TOEFL Reading Practice #027 — The Rock That Cooled Twice

The Rock That Cooled Twice

¶1 Molten rock can cool into some of the hardest material on Earth, or just as easily into rock that shatters like glass. What decides the difference is not what the rock is made of but how quickly it hardens. That single condition splits igneous rock, rock that forms when molten material cools and solidifies, into two clearly separated categories. One kind forms wherever magma reaches the surface and hardens within minutes, hours, or at most a few days. The other forms far underground, where the same material can take thousands or even millions of years to harden completely. In one unusual case, however, a single rock carries evidence of both categories at once.

¶2 Rock that cools at Earth's surface, called extrusive rock, seldom gets more than a day to arrange itself before it freezes solid. Lava erupting from a volcano such as Kilauea in Hawaii can cool into solid rock within hours of leaving the vent. Atoms in the melt have almost no time to link into orderly crystal lattices, so most extrusive rock is fine-grained, its grains too small to see without a microscope. When cooling is fast enough, no orderly structure forms at all. Obsidian, a natural volcanic glass, hardens so quickly that its atoms freeze in a disordered arrangement, leaving no crystals whatsoever. Its edge can be thinner than a surgical scalpel, so some surgeons still choose obsidian blades for certain fine incisions.

¶3 Rock that instead cools far underground, called intrusive rock, is insulated by the surrounding earth and can take an extraordinarily long time to lose its heat. A large body of magma trapped several kilometers below the surface may need thousands to millions of years to harden completely. That much time lets atoms migrate freely through the melt and lock into large, well-formed crystals several centimeters across. Granite, the most familiar intrusive rock, owes its speckled look to just this process; its visible grains of quartz, feldspar, and mica are large enough to identify with the naked eye. The granite cliffs of Yosemite's Half Dome and El Capitan once formed as a single molten mass many kilometers underground, cooling slowly enough for such crystals to grow before millions of years of erosion finally exposed them at the surface.

¶4 Cooling rate alone, however, cannot always predict a rock's category, because a magma chamber does not always cool in a single, uninterrupted stage. Porphyritic texture, a rock structure containing large crystals surrounded by a much finer-grained material, forms when magma cools twice, first slowly and then suddenly. The dacite that erupted from Mount St. [A] Helens in 1980 shows this pattern clearly. [B] Large crystals of plagioclase and hornblende had already formed while the magma sat underground for centuries. [C] When the mountain's north face collapsed and pressure suddenly dropped, the melt rushed to the surface and hardened within hours. [D] That final rapid stage froze a fine-grained matrix around crystals that had already finished growing. In fact, a single fragment of this rock preserves two cooling rates at once, one slow enough to grow visible crystals and one too fast for any to form.

¶5 Grain size, in this way, works like a clock for geologists studying rock they never watched form. A specimen packed with large crystals signals magma that cooled slowly, probably deep underground; glassy or fine-grained rock signals magma that cooled almost instantly near the surface. Porphyritic rock, with two distinct grain sizes locked into one fragment, signals something more specific: a magma chamber that sat quietly for a long stretch before conditions above it changed abruptly. Volcanologists now read that record before anyone drills a borehole, since a rock's texture alone can indicate roughly how deep, and how disturbed, the chamber that produced it once was.