PreACT Practice Test 1

The Woman Who Drew the Seafloor

In 1948, Marie Tharp took a job with a geology research group at Columbia University in New York City. She was hired to assist the male scientists who [1] studied the ocean floor. Women were not yet allowed aboard the group's research ships, so Tharp worked on land at a drafting table.

The ships carried echo sounders; instruments [2] that measure depth by timing a pulse of sound as it travels to the seafloor and back. Tharp's job was to turn thousands of these readings into profiles, side views that showed the rise and fall of the bottom along a ship's route. She plotted each separate individual reading [3] by hand. [4]

In 1952, while comparing six profiles that crossed the North Atlantic, Tharp noticed something odd. Each profile showed a deep notch in the crest of an undersea mountain range, this notch appeared [5] in the same position on every profile. Tharp concluded that the notch was a rift valley, a place where the seafloor was splitting apart. Therefore, [6] her supervisor, Bruce Heezen, dismissed the idea. At the time, most geologists rejected the theory that continents drift.

Meanwhile, a colleague was plotting the locations of undersea earthquakes on maps of the same region. The epicenters of the earthquakes falls [7] almost exactly along the line of Tharp's valley. Convinced at last, the valley became the centerpiece of the maps that Heezen and Tharp published together. [8]

Over the next two decades, the pair mapped the floors of all the world's oceans. Their work, including a 1977 painting of the entire ocean floor by the Austrian artist Heinrich Berann, revealed a chain of ridges winding more than 40,000 miles around the globe. Seen together, the ridges looked like the seams on a baseball. [9] The rift valley that Tharp first spotted in 1952 became key evidence for plate tectonics, the theory that Earth's outer shell is broken into slowly moving plates.