Interlude B - Sediments and Soils

physical weathering/mechanical weathering

breaks rocks into clasts (smaller grains or chunks)

examples of physical weathering

1. jointing - natural crack in a rock that can separate one piece of rock into two

2. frost wedging

3. salt wedging - dissolved salt in groundwater precipitates in open pore spaces in rocks

4. root wedging

5. thermal expansion - when heat of forest fire bakes a rock, the outer layer of a rock expands. when it cools, the layer contracts which breaks the outer part of the rock

chemical weathering

chemical reactions that alter or destroy minerals when a rock comes in contact with water solutions or air

types of chemical weathering

1. dissolution

2. hydrolysis

3. oxidation

4. hydration

dissolution

chemical weathering where water (a solvent) dissolves minerals as it flows through a rock (affecting mostly salts and carbonate minerals)

hydrolysis

chemical weathering where water reacts with minerals and breaks them down to form other minerals

oxidation

chemical weathering where reactions in rocks transform iron-bearing minerals into a rusty-brown mixture of various iron-oxide and iron-hydroxide minerals… rusts rocks

hydration

chemical weathering where the absorption of water into the crystal structure of minerals causes some minerals (like clay) to swell. this expansion weakens rock

how does soil form

chemical and physical weathering produce loose debris and ions in solution and the resulting detritus consists of mineral grains and new weathering products

O and A horizons

dark grey to blackish topsoil, the fertile portion of soil that farmers till for planting crops - organic material

E horizon

in some places, the A-horizon grades downward into the E-horizon, which has undergone leaching but has not yet mixed with organic material

B horizon

ions and clay accumulate in the subsoil

C horizon

consists of the material derived from the substrate that’s been chemically weathered and broken apart, but hasn’t undergone leaching and accumulation. the C-horizon grades downward into unweathered bedrock or sediment

what is sediment?

consists of loose fragments of rocks or minerals broken off bedrock, mineral crystals that precipitate directly out of water, and shells or shell fragments

what is soil?

consists of rock or sediment that has been modified by physical chemical interaction with organic material, rainwater, and organisms at or just below the earth’s surface over time

examples of soil classification

rock cycle