How Continents Move Study Guide
One of the most important discoveries of science in the last fifty years is that the Earth's continents move. Not only the continents, but the oceans move, too. In fact, oceans grow and shrink over tens to hundreds of millions of years. How this all works is described by the grand theory of plate tectonics.
In 1912, German scientist Alfred Wegener proposed that continents could move around—that they could "drift." One of Wegener's clues to the movement was the fact that the east coast of South America could fit into the lower half of the west coast of Africa, almost as if they were two pieces of a puzzle that once were joined together.
Wegener also pointed to evidence in South America, Africa, India, and Australia of the previous existence of ice sheets at about the same time, 300 million years ago. This made no sense with the continents in their present positions, because some of these sites are at today's equator. In Wegener's day, his theory was dismissed because no one could think of any mechanism for how objects as gigantic as continents could move across the earth's surface.
In the 1960s, new lines of evidence bore fruit for the idea that continents and oceans can radically shift. The key measurements came from geological research ships that drilled into the floor of the Atlantic Ocean. Scientists who participated in this Ocean Drilling Project hoisted back up to the ship cylindrical cores of the ocean's rocky floor, part of the oceanic crust. Then they analyzed each core for the direction of magnetism in the rock.
In Lesson 4, we saw that Earth's magnetic field, in the geological past, periodically underwent reversals, in which the south magnetic pole became the north magnetic pole, and the north magnetic pole became the south magnetic pole. When magma comes up from deep in the earth—for example, when a volcano erupts— the liquid lava is not magnetized. Then, as the lava cools into rocky minerals, the rock takes on a slight magnetization. This magnetization records the direction of the earth's magnetic field at the time of cooling when the rock was created.
In a line approximately running down the middle of the Atlantic Ocean, from north to south, is a great underwater mountain chain of volcanoes, the Mid-Atlantic Ridge. The ships measured the magnet ism of the volcanic rocks that form on the sea floor on either side of this Mid-Atlantic Ridge. Some rocks showed a magnetic field similar to that of today's Earth. Other rocks showed a reversed field. You can see a diagram of what was found in Figure 5.1.
This diagram of a cross-section of the Mid-Atlantic Ridge shows how the patterns of the magnetic field directions (normal or reversed) are symmetrical on both sides of the ocean floor, which has been spreading away from the ridge for nearly 200 million years. Note how magma (molten rock) comes up from the asthenosphere to create new ocean crust and lithosphere, and how the lithosphere gets thicker as it moves away from the hot ridge.
When maps were drawn of the ancient magnetic field directions in the ocean floor, the maps showed symmetrical stripes on the two sides of the Mid-Atlantic Ridge. Back in deep time, when molten magma emerged from the volcanic ridge, the rock that formed as part of the new ocean crust recorded the direction of the magnetic field. This rock then moved away from the ridge in both directions as new magma erupted from the ridge. That new magma, as it in turn emerged a few million years later and cooled, experienced a reversed magnetic field and so it recorded that direction of Earth's magnetism.
The process continued for many tens of millions of years as Earth's magnetic field was continuously recorded in the stripes in the rocks of the Atlantic Ocean's floor. The resulting pattern has been compared to a tape recorder whose tape has symmetrical patterns on both sides of the Mid-Atlantic Ridge.
Obviously, material had been emerging from and spreading away from the ridge. This was the great discovery of seafloor spreading. The ocean's floor was growing, which meant that the Atlantic Ocean was widening. Earthquakes along the mid-ocean ridges confirmed that the ridges were spreading.
The molten material that flows upward at an ocean ridge to form new ocean crust (which is part of the lithosphere) comes from the hotter, deeper asthenosphere. When ductile asthenosphere cools enough to behave like an elastic solid, it becomes lithosphere. The new lithosphere on both sides of the ridge spreads away from the ridge. In fact, the hot ridge cools and contracts as it spreads the emerging material outward to the sides. As it cools, it becomes denser and floats deeper than the younger, less dense lithosphere at the ridge. The depth of this slab of lithosphere increases as it cools during its move away from the continuously forming ridge.
Other examples of spreading ridges across Earth's oceans include a zone between Antarctica and Australia, Africa and India, and a faster spreading rise in the eastern Pacific Ocean. How fast do the oceans spread? That depends on the location of the spreading zones.
The spreading rate of the Atlantic Ocean is 2 to 4 centimeters per year (about 1 to 2 inches per year). That won't affect the cost of an airplane ticket to Europe. The spreading rate in the eastern Pacific Ocean is much faster, on the average about 10 centimeters per year (or 4 inches per year). These rates are far less than snail paces. But consider the rates as operating over tens of millions of years. South America, Africa, and Antarctica were all joined as recently as about 200 million years ago.