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beenjoined. Most geologists dismissed these ideas until irrefutable evidence, including actual pictures of deep-sea mountain ridges and the discovery of zebra-like magnetic stripes in the Earth’s crust, confirmed that magma upwells through the crust and forces the plates apart to form the deep-sea ridge.

    Plate tectonics provides more than just the machinery by which volcanoes can spew carbon dioxide into the atmosphere. The collisions of the plates thrust mountains upward, exposing bare rocks critical for the next step in carbon’s life-enabling cycle. Carbon dioxide in the atmosphere dissolves in raindrops, making the rain slightly acidic. When the rain falls, the acid weathers rocks. You can see this on the chiseled names and dates on tombstones as they become weathered and blurred with centuries of rainfall. The weathering process pulls carbon dioxide out of the atmosphere on geologic time scales, countering the carbon dioxide that volcanoes spew in.
    The cycle continues. Carbon-containing minerals from the weathering process eventually wash into streams, then rivers, and ultimately to the ocean. Plankton and other organisms build their shells with thesecalcium- and carbon-containing substances. The organisms die. Some shells sink to the seafloor. The sunken shells become part of the sediment, and the seafloor spreads through plate tectonics, eventually carrying the calcium- and carbon-containing sediment to the edge of another continental plate. The sediment from the sunken shells slips down into the Earth’s interior, taking the carbon with it. With high temperatures and pressures deep below the surface, carbon dioxide dissolved in the magma rises again and enters the atmosphere through volcanic eruptions. The process starts anew. Around it goes. Millions of years go by for a carbonatom to complete the cycle.
    The extraordinary, life-enabling feature of this cycle is that it operates faster or slower depending on the temperature. When times are hot, the acid-producing chemical reactions speed up, eroding more rock. This is what happened during the warm reign of the dinosaurs about 65 million years ago. With faster reactions, more carbon dioxide gets pulled from the atmosphere, and temperatures cool. When times are cool, the process slows down, more carbon dioxide accumulates in the atmosphere, and temperatures warm. The self-correcting cycle oscillates between what geologists call hot houses—times of fervent volcanic activity—and ice houses—times when weathering outpaces volcanoes. Over time scales of millions of years, weathering serves as a thermostat to regulate our planet’s climate. The thermostat prevents a runaway greenhouse like Venus or a frozen state like Mars.
    Humanity cannot change the speed of continental plates moving across the Earth’s surface, the collisions between plates that form mountains, or the frequency of volcanoes, just as we cannot control Earth’s other planetary features. But we can put more carbon dioxide in the atmosphere by burning forests and deep stores of carbon for fuels, creating the current global warming problem.
    Geologists did not always appreciate the dynamism and cycling in nature. The prevailing view in the 1700s was that floods dumped rockson the surface. The much-maligned Scottish gentleman farmer, naturalist, and geologist James Hutton argued that soils, rocks, and mountains were destroyed and created in a perpetual cycle. “We find no vestige of a beginning—no prospect of an end,” he wrote. These words were as true in the 1700s as they were for the previous millions of years and as they will be formillions of years hence.
    Carbon is the key element of life, but plants and animals can’t live from carbon alone. Like animals, plants need a host of nutrients in order to grow and thrive. Protein-enabling nitrogen and bone-building phosphorus are crucial elements for life on Earth. Plants draw their nutrients from the soil, and animals get theirs from eating