4.7: Hot Spots

Scattered throughout the world are locations where heat from the mantle flows outward through the crust at a much higher rate than in the surrounding crust. Some of these hot areas are within plates, and some are at plate boundaries. Most of them are beneath oceanic crust, but others are under continental crust (Fig. 4-11). Most hot locations occupy only a very limited area at the Earth’s surface and are, therefore, called hot spots.

Many, but not all, of Earth’s hot spots are known to be located where convection plumes upwell from the deep mantle (Fig. 4-6, CC3). Some of these may be plumes that extend all the way to the boundary between the core and mantle. Others may originate from a superplume in the lower mantle. Yet other hot spots, such as that at Yellowstone Park, seem to be linked to shallow convection processes in the upper mantle only. Finally, some locations where the hot area under the lithosphere is more extensive may not themselves be hot spots (although hot spots may occur within such regions). Instead, they may be locations where continents have remained for prolonged periods of time. At such locations, the continents may act as a blanket to trap the heat flowing upward through the upper mantle, and the heat may melt and thin the crust. The East African Rift may be an example. However, a superplume is believed to be located beneath the region in which the East African Rift occurs, and this rift may be the result of several interacting processes.

Most oceanic hot spots are characterized by active volcanoes that rise through the ocean floor to form islands. Examples of islands located at hot spots are Iceland and the island of Hawaii. Both have active volcanoes that generally erupt relatively quietly, without explosions, and bring copious amounts of magma to the surface (Fig. 4-24). The magma solidifies and steadily builds the island.

Figure 4-24. The 1983 eruption of Pu‘u‘ō‘ō, the active vent of Kilauea Volcano in Hawaii. Kilauea is a hot-spot volcano that ejects small amounts of ash and large amounts of lava in a relatively smooth, non-explosive manner. This eruption of Kilauea continued for 35 years, ending in 2018.

The quantity of lava produced by hot-spot volcanoes is so large that such volcanoes are the tallest topographic features on Earth. For example, the island of Hawaii rises about 5500 m from the seafloor to the ocean surface and then rises another 4205 m above the ocean surface to its highest mountain peak. Its total elevation, about 9700 m, is gained over a horizontal distance of less than 200 km and far exceeds the 8848-m elevation of Mount Everest. The enormous weight of Hawaii and the adjacent islands has depressed the Pacific Plate through isostatic leveling (CC2). As a result, the seafloor around the islands has a broad moat-like depression some 500 m deeper than the surrounding seafloor. The downward deformation of the Pacific Plate extends about 300 km from Hawaii. Surrounding the “moat” region is another broad area where the seafloor is deformed upward as the crust is compressed outward by the island rising through its center.

Until recently, it was thought that hot spots remain fixed in place with respect to the Earth’s rotational axis for tens or hundreds of millions of years as the lithospheric plates move over them. However, it is now known that at least some hot spots do migrate independently within the mantle, so two independent processes change the location of individual hot spots, movement of the hot spot plumes and movement of the lithospheric plates.

As a lithospheric plate moves over (or in relation to) an oceanic hot spot, the most recently formed volcanic island moves away from the hot spot. With the migration of the island from the hot spot, another segment of plate is brought over the hot spot, new volcanic vents open in it, and eventually another island may be formed. Continued plate movement takes this island away from the hot spot, and yet another island is formed, and so on. The results of that process can be seen in the seafloor topography as a trail of islands and seamounts (undersea cone-shaped mountains). In the Pacific Ocean, the chains of islands and seamounts that align northwest of Macdonald Seamount, Easter Island, and Hawaii are all remnants of their respective hot spots (Fig. 4-11).

The island and seamount trails provide a history of the rate and direction of movement of the plate relative to the hot spot. Of the Hawaiian Islands, the island of Hawaii is the youngest and has several active volcanoes, including Kilauea (Fig. 4-24) and Mauna Loa. About 20 km southeast of Hawaii, a 3.5 km tall seamount called Loihi has been built on the sea floor and continues to grow by volcanic action (Fig. 4-25). Its current peak is about 970 m below sea level, but if volcanic activity continues at its present rate, Loihi may become the next Hawaiian island 10,000 to 40,000 years from now.

Figure 4-25. (a) The Hawaiian Island–Emperor Seamount chain. (b) The islands and seamounts are formed at the hot spot. As the plate moves relative to the hot spot, new islands are formed, and the older islands move off the hot spot, cool, sink isostatically, and are eroded.

Radioisotope dating (CC7) of the volcanic rocks in the Hawaiian Island–Emperor Seamount chain shows each island or seamount to be progressively older with distance northwest from Hawaii (Fig. 4-25a). For example, Oahu was formed about 2 to 3 million years ago and only has inactive, although not yet necessarily extinct, volcanoes. As hot-spot volcanic islands migrate away from the hot spot, their volcanoes become inactive, the islands cool and sink isostatically, and they are subjected to erosion (Fig. 4-25b).

Some hot spots lie on divergent plate boundaries where two plates are moving apart. A volcanic island formed at such a location is steadily broken apart as the plates diverge. Iceland is a good example. Each side of the island, with its cooling volcanoes, migrates away from the hot spot on its respective plate. Evidence of this process can also be seen in the ocean floor topography. For example, the Icelandic Ridge stretching between Greenland and Europe consists of sediment-buried remnants of volcanoes that occupied the Icelandic hot spot when the Atlantic Ocean was narrower. Similarly, the Walvis Ridge and Rio Grande Rise in the South Atlantic Ocean are remnants of the Tristan da Cunha hot-spot volcanoes (Fig. 4-11).

A Lesson about Science

The rate of Pacific Plate movement appears to have varied relatively little during the past 50 million years. However, a distinct change in the direction of the Hawaiian Island–Emperor Seamount chain occurs at islands formed about 50 million years ago (Fig. 4-25). Seemingly identical changes of direction are also seen in the Easter Island and Macdonald Seamount chains.

Studies of the change in direction of the Hawaiian Island–Emperor Seamount chain provide a lesson for this author and the ocean and geological science community. This story provides you, the readers of this text, with a perfect example of why critical thinking skills are so important. It also illustrates that, while the material in this book represents the most recent scientific consensus regarding what is known about our ocean world, that consensus is always evolving.

As recently as 1998 (when the first edition of this text appeared), the consensus view of the scientific community was that hot spots remained fixed in place for tens or hundreds of millions of years as the lithospheric plates moved over them. At that time, the scientific consensus was also that the distinct change in the direction of the Hawaiian Island–Emperor Seamount chain indicated an abrupt (in geological time) change in the Pacific Plate’s direction of motion. Experts in the field agreed that the available data fit this conclusion. For example, the ages of the islands in the chain had been measured and did increase with distance from the hot spot. Other hot-spot trails were also found on the Pacific Plate, and they mirrored both the direction and the change in direction of the Hawaiian Island chain.

Fortunately, some scientists continued to think critically, even when the available data seemed to fit these explanations very well. These scientists were uncomfortable that nobody could explain how or why the direction of the Pacific Plate movement changed so abruptly. Thus, when the opportunity presented itself, they examined cores drilled into three seamounts of the Emperor chain that were formed before the “change in direction of motion.” They analyzed these cores for many different parameters, but the key data were obtained by examination of the paleomagnetic signatures of the magmatic rocks that form these seamounts. These data allowed the researchers to determine the rocks’ paleolatitudes (the latitudes at which these rocks had formed by cooling and solidification of liquid magma). 

To explain how paleolatitude was measured, we have to look carefully at Earth’s magnetic field characteristics. We normally hold a compass horizontally so that its needle can rotate to align north and south. However, if we turn the compass on its side and orient it toward the pole, we find that the Earth’s magnetic field is not parallel to the ground (except at the magnetic equator). In the Northern Hemisphere, the compass needle points at an angle toward the ground (the dip angle) in the direction of the north magnetic pole (Fig. 4-26). The dip angle increases as we move toward the magnetic pole. At the magnetic pole, the needle points at a 90° angle directly into the ground. At the magnetic equator, the dip angle is zero and the needle is parallel to the ground. Because the magnetic and geographic poles are always reasonably close together, the dip angle of the compass needle is a measure of latitude. Larger angles correspond to higher latitudes.

Figure 4-26. The principles of paleomagnetic dip angle. The dip angle is the angle between the rock’s direction of magnetization and the Earth’s surface (if the rock has not been tilted since being formed). Because the dip angle changes with latitude, the original dip angle of a rock’s magnetization reveals the latitude at which it was formed.

Magnetic materials in magma orient both horizontally and vertically to align with the Earth’s magnetic field when the magma solidifies. Therefore, rocks carry paleomagnetic information about both their horizontal orientation with respect to the pole at the time they were formed, and the latitude at which they were formed, the paleolatitude.

What the measured paleolatitudes of the three Emperor chain seamounts revealed was that these islands had not been formed at the same latitude that Hawaii now occupies. Instead, each older seamount had been formed progressively farther north. How could this be if hot spots are fixed in place? Of course, it cannot. Most scientists had accepted the hypothesis that hot spots remain fixed in place relative to the Earth’s axis of rotation. That hypothesis now appears to be wrong. This hot spot must have moved south during a period about 50 to 80 million years ago, when the Emperor seamounts were formed. The lesson here is that, when a new hypothesis is proposed, there are almost always a few pieces of data missing, or data that do not exactly fit the “facts” of the hypothesis. These minor inconsistencies are the clues to how a hypothesis might be wrong.

Where does this new evidence leave our understanding of hot spots and how they fit into the jigsaw puzzle of plate tectonics? First, it raises many more questions. For example, why are there almost identical changes in direction in other Pacific hot-spot trails? Did all the Pacific hot spots move in the same direction at the same time? Scientists must answer these questions, and many more, if we are to better understand plate tectonic processes.