The Rubber and Foam Page
Background chemistry of Rubber-
Shoe outsoles are made of synthetic rubbers. The basic elements that make a part of the synthctic rubbers is Silicon.  Silicon with Oxygen is the backbone of Silicone rubber. 
This silicon-oxygen linkage is identical to the chemical bond found in highly stable materials such as quartz,glass, and sand, and is responsible for the outstanding high temperature performance of silicones and their general resistance to dterioration factors such as ozone, corona, weathering, and radiation
The basic running shoe is made of three parts Outsole, Midsole, and the Upper.
The OUTSOLE "bottom of the shoe"
It depends of the company that is producing the shoes but their is usally two tpyes of rubber that could be used.
Carbon Rubber- Durible, somewhat stiff and relativey heavy material most likely found in a trail running shoe.
Blown Rubber- This tpye is lighter then the carbon rubber, it's provides more cushining and flexiiblility.  Blown rubber is made by injecting air into the rubber compound which makes it have a light weight.
For info on Foam
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