Instruction 2-1

Water Movement | Rivers and Streams | Beaches | Habitats and Geological Events | Summary

The surface of the Earth is constantly changing. Sometimes material is added, like when lava erupts from a volcano. Most changes occur as a result of either weathering or erosion.
 
What's the difference?
 
Weathering vs. Erosion
http://www2.nature.nps.gov/geology/usgsnps/misc/gweaero.html

Weathering is the process by which exposed rocks are broken down on the spot by some element of the weather. This could be frost, wind or rain. There are two types of weathering -- physical weathering and chemical weathering.
 
One type of physical weathering is called freeze-thaw, which is the freezing and thawing of water trapped in cracks in the rocks. The other type is exfoliation, which is the flaking off of the surface of rocks due to extreme changes in temperature.
 

Chemical weathering involves a chemical change in the rocks themselves. This happens when they are soaked by rainwater that has absorbed carbon dioxide from the atmosphere. Carbolic acid is then formed and it reacts with certain minerals in the rocks to break them down into small particles.
 
Sometimes physical weathering and chemical weathering occur together. When this happens, geologists oftentimes don't know which kind of weathering caused what.
 
Ayers Rock (Uluru in the Aboriginal language) in Australia is a good example of mixed weathering. Was its shape caused by exfoliation, which is the flaking off of surface layers of rock from the heat of the Sun? Or was it the result of the chemical reactions of minerals with the area's infrequent rains? We don't know.
 
With either physical or chemical weathering, however, the decomposed rock particles stay put.
 
Once the particles begin to move, mass wasting or erosion occurs. Mass wasting is when material moves down a slope solely because of gravity (like water moves down a hillside). Rock-falls, slumps and debris-falls are examples of mass wasting.
 
Erosion occurs when some “flowing agent”, like air, ice or water, moves the rock particles. As we said, many things cause changes in the Earth's surface, but the most effective is when running water causes erosion.
 
The Water Cycle
http://ga.water.usgs.gov/edu/watercycle.html
 
Before we tell you about erosion, though, we want to remind you about The Water Cycle.
 
As you know, the Earth has a limited amount of water, and that water keeps going round and round in the continuous Water Cycle. The Water Cycle consists of:

• evaporation,
• condensation,
• precipitation, &
• collection.

Evaporation. Evaporation takes place when the Sun heats up the water in a stream, river, lake or ocean. Some of the water then turns into water vapor, which is a gas, and mixes with the atmosphere. Plants lose moisture through their leaves, like people lose water through perspiration. This process is called transpiration. Transpiration from plants also adds water vapor to the atmosphere.
 
Condensation. When the water vapor in the air gets cold, it turns back into a liquid water and forms clouds. This process is called condensation.
 
Precipitation. When so much water has condensed that the air cannot hold it any longer, it falls back to Earth as rain, hail, sleet or snow. This is called precipitation.
 
Collection. The water that falls back to Earth may land on oceans, lakes or rivers, or it may end up back on land. If it falls on land, the water may soak into the Earth and become "ground water" (a term that describes underground reservoirs of water). The fallen water may also run over the soil and collect in the oceans, lakes or rivers. This starts The Water Cycle all over again.
 
 
Running Water
http://www.uh.edu/~jbutler/physical/chapter13.html

Nothing causes more, or more dramatic, changes in the Earth's surface than running water. Just think of the Grand Canyon, which was carved out over centuries by the running water of the Colorado River.

We'll tell you more about streams and rivers in our next Instruction.

Experiments for Home and Classroom

In this Instruction, students learn about weathering. There were some excellent experiments on weathering at the end of the previous Instruction (Instruction 1-4). Go to:
http://www.consrv.ca.gov/cgs/information/kids_geozone/do_rocks_last_forever.htm

The Water Cycle is important for students to understand. For another simple demonstration of the Water Cycle, as well as suggestions about how to explain rain to children, click:
http://www.ehow.com/how_13750_explain-child-why.html

Now let's do Practice Exercise 2-1 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.