There have been a number of strange theories regarding the conditions deep within Earth's interior circulating around the internet. Claims that solar flares will cause nuclear reactions deep below our feet are perhaps the most ludicrous, but fairly easy to dismiss. More timely, perhaps, is the sudden conversion of global warming guru Al Gore into a geothermal energy booster. Evidently Gore thinks it's bad to drill for oil, but good to drill for heat. TV documentaries threaten mega-volcano eruptions and talk about mantle plumes underneath Hawaii, but just what does science tell us about our planet's interior?

Gore, trying to explain geothermal energy to Conan O’Brien, stated that geothermal energy is plentiful because the Earth’s core temperature is millions of degrees. That's not quite accurate, the inner core calculates out at about 4100 to 4200°C. Kei Hirose, from the Department of Earth and Planetary Sciences, Tokyo Institute of Technology, points out that the rocks and minerals of the deep mantle are not accessible in nature, except those occurring infrequently as inclusions in diamond. Writing in a perspective article in the January 8^th issue of Science, Dr. Hirose explains that a paper by Tetsuo Irifune et al. in that same issue could have profound implications for predicting the properties and dynamics of the deep mantle.

“Recent such experimental investigations, as well as theoretical calculations, have suggested that the properties of lower-mantle minerals vary with increasing depth much more than was previously thought,” states Hirose. Irifune et al. report changes in mantle composition for conditions corresponding to depths below 1100 km involving iron (Fe) partitioning between the two main lower-mantle constituents, iron-magnesium silicate perovskite (Pv) and iron-magnesium oxide (ferropericlase, Fp).

Traditional view of Earth's internal structure. USGS.

“Phase transitions and the chemical composition of minerals in Earth’s interior influence geophysical interpretations of its deep structure and dynamics,” state Irifune et al. “A pressure-induced spin transition in olivine has been suggested to influence iron partitioning and depletion, resulting in a distinct layered structure in Earth’s lower mantle.” The lower mantle is typically considered relatively homogenous but now it seems that the planet's internal structure is a bit more complicated than previously thought. Recent seismological studies have demonstrated that there are some minor discontinuous changes in seismic velocities in the upper to middle parts of the lower mantle as well. But what about those mantle plumes reported to cause volcanic hotspots in various places around the world?

Volcanic hot spots are thought to form by one of two mechanisms: Either mantle plumes bring hot, buoyant material to the surface from deep within the Earth's interior, or extensive processing of the upper mantle by plate tectonics causes localized volcanism in stressed or heterogeneous crust. Plumes have been proposed as the cause of a number of major volcanic events in the distant past. Hot, liquid magma welling up from deep inside the planet is thought to have caused a period of sustained, gigantic volcanic eruptions some 251 million years ago, around the time of the Permian-Triassic Extinction. As we said in The Resilient Earth:

The Permian-Triassic Extinction is widely considered the worst of all the major extinction events, killing off an estimated 95% of terrestrial life. From rock layers in Texas and Utah comes evidence that this extinction came in two parts, called extinction pulses, separated by about 10 million years. Either of the two events alone was worse than the KT Extinction that killed off the dinosaurs. Between the two events, 82% of marine genera and 50% of all marine families were extinguished. In earlier chapters, we mentioned the extent of damage this extinction inflicted on Earth's creatures, but the most telling feature was the length of time needed for life to recover. Well into the Triassic, as many as 20 million years later, the effects were still felt. Geologists and paleontologists consider this extinction a major turning point in the history of life on Earth.

One of the proposed causes of that worst of all mass extinctions is the eruptions that took place in what is now Siberia. The time of the Permian-Triassic extinction, 251 million years ago, is marked by massive floods of volcanic rock in Siberia that cover an area 1.6 million square kilometers, roughly the size of western Europe. This area, known as the Siberian Traps, contains rock deposits a mile thick in places.

The volcanic activity in Siberia did not resemble the spectacular, explosive eruptions of recent volcanoes like Mount St. Helens or Mount Pinatubo. Unlike cone-shaped mountains spewing out smoke and molten rock, the eruptions of the Siberian Traps were characterized by large amounts of lava oozing out of cracks in the ground. The volume of rock contained in these lava flows, called flood basalts, was so great it could have paved the entire Earth with a layer 20 ft (6 m) thick.

During this period of near constant eruptions, lasting a million years, Siberian volcanoes are thought to have pumped out 10,000 gigatons of carbon dioxide into the atmosphere, 14 times the amount present today. This injection of CO₂ may have been the trigger that ended the Karoo Ice Age and there are those who think that volcanoes, not an asteroid collision or nearby supernova, were the cause of the Permian-Triassic extinction.

Nothing like a million year long series of mega-volcanic eruptions to cause real climate change. A similar volcanic event around the time of the Cretaceous-Tertiary Extinction, 65 million years ago, created the Deccan Traps in the northwestern part of India. This eruption, along with a couple of asteroid impacts, may also have played a part in the most recent mass extinction event (see “Shiva The Dinosaur Killer”). These links to mass extinctions make the possible existence of mantle plumes of more than academic interest.

Such a plume may be lurking under Yellowstone National Park, where it is reputed to have caused a sequence of mega-volcanic eruptions. These eruptions occur every 450,000 years or so. A new eruption, which by most counts is overdue, would devastate North America and profoundly affect Earth's climate for centuries. Both PBS and the Discovery Channel have done documentaries on these types of eruption.

Given how significant an impact Earth's active geology has had on life and the environment, one would think that science would have figured out what lies below our feet by now. Surprisingly, even the well known theory of plate tectonics was only accepted by mainstream geology in the 1960s. Arguments still rage about what the internal processes of Earth's interior are like. In an earlier article, “Sea-Floor Study Gives Plumes From the Deep Mantle a Boost,” Science writer Richard A. Kerr framed the question almost poetically:

Earth's interior is like a pot of boiling water—very viscous, very slowly churning water. The great debate about how Earth's interior operates to shed internal heat and shape the surface began with disagreements over whether two layers in the pot—the upper and lower mantle—always remain separate, like oil and water.

That debate ended when seismologists imaged Earth's cold, brittle surface scum, the tectonic plates, and saw some of them diving all the way into the lower mantle. For the past decade, geoscientists have been focusing on the opposite question: whether plumes of hot, buoyant rock from the lower mantle are rising to the surface to fuel volcanic hot spots.

Reporting in the December 4, 2009, issue of Science, eight researchers described the most detailed seismic imaging ever taken of the hot spot beneath the island of Hawaii. “I do think it's a strong case" for a deep plume, said lead author Cecily Wolfe of the University of Hawaii, Manoa. The quality of the data and the Hawaiian plume's resemblance to theorists' expectations have bolstered claims for the existence of plumes but questions remain.

Scientists think that the Hawaiian Islands were formed when the Pacific Plate of the Earth's crust moved over a hot spot below it. Although Kaua'i is the oldest of the eight major Hawaiian Islands, it is a younger member of the Hawaiian-Emperor Volcanic Chain, a string of islands and submerged seamounts that stretch northwestward across the Pacific. The chain has a kink in it because about 40 million years ago, the Pacific Plate changed direction from north to northwest.

Island formation has been occurring over this hot spot for at least 82 million years, starting with the formation of the Meiji seamount on the outer slope of the Kuril-Kamchatka Trench. Meiji was created volcanically by the Hawaiian hotspot, grew to become an island, and later subsided to below sea level. As with the other Emperor seamounts, this all took place while the motion of the Pacific Plate carried it first north and then northwest across the Pacific. Present day islands in the same chain include Midway Island, still keeping its head above water after 27.7 million years. Kaua'i is 5.1 million years old, and the Big Island of Hawaii is less than half a million years old. The Hawaiian hot spot is presently under the island of Hawaii, but eventually the Pacific plate will move on and a new island will form.

Given the titanic energy contained in Earth's molten interior is it possible to harness that heat to produce energy for mankind? The answer is yes, with some conditions attached. As you go deeper into our planet, pressure and temperature steadily increase—the rock become hotter and denser the deeper you go. The weight of the overlying rock causes the pressure to increase and the decay of heavy radioactive elements like uranium deep in the core produces the increase in heat. Note that this is radioactive decay, not a chain reaction as found in a nuclear power reactor. The last time that uranium concentrations were high enough to create a natural chain reaction was more than a billion years ago.

However, there are some places on Earth where the decay of natural radioactive elements heats rock close enough to the surface to provide a suitable source of geothermal energy (see “Australian Hot Rocks could provide Green Energy”). In other places, hot magma welling up from the interior provides the heat. This is the case in Iceland, the Philippines and the American southwest.

Geothermal energy is one of the oldest power sources known to man. The Maoris of New Zealand use hot rocks to cook food in the ground. Around the world people also swim in warm natural springs to help soothe body aches and pains. In a more modern context, geothermal energy can be used to generate electricity by utilizing two main types of geothermal resources. Hydrothermal resources use naturally-occurring hot water or steam circulating through permeable rock, and Hot Rock resources produce super-heated water or steam by artificially circulating fluid through the rock.

This involves the circulation of water down an injection well, through deep (2 to 5 km) hot rocks with temperatures typically around 200°C. In order to allow the flow of water, tiny fractures are engineered creating an underground heat exchanger. As the water passes through the heat exchanger, it is rapidly heated to a high temperature by contact with the hot rock. The superheated water, is then returned via a production well to surface where the heat energy is converted to electricity.

While there are a number of places around the globe where the conditions are right for generating geothermal power—Iceland and California to name two—geothermal is not a panacea for the world's energy problems. The US Geological Survey (USGS) found 20,000 - 26,000 megawatts of known geothermal sites that exist throughout the United States. The USGS assessment estimates a geothermal resource base of between 95,000 and 150,000 MW. Total US generation capacity in 2007 was 986,000 MW so even maximizing geothermal potential could not replace current dirty generation capacity.

Even so, the US continues to produce more geothermal electricity than any other country, comprising approximately 30 percent of the world total. Availability of geothermal energy in the US is largely confined to the southwestern Rocky Mountain parts of the country, as can be seen from the map below. Again, to be viable with current drilling technology you need to have rock with a temperature of 200°C within 5-6 km of the surface that can be fractured to allow water flow. Not all rock formations are suitable even if they are hot enough.

In 2007, geothermal was the fourth largest source of renewable energy in the United States. Today the US has about 3,000 MW of geothermal electricity connected to the grid. Geothermal energy generated 14,885 gigawatt-hours of electricity in 2007, which accounted for 4% of renewable energy-based electricity consumption in the US. So aside from mass extinctions, killer mega-volcanoes, and new tropical islands it is possible that the heat from deep underground can provide warmth and electricity for people, at least in some locations.

Returning to the question of Earth's deep structure, has science resolved the outstanding questions? Are we closer to being able to predict volcanic outbursts and seismic activity caused by plumes from below? All the evidence is not in. “Supposed strong chemical heterogeneities in plume upwelling regions underneath Africa and the Pacific are yet to be examined,” reports Herose, “further progress in high-pressure experimental techniques will allow us to tackle these unsolved problems in the deep Earth.” It appears that the resilient Earth still holds secrets that science has yet to uncover.

Be safe, enjoy the interglacial and stay skeptical.

The Big Island of Hawaii keeps on growing. Photo USGS.