What the Landslides Uncovered
1Thornbury is a small town high in a coastal mountain range, about 60 km inland from a deep ocean trench. Small earthquakes rattle the town most months, and every few winters heavy rain sets off landslides along the mountain road. Last winter, after three days of storms, four slides tore the soil and forest off the slopes above the road and showed bare rock that is usually hidden. In March, Mr. Desmond Ekwueme's seventh-grade class walked the road with Dr. Yuki Lindahl, a geologist from the state survey. The class stayed on the pavement while she pointed out the rock in each scar (Table 1).
2Dr. Lindahl explained what each rock can tell them. Chert is a hard rock made mostly of the tiny glassy shells of radiolarians, plankton that drift in the open ocean. Today, thick layers of these shells build up mainly on the deep ocean floor far from land, where little mud from rivers reaches, and they build up very slowly, a few millimeters every thousand years. Pillow basalt forms when lava erupts underwater and cools into rounded blobs; much of the ocean floor is basalt that formed this way at mid-ocean ridges. Mudstone forms from mud, and sandstone from sand, often in rivers or near a shore.
3Offshore, a mid-ocean ridge makes new ocean floor. Drill cores show that the ocean floor is youngest at the ridge and about 10 million years old at the trench. GPS stations on the ocean floor show the plate moving toward the coast at about 4 cm per year. At the trench, the ocean plate sinks under the continent. As it slides down, some of its rock and mud can be scraped off and pressed against the edge of the continent, the way snow piles up in front of a plow.
4Earthquake records from the past 20 years show how deep the rock breaks at different distances inland (Table 2). About 200 km inland from the trench stands a line of volcanoes, where magma forms above the sinking plate. The oldest lava in that line of volcanoes is about 30 million years old.