Sun or Inner Heat? Reading a Canyon Wall's Rock Cycle

A Stop at Lookout Point

1Over spring break, Rafael Lindqvist and his stepmom, Dana, drove to Vermilion Gorge National Park, where a river has cut a canyon about 800 m deep. At Lookout Point, Ranger Imani Okafor pointed out the layers in the wall across the canyon (Table 1). The canyon, she said, is a window into the rock cycle: Earth's materials keep changing from one kind of rock into another, driven by energy from the Sun and from Earth's hot interior.

2Ranger Okafor explained the clues. Large crystals grow when magma cools slowly deep underground, while tiny crystals and gas holes form when lava cools quickly at the surface. Sandstone, shale and limestone are made of sand, mud and shells that piled up on beaches, lake floors or shallow sea floors and were later buried, pressed and cemented together. Schist forms when older rock is buried deep, heated and squeezed, so that new minerals grow in wavy bands without the rock melting. "The schist at the bottom was once layers of mud and sand," she said. "Its minerals grow only at about 300 °C or hotter."

3Near the park entrance, a test well measured the temperature of the rock at several depths (Table 2). Heat from Earth's interior flows slowly outward, so rock is warmer the deeper you go. Forces powered by that inner heat have also pushed this whole region up by more than 2 km over millions of years, and the river has kept cutting down as the land rose.

4On the second day, Rafael and Dana hiked a steep trail to the river. On a gravel bar, Rafael picked up 100 pebbles one at a time and sorted them by rock type, using the clues in Table 1 (Graph 1). Rain, frost and the roots of plants break pieces off the canyon walls, and floods roll them downstream. Far downstream, the river flows into a reservoir, where its sand and mud settle to the bottom in still water.