After the Landslide: Modeling the Rock Cycle in a Mountain Town

The Tamarack Slide

1Tamarack is a small town in a steep mountain valley. Small earthquakes shake the area several times a year, and hot springs bubble up at the edge of town. Geologists measure the nearby peaks rising a few millimeters each year as huge slabs of rock deep below are slowly pushed together. Last spring, after a week of heavy rain and fast-melting snow, a landslide sent thousands of tons of broken rock and soil onto the valley road.

2Ms. Adeyemi's seventh-grade class studied the slide with a geologist from the state survey. She explained three ways rock forms. Igneous rock forms when melted rock cools and hardens. If it cools slowly deep underground, it has large crystals that fit together like puzzle pieces; if it cools quickly at the surface as lava, its crystals are tiny. Sedimentary rock forms when bits of rock, called sediment, settle in layers and are buried, pressed and cemented together. Metamorphic rock forms when existing rock is heated and squeezed deep underground without melting, and its crystals are often lined up in bands.

3The class collected five samples from the slide and the valley (Table 1). The geologist pointed out that frost, rain and flowing water break exposed rock into smaller pieces, and that after big storms the Tamarack River runs brown with mud. The river carries this sediment to Lake Ridley, a reservoir 8 km downstream, where it settles to the bottom. As new layers pile up over thousands of years, the deepest layers end up buried under great weight.

4Each fall, the reservoir crew measures the thickness of the new sediment on the lake bottom near the dam, at the same marked spot every year so the numbers can be compared. They shared five years of measurements with the class, along with the total precipitation (rain plus melted snow) for each year (Table 2 in Section 2).