Some of the evidence supporting continental drift was presented earlier in this chapter. With the development of plate tectonics theory, researchers began testing this new model of how Earth works. In addition to new supporting data, new interpretations of already existing data often swayed the tide of opinion.

Evidence:
Ocean Drilling

Some of the most convincing evidence for seafloor spreading came from the Deep Sea Drilling Project, which operated from 1966 until 1983. One of the early goals of the project was to gather samples of the ocean floor in order to establish its age. To accomplish this, the Glomar Challenger, a drilling ship capable of working in water thousands of meters deep, was built. Hundreds of holes were drilled through the layers of sediments that blanket the oceanic crust, as well as into the basaltic rocks below. Rather than use radiometric dating, which can be unreliable on oceanic rocks because of the alteration of basalt by seawater, researchers dated the seafloor by examining the fossil remains of microorganisms found in the sediments resting directly on the crust at each drill site.

When researchers recorded the age of the sediment from each drill site and its distance from the ridge crest, they found that the sediments increased in age with increasing distance from the ridge. This finding supported the seafloor-spreading hypothesis, which predicted that the youngest oceanic crust would be found at the ridge crest—the site of seafloor production—and the oldest oceanic crust would be located adjacent to the continents.

The distribution and thickness of ocean-floor sediments provided additional verification of seafloor spreading. Drill cores from the Glomar Challenger revealed that sediments are almost entirely absent on the ridge crest and that sediment thickness increases with increasing distance from the ridge (Figure 25A). This pattern of sediment distribution should be expected if the seafloor-spreading hypothesis is correct.

The data collected by the Deep Sea Drilling Project also reinforced the idea that the ocean basins are geologically young because no seafloor older than 180 million years was found. By comparison, most continental crust exceeds several hundred million years in age, and some samples are more than 4 billion years old.

In 1983, an ocean-drilling program was launched by the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES). Now ocean-drilling research is also conducted by the International Ocean Discovery Program (IODP), an ongoing international effort that uses multiple vessels for exploration, including the massive 210-meter-long (nearly 690-foot-long) Chikyu (“planet Earth” in Japanese), which began operations in 2007 (Figure 25B). One of the goals of the IODP is to recover a complete section of the oceanic crust, from top to bottom.

Evidence:
Mantle Plumes and Hot Spots

Mapping volcanic islands and seamounts (submarine volcanoes) in the Pacific Ocean revealed several linear chains of volcanic structures. One of the most-studied chains consists of at least 129 volcanic structures that extend from the Hawaiian Islands to Midway Island and continue northwestward toward the Aleutian trench (Figure 26). Radiometric dating of this linear feature, called the Hawaiian Island–Emperor Seamount chain, showed that the volcanoes increase in age with increasing distance from the Big Island of Hawaii. The youngest volcanic island in the chain (Hawaii) rose from the ocean floor less than 1 million years ago, whereas Midway Island is 27 million years old, and Detroit Seamount, near the Aleutian trench, is about 80 million years old (refer to Figure 26).

One widely accepted hypothesis* proposes that a roughly cylindrical upwelling of hot rock that originates deep in the mantle, called a mantle plume, is located beneath the island of Hawaii. As the hot, rocky plume ascends through the mantle, the confining pressure drops, which triggers partial melting. This process is called decompression melting. The surface manifestation of this activity is a hot spot, an area of volcanism, high heat flow, and crustal uplifting that is a few hundred kilometers across. As the Pacific plate moved over the hot spot, which is thought to maintain a relatively fixed position within the mantle, a chain of volcanic structures known as a hot-spot track was built. As shown in Figure 26, the age of each volcano indicates how much time has elapsed since it was situated over the mantle plume. Of approximately  hot spots on Earth that are thought to have formed because of upwelling of hot mantle plumes, most, but not all, have hot-spot tracks.

A closer look at the five largest Hawaiian Islands reveals a similar pattern of ages, from the volcanically active island of Hawaii to the inactive volcanoes that make up the oldest island, Kauai (refer to Figure 26). Five million years ago, when Kauai was positioned over the hot spot, it was the only modern Hawaiian island in existence. Kauai’s age is evident in the island’s inactive volcanoes, which have been eroded into jagged peaks and vast canyons. By contrast, the relatively young island of Hawaii exhibits many fresh lava flows, and one of its five major volcanoes, Kilauea, remains active today. About 30 kilometers (19 miles) southeast of the coast of Hawaii, a new volcanic island is forming from hot-spot volcanism. The Kama’ehuakanaloa Seamount (formerly known as Lōʻihi) is only about 970 meters (about 3200 feet) below sea level. Geologists estimate it will break above sea level in about200,000  years.

Although the mantle plume hypothesis provides a compelling explanation for volcanism that occurs in the middle of a tectonic plate, the existence of slim mantle plumes that originate near Earth’s core–mantle boundary has not been verified by seismic studies. As a result, some geologists have proposed that the source of magma that generated the Hawaiian chain originated from localized melting in the upper mantle.

Evidence:
Paleomagnetism

You are probably familiar with how a compass operates and know that Earth’s magnetic field has north and south magnetic poles. Today these magnetic poles roughly align with the geographic poles that are located where Earth’s rotational axis intersects the surface. Earth’s magnetic field is similar to that produced by a simple bar magnet. Invisible lines of force pass through the planet and extend from one magnetic pole to the other (Figure 27). A compass needle, itself a small magnet free to rotate on an axis, becomes aligned with the magnetic lines of force and points to the magnetic poles.

Earth’s magnetic field is less obvious to us than the pull of gravity because we cannot feel it. Movement of a compass needle, however, confirms its presence. In addition, some naturally occurring minerals are magnetic and are influenced by Earth’s magnetic field. One of the most common is the iron-rich mineral magnetite, which is abundant in lava flows of basaltic composition.* Basaltic lavas erupt at the surface at temperatures greater than  , exceeding a threshold temperature for magnetism known as the Curie point (about 585°C [1085°F])). The magnetite grains in molten lava are nonmagnetic, but as the lava cools, these iron-rich grains become magnetized and align themselves in the direction of the existing magnetic lines of force. Once the minerals solidify, the magnetism they possess usually remains “frozen” in this position. Thus, they act like a compass needle because they “point” toward the position of the magnetic poles at the time of their formation. Rocks that formed thousands or millions of years ago and contain a “record” of the direction of the magnetic poles at the time of their formation are said to possess paleomagnetism, or preserved magnetism.

Apparent Polar Wandering

A study of paleomagnetism in ancient lava flows throughout Europe led to an interesting discovery. Taken at face value, the magnetic alignment of iron-rich minerals in lava flows of different ages would indicate that the position of the paleomagnetic poles had changed through time. A plot of the location of the magnetic north pole, as measured from Europe, seemed to indicate that during the past 500 million years, the pole had gradually “wandered” from a location near Hawaii northeastward to its present location over the Arctic Ocean (Figure 28). This was strong evidence that either the magnetic north pole had migrated, an idea known as polar wandering, or that the poles had remained in place and the continents had drifted beneath them—in other words, Europe had drifted relative to the magnetic north pole.

Although the magnetic poles are known to move in a somewhat erratic path, studies of paleomagnetism from numerous locations show that the positions of the magnetic poles, averaged over thousands of years, correspond closely to the positions of the geographic poles. Therefore, a more acceptable explanation for the apparent polar wandering was provided by Wegener’s hypothesis: If the magnetic poles remain stationary, their apparent movement is produced by the drift of the seemingly fixed continents.

Further evidence for continental drift came when a polar-wandering path was constructed for North America (refer to Figure 4.28A). For the first 200 million years or so, the paths for North America and Europe were found to be similar in direction—but separated by about 5000 kilometers (3000 miles). Then, during the middle of the Mesozoic era (180 million years ago), they began to converge on the present North Pole. The explanation for these curves is that North America and Europe were joined until the Mesozoic, when the Atlantic began to open. From this time forward, these continents continuously moved apart. When North America and Europe are moved back to their pre-drift positions, as shown in Figure 28B, these paths of apparent polar wandering coincide. This is evidence that North America and Europe were once joined and moved relative to the poles as part of the same continent.

Magnetic Reversals and Seafloor Spreading

More evidence for Wegener’s hypothesis emerged when geophysicists learned that over periods of hundreds of thousands of years, Earth’s magnetic field periodically reverses polarity. During a magnetic reversal, the magnetic north pole becomes the magnetic south pole and vice versa. Lava that solidified during a period of reverse polarity has been magnetized with the polarity opposite that of volcanic rocks being formed today. When rocks exhibit the same magnetism as the present magnetic field, they are said to possess normal polarity, whereas rocks exhibiting the opposite magnetism are said to have reverse polarity.

Once the concept of magnetic reversals was confirmed, researchers set out to establish a time scale for these occurrences. The task was to measure the magnetic polarity of hundreds of lava flows and use radiometric dating techniques to establish the age of each flow. Figure 29 shows the magnetic time scale established using this technique for the past few million years. The major divisions of the magnetic time scale, chrons, last roughly  million years each. As more measurements became available, researchers realized that several short-lived reversals (less than 200,000 years long) sometimes occurred during a single chron.

Meanwhile, oceanographers had begun magnetic surveys of the ocean floor in conjunction with their efforts to construct detailed maps of seafloor topography. These magnetic surveys were accomplished by towing very sensitive instruments, called magnetometers, behind research vessels (Figure 30A). The goal of these geophysical surveys was to map variations in the strength of Earth’s magnetic field that arise from differences in the magnetic properties of the underlying crustal rocks.

The first comprehensive study of this type was performed off the Pacific coast of North America and had an unexpected outcome. Researchers discovered alternating stripes of high- and low-intensity magnetism, as shown in Figure 30B. This relatively simple pattern of magnetic variation defied explanation until 1963, when British geologists Fred Vine and Drummond Matthews demonstrated that the high- and low-intensity stripes supported the concept of seafloor spreading. Vine and Matthews suggested that the stripes of high-intensity magnetism are regions where the paleomagnetism of the ocean crust exhibits normal polarity (refer to Figure 29A). Consequently, these rocks enhance (reinforce) Earth’s magnetic field. Conversely, the low-intensity stripes are regions where the ocean crust is polarized in the reverse direction and therefore weaken the existing magnetic field. But how do parallel stripes of normally and reversely magnetized rock become distributed across the ocean floor?

Vine and Matthews reasoned that as magma solidifies at the crest of an oceanic ridge, it is magnetized with the polarity of Earth’s magnetic field at that time (Figure 31). Because of seafloor spreading, this strip of magnetized crust would gradually increase in width. With a reverse in the polarity of Earth’s magnetic field, any newly formed seafloor having this reverse polarity would form in the middle of the old strip. Gradually, the two halves of the old strip would be carried in opposite directions, away from the ridge crest. Subsequent reversals would build a pattern of normal and reverse magnetic stripes, as shown in Figure 31. Because new rock is added in equal amounts to both trailing edges of the spreading ocean floor, we should expect the pattern of stripes (width and polarity) found on one side of an oceanic ridge to be a mirror image of those on the other side. In fact, a survey across the Mid-Atlantic Ridge just south of Iceland reveals a pattern of magnetic stripes exhibiting a remarkable degree of symmetry in relation to the ridge axis.

• Multiple lines of evidence have verified the plate tectonics model. For instance, the Deep Sea Drilling Project found that the age of the seafloor increases with distance from a mid-ocean ridge. The thickness of sediment atop this seafloor is also proportional to distance from the ridge. Older lithosphere has had more time to accumulate sediment.

• A hot spot is an area of volcanic activity where a mantle plume reaches Earth’s surface. Volcanic rocks generated by hot-spot volcanism provide evidence of both the direction and rate of plate movement over time.

• Magnetic minerals, such as magnetite, align themselves with Earth’s magnetic field as rock forms. These preserved magnets are records of the ancient orientation of Earth’s magnetic field. This is useful to geologists in two ways: (1) It allows a given stack of rock layers to be interpreted in terms of their orientation relative to the magnetic poles through time, and (2) reversals in the orientation of the magnetic field are preserved as “stripes” of normal and reversed polarity in the oceanic crust. Magnetometers reveal this signature of seafloor spreading as a symmetrical pattern of magnetic stripes parallel to the axis of the mid-ocean ridge.

• Curie point: The temperature above which a material loses its magnetization.

• hot spot: A mantle plume that extrudes onto Earth’s surface in an area of volcanism, high heat flow, and crustal uplift.

• hot-spot track: A chain of volcanic structures produced as a lithospheric plate moves over a stationary mantle plume.

• magnetic reversal: A change in Earth’s magnetic field from normal to reverse or vice versa.

• magnetic time scale: A record of the alternation of normal and reversed polarity over Earth’s geologic past.

• magnetometers: Sensitive instruments used to measure the strength and direction of a magnetic field at various points on Earth.

• mantle plume: A solid, mobile mass of hotter-than-normal mantle material that originates as deep as the core-mantle boundary, and ascends toward the surface, where it may lead to igneous activity.

• normal polarity: A magnetic field in which the north and south magnetic poles are at the  same geographical poles of Earth (relative to the rotational axis) as at present.

• paleomagnetism (preserved magnetism): The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized. Also called preserved magnetism.

• reverse polarity: A magnetic field opposite to that of Earth’s present magnetic field.