Weathering and Soil

Weathering is the breakdown of rock, minerals, and soil right where they sit, at or near Earth's surface. Nothing has to move for weathering to happen. A granite headstone in a churchyard weathers without going anywhere. The sharp edges of its lettering slowly soften as rain, air, and temperature swings wear at the stone.

That is the line between the two ideas people mix up most in this unit. Weathering breaks rock down in place. Erosion is the next step, when something moving, such as a stream, a glacier, or the wind, picks up the loosened pieces and carries them away. Weathering makes the sediment. Erosion moves it. This chapter is about the first half.

Weathering, Erosion, and Deposition (Part 1)

Mechanical weathering, also called physical weathering, breaks rock into smaller pieces without changing what the rock is made of. A boulder shattered into gravel is still the same minerals, just in smaller pieces. The most important agents in New York are:

Frost wedging. Water seeps into a crack, freezes, and expands about nine percent as it turns to ice. That expansion pushes the crack wider. Melt, refreeze, repeat, and the crack splits the rock apart. Frost wedging needs temperatures that swing back and forth across the freezing point, which is exactly what a New York winter delivers.

Abrasion. Rock grinds against rock. Sand carried by a stream scours the channel. Pebbles tumbling in a river wear each other round. Wind-blown grit polishes desert rock. Every collision knocks off tiny pieces.

Exfoliation. Deeply buried rock is under enormous pressure. When erosion strips away the material above it, that pressure is released. The rock expands and peels off in curved sheets, like the layers of an onion.

Biological action. Tree roots grow into cracks and pry them open. Burrowing animals break up rock and soil. Even lichens loosen grains.

Chemical weathering does not just break rock into smaller pieces. It changes the minerals into new substances by reacting with water, air, and acids. The three reactions worth knowing:

Dissolution. Rainwater is slightly acidic, because it absorbs carbon dioxide from the air to form weak carbonic acid. That acid dissolves calcite, the mineral in limestone and marble. Over long spans of time it hollows out the caves, sinkholes, and underground streams of a karst landscape. New York has real examples, including caverns in the limestone of the state's central and eastern regions.

Oxidation. Minerals that contain iron react with oxygen, usually with water present, to form iron oxides. This is rust. It is why rock rich in iron weathers to a red or orange color, and why those minerals fall apart more easily once oxidized.

Hydrolysis. Water reacts with feldspar, the most common mineral in the crust, and turns it into clay. Since feldspar is everywhere, this single reaction produces an enormous share of the world's clay and is a major reason granite crumbles over time.

Mechanical and chemical weathering work together, and the link between them is surface area. Chemical weathering can only work where air and water touch the rock, which means the outside surfaces. Mechanical weathering breaks a rock into smaller pieces, and smaller pieces have far more surface for the chemistry to work on.

Break one block into eight smaller blocks and you have not lost any rock, but you have roughly doubled the total exposed surface. Keep breaking and the surface area keeps climbing. That is why a pile of gravel weathers chemically much faster than a single boulder of the same rock. It is also why the two kinds of weathering speed each other up.

Two identical rocks can weather at wildly different speeds depending on where they sit. Four factors set the pace:

Climate. This is the big one. Chemical weathering is fastest where it is warm and wet, because heat and water drive the reactions. Frost wedging is fastest where temperatures cross the freezing point often. A hot desert weathers slowly because it is dry. The frozen poles weather slowly because the reactions nearly stop in the cold.

Rock and mineral type. Minerals differ in how easily they break down. Quartz is extremely resistant, which is why beach sand is mostly quartz. It is what is left after everything softer has weathered away. Calcite, on the other hand, dissolves easily in acid. So a quartz sandstone holds up far better than a limestone in the same climate.

Surface area. As we saw, smaller pieces weather faster because more of the rock is exposed.

Time. Weathering is slow. The longer a rock is exposed at the surface, the more thoroughly it breaks down.

Set a climate and see how it drives the two kinds of weathering. Chemical weathering climbs with heat and moisture. Frost wedging needs temperatures that swing across freezing, so it switches on in cool, wet conditions and off in a warm or bone-dry climate.

Soil is where weathering meets life. It is a mixture of weathered rock, broken down into small mineral grains, and organic material from dead and decaying plants and animals, called humus. Add water and air in the pore spaces, and you have the thin living layer that nearly all land life depends on. Building a few centimeters of true soil can take hundreds to thousands of years.

Where soil sits on top of the bedrock it came from, it is called residual soil. Where it was carried in from somewhere else and dropped, it is transported soil. Most of New York's soil is transported. It is glacial material, ground up and spread across the state by the ice sheets of the last ice age. That is why it often does not match the bedrock directly beneath it.

Mature soil sorts itself into layers called horizons, from the organic litter at the top down through the topsoil and subsoil to the weathered parent rock and finally solid bedrock below.

Weathering breaks rock down in place. Erosion is the separate step that carries the pieces away. Mechanical weathering makes smaller pieces without changing the minerals, and frost wedging is the New York classic. Chemical weathering changes the minerals themselves through dissolution, oxidation, and hydrolysis. The two work together, because breaking rock into small pieces exposes more surface for the chemistry to work on. Climate sets the overall pace. Warm and wet means fast chemical weathering, and repeated freezing and thawing means fast frost wedging. Soil is the end product, made of weathered rock plus humus, and here in New York most of it was carried in by glaciers.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.