Slip Sliding Away

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Slip Slidin' Away

In the early 1960's, a new theory that suggested the surface of the Earth was composed of many separate sections of crust, called plates, became popular. These plates were moved around on the Earth's surface by convection currents and might explain earthquakes, volcanoes, seemingly out-of-place fossils, and "trails" of volcanic features. The movement of these plates is called plate tectonics. Plate boundaries are located throughout the world.

Two plates can be in contact in three different ways. A convergent zone occurs when two plates collide and usually one subducts or is forced beneath the other. In a divergent zone two plates pull away from one another forming new crust in the rift. When two plates slide past one another it forms a transform zone.

tectonic zones

In California, the North American Plate and the Pacific Plate are sliding or grinding past each other along the San Andreas Fault. Under each of those plates is located a hotspot which is blowtorching a path across the plate as the plate moves. Above the hotspot on the Pacific Plate you will find the underwater volcano Loihi, and in the past, the Hawaiian Islands and Emperor Seamounts. These oceanic hotspots are not uncommon. Above the hotspot on the North American Plate you will find Yellowstone National Park, and in the past, southern Idaho. These continental hotspots are a bit more unusual. It would appear that a hotspot is moving underneath the plates. In reality the plate is moving over the stationary hotspots.

According to many geologists, if a part of the hot, molten mantle finds its way to the surface, it will melt the overlying crust, forming magma, and possibly produce a volcanic eruption. This is known as the "Hotspot Theory." Over time, a series of these volcanic eruptions will leave a trail of volcanic evidence marking the movement of the plate over the hotspot. One such trail is believed to be the Snake River Plain. In the following lessons you will investigate the movement of plates.

Lesson 1

The movement and direction of the North American Plate is recorded in southern Idaho as a series of volcanic eruptions. For reasons not fully understood, very large caldera eruptions began in southwest Idaho and southeast Oregon about 17 Ma. These eruptions occurred when fissures in the crust allowed magma from an underground chamber to reach the surface. This caused the collapse of the overlying crust, creating a caldera or hole in the Earth's crust. These eruptions were very explosive because the silica-rich crust melted and mixed with the magma. Silica-rich magma is stiff and causes very explosive eruptions. The largest volcanic eruptions the world has ever known were caldera eruptions. The calderas were later filled in with sediment and lava flows so they are difficult to see today.

Download and print the worksheet, map and graph to complete this activity.

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Lesson 2

The movement and direction of the Pacific Plate has been recorded in the Hawaiian Islands and the Emperor Seamounts. The hotspot formed magma by melting the oceanic crust. Oceanic crust is basalt and is silica-poor, resulting in silica-poor eruptions. Basalt eruptions (silica-poor) are very fluid and not very explosive but rather can flow like a river for many miles. The Hawaiian Islands are large basaltic mountains. The Emperor Seamounts were just like the Hawaiian Islands at one time but had their tops shaved off by waves of the ocean. The tops of those old islands are now underwater and are called seamounts.

Download and print the worksheet, map and graph to complete this activity.

These documents are Adobe® Acrobat® PDF files. Acrobat® Reader for Mac and PC is available free at

Last updated: February 28, 2015

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