I bet you've never thought about it, but dirt has to be made. If you look at road cut or a canyon cut by a river, or if you've ever taken a shovel and started to dig in your backyard, you know that you don't have to go very far before you hit rock. In fact our planet is misnamed -- it shouldn't be Earth because there is very little earth on it. There is only a thin layer of dirt, a few feet thick at most. Our planet is rock, hard, solid rock.

So where does the dirt come from -- how is it made? Mostly it is made of rock. Small and larger pieces of rock mixed with decayed and decaying leaves, roots, dead bugs, etc. So to really understand where dirt comes from, we need to look at the processes that break the rock to make particles of sand and clay that we find in dirt or soil. Geologists refer to this breaking of the rock as weathering because moisture and temperature, key elements of weather, control the process.

Link to images

I. Dirt and soil, a description

A. Bedrock
B. Soil -- a mix of rock, loose mineral grains and organic matter.
C. Slippery slopes

II. The principles of weathering

A. Physical processes
1. The role of water -- freeze and thaw

2. Other physical processes

B. Chemical weathering

1. The role of water
a. reactions with feldspars

b. reactions with limestone

2. Organic acids

III. Climate and weathering rate

A. Conditions that favor physical weathering

B. Conditions that favor chemical weathering

C. A world map of soil thickness

1. Slope and climate


1. Provide evidence that soil/dirt must be formed continually for there to be dirt on the earth's surface.

2. Explain why granite will not weather as fast as shale in the mountains of the northwest.

3. Explain why granite will weather faster than shale in the Amazon rainforest.

4. Compare expected weathering rates in Pennsylvania and Florida. In Pennsylvania and Arizona. In Pennsylvania and Rainier National Park in Washington.

5. Sinkholes are a common problem in the area near Valley Forge and have caused structural damage to the King of Prussia Mall. Describe how sinkholes form.

6. Explain the formaton of 

cliffs like the one shown in this photo.

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