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A Closer Look

Measuring Plate Movement
ccording to the theory of plate tectonics, Experiment, which was designed to reflect to an accuracy of 0.5 mm, about the size of
AEarth’s outer shell is made up of moving pulses of laser light from Earth. Three more the period at the end of this sentence.
plates. The plates making up the continents reflectors were later placed on the Moon, Reflected laser light experiments have
are about 100 km thick and are gradually drift- including two by other Apollo astronauts found that the Moon is pulling away from
ing at a rate of about 0.5 to 10 cm (0.2 to 4 in) and one by an unmanned Soviet Lunakhod Earth at about 4 cm/yr (about 1.6 in/yr);
per year. This reading is about one way that 2 lander. that the shape of Earth is slowly changing,
scientists know that Earth’s plates are moving The McDonald Observatory in Texas, undergoing adjustment from the compres-
and how this movement is measured. the Lure Observatory on the island of Maui, sion by the glaciers during the last ice age;
The very first human lunar landing Hawaii, and a third observatory in south- and that the observatory in Hawaii is slowly
mission took place in July 1969. Astronaut ern France have regularly sent laser beams moving away from the one in Texas. This
Neil Armstrong stepped onto the lunar sur- through optical telescopes to the reflectors. provides a direct measurement of the rela-
face, stating, “That’s one small step for man, The return signals, which are too weak to tive drift of two of Earth’s tectonic plates.
one giant leap for mankind.” In addition to be seen with the unaided eye, are detected Thus, one way that changes on the surface
fulfilling a dream, the Apollo project carried and measured by sensitive detection equip- of Earth are measured is through lunar
out a program of scientific experiments. ment at the observatories. The accuracy of ranging experiments. Results from lunar
The Apollo 11 astronauts placed a number these measurements, according to NASA ranging, together with laser ranging to arti-
of experiments on the lunar surface in the reports, is equivalent to determining the ficial satellites in Earth orbit, have revealed
Sea of Tranquility. Among the experiments distance between a point on the east and a the small but constant drift rate of the plates
was the first Laser Ranging Retro-reflector point on the west coast of the United States making up Earth’s dynamic surface.

Island
arc
Ocean
Continental Continental
lithosphere lithosphere

Trench
Oceanic
lithosphere
FIGURE 18.17 Continent-continent plate convergence. Rocks
are deformed and some lithosphere thickening occurs, but neither
Oceanic Magma plate is subducted to any great extent.
lithosphere
Shallow Deep-seated
earthquakes
earthquakes
FIGURE 18.16 Ocean-ocean plate convergence. This type of
plate convergence accounts for shallow and deep-focused earthquakes,
an oceanic trench, and a volcanic arc above the subducted plate.

seated earthquakes. These islands form where the melted sub-
ducted material rises up through the overriding plate above
sea level. The Japanese, Marianas, and Indonesians are similar
converges with the oceanic part of the North American Plate groups of arc islands associated with converging oceanic-
near the Bering Sea. The Pacific Plate is subducted, forming oceanic plate boundaries.
the Aleutian oceanic trench with a zone of earthquakes that During continent-continent plate convergence, subduction
are shallow near the trench and progressively more deep-seated does not occur as the less dense, granite-type materials tend to
toward the continent. The deeper earthquakes are associated resist subduction (Figure 18.17). Instead, the colliding plates
with the movement of more deeply subducted crust into the pile up into a deformed and thicker crust of the lighter material.
mantle. The Aleutian Islands are typical island arcs, curving Such a collision produced the thick, elevated crust known as the
chains of volcanic islands that occur over the belt of deep- Tibetan Plateau and the Himalayan Mountains.

466 CHAPTER 18 Plate Tectonics 18-12