Teo's Travel Journal
1My aunt Rosalind is a geologist, and this summer I rode along on her work trip across canyon country in the western United States. The canyon walls look like a stack of pancakes: tan sandstone, crumbly gray shale and, near the bottom of one canyon, a black layer about 2 m thick. Aunt Rosalind says the sandstone formed from sand dunes and beaches, and the shale formed from fine mud on the floors of ancient lakes and shallow seas.
2Our first stop was a ranch where the owners were deciding where to drill a new well. A water company had tested six wells nearby, and the table below shows its results. Groundwater fills the tiny open spaces, called pores, between grains of rock, and it can also fill cracks. A well only gives a lot of water if water can flow through the rock toward it.
3Our second stop was the black layer. Up close it was coal, with the shapes of flattened leaves and bark in it, and fossil tree stumps stood in the shale just above it. Aunt Rosalind explained that coal forms from thick piles of dead plants in swamps that get buried under mud and sand. Over millions of years, heat and pressure from the layers above slowly change the plant material into coal. She said geologists here estimate that about 10 m of packed plant material was needed to make each 1 m of coal.
4Next we drove past an oil field. Oil and natural gas formed from the remains of tiny ocean organisms buried in seafloor mud long ago. Over time they move upward through porous rock until a tight layer, such as thick shale, traps them underneath it.
5Our last stop was an old copper-mining town in hills of granite, a rock that forms when magma cools slowly deep underground. When the magma was cooling, very hot water carrying dissolved metals moved through cracks in the surrounding rock. As that water cooled, copper minerals were left behind as veins of ore.