A Tunnel, a Plan and a Cemetery
1The city of Westbury is growing, and its planners have two big projects. A new 12 km water tunnel will carry water from a reservoir on the far side of Pike Mountain to the city's treatment plant. The planners also hope to use the warm water that seeps into the tunnel to heat buildings near its entrance. As the boring machine cut through the mountain, geologist Dr. Ottilie Grunwald logged the rock and measured the temperature of the tunnel wall (Table 1). The east half of the tunnel passes through the same rocks in reverse order. Before the water was let in, seventh graders from Westbury Middle School toured the tunnel with her.
2Dr. Grunwald explained that the shale near the entrances formed from mud that settled on an ancient sea floor, and some of its layers hold fossil shells. Farther in, the rock changes. Slate, schist and gneiss are metamorphic rocks: they formed when older rock was heated and squeezed deep underground but did not melt. Lab tests show that shale usually turns into slate at about 250 to 300 °C, slate turns into schist at about 450 to 500 °C, and schist turns into gneiss at about 600 to 700 °C. As the rock changes, new crystals grow, and they often line up in bands. At about 700 °C or more, rock that holds a little water can begin to melt into magma.
3The granite in the middle of the mountain cooled slowly from magma several kilometers underground. Today all of these rocks lie at or near the surface. Forces powered by Earth's hot interior pushed this region upward, and over millions of years, rain, ice and streams wore away the rock that once covered them. The temperature of rock goes up with depth, so the tunnel wall is warmest where the most rock lies above it.
4The city also has a 150-year-old cemetery with headstones made of four kinds of stone (Table 2). Marble is a metamorphic rock that forms from limestone. Westbury's rain is slightly acidic, so it slowly dissolves marble, while frost and rain break the other stones into tiny grains. Rainwater washes the worn bits into Pike Creek, which carries them, some as solid grains and some dissolved, all the way to the sea. In the sea, animals such as clams use dissolved minerals from the water to build their shells.