Three Lives of a Quartz Grain: Reading a Rock's History

Three Lives of a Quartz Grain

[1]Pick up a single grain of sand from a river bar and hold it up to the light. If it is clear and glassy, it is probably quartz, one of the toughest common minerals on Earth. That tiny grain may be older than any mountain you can see, and it may have been part of at least two different rocks. Geologists cannot watch a grain's whole story unfold, because most of it happened too slowly, too deep underground or too long ago. But by reading the clues in rocks and applying the rock cycle, they can reconstruct a history like the one that follows. Treat it as a true-to-life example, not a report on one real grain.

[2]The grain's first life began far underground, inside a body of magma the size of a city. Heat from Earth's interior had melted older rock to make the magma, and the magma was wrapped in warm rock that let its heat escape only slowly. As the melt cooled over roughly a million years, atoms of silicon and oxygen locked together in a repeating pattern, and quartz crystals grew until they pressed against the crystals of other minerals growing beside them. The result was granite, an intrusive igneous rock with interlocking crystals large enough to see. Our grain was one of those crystals, a few millimeters across, sitting several kilometers below the surface.

[3]The granite did not stay buried. Over tens of millions of years, tectonic forces pushed the region upward while rain, rivers and ice stripped away the rock on top. Eventually the granite lay in the open air. There, water seeped into cracks and froze on cold nights, wedging the rock apart. Rainwater reacted with the feldspar crystals and slowly turned them into soft clay, but the quartz resisted. One day the grain fell free of the crumbling granite. A stream carried it downhill, bumping it against other grains until its corners were worn round, and dropped it on a river delta when the current slowed. Every step of this second life ran on energy from the Sun, which drives the rain, the rivers and the wind, and on gravity, which pulled water and grains downhill.

[4]On the delta the grain was buried. Layer after layer of newer sand and mud piled on top, about 2,000 meters of it, accumulating at an average rate of about 0.1 millimeter per year. The weight of those layers squeezed the grains together, pushing out water, in the step called compaction. Water moving between the grains left behind thin coats of minerals that glued neighbor to neighbor, the step called cementation. The loose sand had become sandstone, a sedimentary rock, with the original layers still visible and our grain still round, still quartz, and still recognizable.

[5]The grain's third life began when two plates collided and the sandstone was dragged much deeper. The temperature climbed to several hundred degrees Celsius and the pressure from the rock above and the sideways squeeze of the collision became enormous. The rock never melted; if it had, the layers and the grains would have been destroyed and a new igneous rock would have formed. Instead, while the rock stayed solid, the atoms at the edges of each quartz grain rearranged and the grains grew into one another until the old round outlines disappeared. The sandstone had become quartzite, a metamorphic rock, by recrystallization. The energy for this change came again from Earth's interior and from the motion of the plates.

[6]Today that quartzite forms a ridge, because uplift has brought it to the surface once more and it resists weathering better than the softer rocks around it. But resisting is not the same as lasting forever. Frost and rain are loosening grains from the ridge, and a stream at its base is carrying them away. Our grain may be buried into a new sandstone, squeezed into a new quartzite, or carried into a zone where it finally melts and starts the whole story again as magma. The rock cycle has no required order and no finish line; the only rule is that each change needs the right energy, from the Sun at the surface or from Earth's interior below.