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 How
Rocks Are Made
Igneous, metamorphic, and sedimentary rocks form in a self-perpetuating cycle. Volcanic activity creates rocks at the Earth's surface. Erosion of all surface rocks produce sediments, which burial transforms into sedimentary rocks. Metamorphism induces further changes, and uplift and erosion expose them all. |
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 Gas
Emissions
The toxic gas sulphur dioxide is emitted by active volcanoes. At Mount St. Helen's, sulphur dioxide emissions reached peak levels during the 1980 eruption, and then gradually subsided. The measurements were made by a sampling aeroplane. |
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 Carbon
Cycle
The pathways by which carbon is cycled through ecosystems are complex. As organisms die and their cells rot, some of the carbon in them enters the soil on land or the sediments on the sea floor. Here it can be used by micro-organisms, or taken in as mineral substances such as carbonates through roots, for new plant growth. In some cases carbon is transformed into fossil fuels such as coal or oil. In addition to the natural routes, which include volcanic eruptions, humans add carbon dioxide to the atmosphere, mainly through the burning of fossil fuels.
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 Causes
of Recent Extinction
Three main factors are behind the more than 400 cases of recent animal extinction dating back to the year 1600. In the last 50 years, habitat destruction has become the most important cause of biodiversity decline. |
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