice carry them far away.

Once rock is broken, erosion carries the pieces away. BGS lists six main ways, and they often work together:

Gravity pulls loose rock downhill, sometimes in landslides. Wind carries sand and dust. Rain splashes soil loose and washes it into streams. Rivers carry sediment and wear away their banks. Waves and ocean currents move sand along the coast. Glaciers, slow rivers of ice, scrape and carry rock of every size, from clay to giant boulders.

Rivers even change shape this way. On a bend, the water flows faster on the outside, wearing away that bank, and slower on the inside, where it drops sand. So the bend slowly grows into a big loop called a meander.

BGS identifies six interconnected erosion processes. Gravity moves weathered material downslope, as in landslides, to where other agents pick it up. Wind, carrying sand and dust, abrades and removes material. Rain dislodges particles by rain-splash and, with snowmelt, washes sediment into streams. Rivers erode and also deposit: on a meander, faster flow on the outer bank erodes and deepens the channel, while slower flow on the inner bank deposits sediment, so the bend migrates sideways. Waves and tides attack the shore at different levels, and longshore currents move sand along it.

Glaciers are the heavyweights. Continental ice sheets erode large parts of continents and alpine glaciers whole valleys; the weight of ice and embedded debris grinds the bedrock as it flows, and the debris is dumped where the ice melts or retreats. Glaciers can carry boulders for very long distances and leave them as glacial erratics (BGS).

When the water or wind slows down, it drops the pieces.

Sand and mud pile up. They can make beaches, sand dunes and new land at the mouth of a river.

Everything that erosion carries has to stop somewhere. When wind or water slows down, it drops what it is carrying. This is called deposition. Flowing water drops the biggest, heaviest pieces first and the tiniest ones last, as it slows down more and more.

That is how many landforms are made. A mountain stream that rushes onto flat land drops pebbles and sand in a fan shape. Gentle waves drop sand along the coast to build beaches. Wind drops sand into dunes. Melting glaciers leave piles of mixed-up rock called moraines. Over millions of years, buried layers of sediment can harden into new rock.

Deposition is the laying down of sediment carried by wind, flowing water, the sea or ice; sediment travels as pebbles, sand and mud, or dissolved as salts (BGS). Flowing water sorts its load, dropping the largest particles first as it slows: a stream leaving a steep slope for flat ground builds an alluvial fan. Low-energy waves deposit beaches, spits and shoals, which gain and lose sand with the seasons. Wind-blown sand bounces along by saltation and settles into dunes. Glaciers deposit till, an unsorted mix of clay to boulders, which forms moraines.

Dissolved ions may be deposited on the way or carried to the ocean, where they add to its saltiness. Buried and compacted, sediments become sedimentary rock, completing one loop of the rock cycle.

A river can carve a deep canyon.

The Colorado River carved the Grand Canyon. It took millions of years. The river is still at work today.

The Grand Canyon shows what erosion can do with enough time. The U.S. National Park Service (NPS) uses a memory word for its story: DUDE.

D is for deposition: layers of sediment were laid down, some at the bottom of ancient seas. U is for uplift: between 70 and 30 million years ago, the moving plates lifted the whole region into a high, flat plateau. D is for down cutting: starting just 5 to 6 million years ago, the Colorado River began cutting downward. E is for erosion: side streams widened the canyon. The NPS says these forces are still deepening and widening it today.

The NPS summarizes Grand Canyon's history as Deposition, Uplift, Down cutting and Erosion. Its rock layers were deposited over long ages; the topmost, the Kaibab Limestone, formed on the sea floor yet now lies at up to 9,000 feet. Plate tectonics, very likely during the Laramide Orogeny (about 70 to 40 million years ago), uplifted the Colorado Plateau high and flat; the NPS gives 70 to 30 million years ago for the uplift.

Beginning only 5 to 6 million years ago, the Colorado River cut downward through the plateau, and tributary streams widened the gorge. The NPS adds a twist: when erosion removes a great deal of material, the crust rises in response, a process called isostatic rebound. The canyon is still being deepened and widened by the same forces, a clear example of uplift and erosion working together.