Decomposition of a substance requires chemical changes in its structure, leading to formation of subsequently simpler chemicals with time. Depending on the substance, this process can take place on its own (with very little effort from us or the nature) or may require intervention (human or environmental) to speed up the process. Decay of organic matter (think about our food, leaves, etc.) is a very rapid process, while the decomposition of uranium takes billions of years. In case of glass, it is really tough to decompose. Glass is made from silica (the same material sand is made of), which is very stable and a prominent constituent of Earth. It is extremely difficult to bring about a change in glass's structure (can be done to some degree with alkaline solutions). Since such intervention may not be possible by environment and would be very costly for us, glass is generally crushed and disposed off at landfill sites.
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