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What Happens During Weathering?

Weathering is a blanket term for the physical and chemical forces that act on rocks and minerals. Rock generally forms deep underground, but once at the surface a variety of processes go to work breaking it down. All of these processes are slow, some requiring millions of years to transform solid rock into new materials.

Physical Weathering
- Flowing water polishes rocks into pebbles.
  
  Several physical or mechanical process act to break apart rock. Water freezes and expands to form linear cracks in a process known as frost wedging. Heat from a fire causes rocks to splinter. Ocean waves ceaselessly batter shoreline rocks. Wind carries particles that etch patterns into soft rocks, especially in deserts. Even the cycle of warmth and cool that happens in a normal day has a cumulative effect on a rock's structural integrity. All exposed rock experiences some combination of these physical weathering processes.
Chemical Weathering
- Rust is an example of chemical weathering.
  
  Natural chemicals at the Earth's surface react with rocks to alter their chemical structures. Water is especially reactive. Some types of rock will completely dissolve if exposed to water long enough. Oxygen in the air can also break down exposed rocks and minerals. The energetic oxygen atoms cause the rock's atoms to lose electrons, weakening the overall molecular structure. Temperature and moisture are the two biggest variables in chemical weathering with the hot, humid tropics seeing the highest rates.
Biological Weathering
- Many biological process contribute to the breakdown of rocks. Plant roots push rocks apart, providing a head start to other forms of weathering. Burrowers and insects displace rocks and sediments and sometimes move them great distances. Plant and animal waste products alter the chemical balances of air, water and soil. Some microorganisms make their homes on rocky surfaces, and their life processes fundamentally alter the rocks on which they live.
Transport
- Powerful rivers carry rock downstream.
  
  As rocks break down into smaller pieces, wind and water move the particles to new areas. The bumping together of rocks in a riverbed represent a further example of weathering. Chemical weathering deposits minerals into the air or water at a much slow rate, but the process is continuous. Through transport, minerals and nutrients are spread throughout the environment. In many cases, these materials are vital to the cycle of life.
Results of Weathering
- Without weathering, there would be no soil - and no life.
  
  Topsoil is a product of weathering as well as a requirement for most plant growth. A thick layer of healthy topsoil represents many years of slow accumulation. Material that is transported away often ends up as part of a riverbed or river delta. The nutrients contained in the sediment are essential to aquatic life. If sediments are subsequently covered up and compressed, new layers of rock form. Later, these sedimentary rocks may resurface and begin the cycle over.

Rocks & Minerals