Monday, May 02, 2011

When our global engine was fueled

kw: book reviews, nonfiction, geology, geological history, plate tectonics

When I was quite young, I was told that the Earth has mountains because the planet is still shrinking as it cools down internally. This had been the prevailing theory for about a century. Then in 1969 I changed my college major from Physics to Geology, and the University was just implementing a new curriculum based on plate tectonics as explained in the book The New View of the Earth: Moving Continents and Moving Oceans by Seiya Uyeda (the latest edition is 1978, but it was first translated and published in English in 1968).

This was a new understanding, that the crust of the Earth is composed of about a dozen stiff plates that grow by volcanic accretion (spreading) on some boundaries, grind against one another along others, and are consumed into the mantle or squeezed up into mountains at others. The most prominent and still rising mountain chain is the Himalayas, pushed up as the Indian plate jams northward into the Asian plate. The "grinding against" region of greatest familiarity to Americans is the San Andreas fault zone that runs from the Gulf of California between Baja and the rest of Mexico, northward through San Francisco and out to sea near there. Nearly all the spreading/accretion regions are beneath the sea, most notably the Atlantic, but Iceland is a portion of the mid-Atlantic Rise that emerges above sea level.

It is now thought that plate processes have proceeded for at least two or three billion years. In that time, the continents alternated between a scattered condition, even more scattered than today, and a merged condition where there was primarily one large supercontinent. The most recent merging produced a supercontinent we call Pangaea, for "all Earth". The rest of the planet was oceanic, and nearly all this global ocean was a single superocean we call Panthalassa, for "all sea". But Pangaea was C-shaped, and there were large islands that almost completed the outline of a circle closing the C, so this is considered a separate ocean has has been given the name Tethys, from the name of the wife of Oceanus, the original Greek sea god.

Tethys Ocean was first enclosed about 260 million years ago (mya), and grew in importance and size for 200 million years, then waned and was squeezed out of existence by about 6 mya. Among other things, this "temporary" ocean is responsible for the high-energy economy this generation enjoys. The growth, career and waning of Tethys is the subject of a remarkable book, Vanished Ocean: How Tethys Reshaped the World by Dorrik Stow.

Dr. Stow has done what I once hoped to do: become a globe-trotting professor of Geology. As I now judge things, I lack the strategic vision to synthesize the many threads of Earth science he has mastered and expressed in this book, so I have gratefully, and rather slowly, read through every bit. Of the many messages he brings, I'll briefly outline just one, the physiography of the continents at a few key epochs, and what that means to the modern economy. First, we will look at this figure from the book (p 24), including its caption.

The great continent that lies athwart the equator blocks an equatorial current that would otherwise run right around the planet. Whenever this has been the case, the Earth has had a period of global cooling with ice ages. Such is the condition today, with the North-South American landmass crossing the equator, and a significant secondary blockage by Africa. We are actually in the midst of an ice age that has lasted for two million years, with periodic interglacial warmings.

Between these two "icehouse condition" stages, the Earth's climate was much warmer. A circum-equatorial ocean current allows a much greater buildup and distribution of heat and a "greenhouse condition" to prevail. Tethys grew, broke through Pangaea as the supercontinent broke apart into Laurasia and Gondwana, and presided over more than 200 million years of greenhouse warming.

This growing ocean was ringed with areas of upwelling from deeper, nutrition-laden waters, wherever its currents impinged on the continental margins. Other areas of upwelling ringed the continents, until by the time tyrannosaurs were chasing Triceratops about, a very active period of organic accumulation on the ocean floors had been going on for 50-100 million years. The accumulation was facilitated by anoxic conditions in the deep ocean because of the very high surface growth of plankton. When lots of plankton die and rain down to the ocean floor under anoxic conditions, organic-rich sediments get laid down, and once they are buried and heated to about 100°C, they produce a lot of petroleum. Further heating cracks this to natural gas.

Thus, the height of the Tethys is marked by large accumulations of oil-producing sediments, which were overlaid by capping layers of clay and silt as Tethys began to be squeezed out of existence. The map below shows the major oil-bearing zones, and it is easy to see that about half of them are marginal to Tethys. As Tethys was squeezed by the later episode of continental convergence that led to today's configuration, these were concentrated into a belt that is centered on the Middle East, but also includes the Gulf of Mexico. Major secondary belts are the North Sea and offshore California-to-Alaska. The storage of all this oil was completed before 15 mya, and most of that much earlier.

Here we see the results of the second great energy-storing episode in Earth's history—the first was the laying down of great coal deposits 50-150 million years earlier, when terrestrial and swamp vegetation was buried in large amounts. The energy-rich economy we enjoy today is largely due to the growth and shrinkage of Tethys. The huge salt deposits in many of the oil provinces testify to the drying of the remnants of this great ocean, and incidentally provided trapping layers for the oil forming from its deeper sediments.

Today, the north-south axis of the Americas plus Africa is holding Earth in an icehouse condition, with periodic interglacial episodes. All of the history of civilization is a history of the latter two-thirds of the latest interglacial episode. We are possibly extending its duration by burning the legacy of millions of years of organic accumulation and releasing carbon dioxide to the atmosphere. In a few decades, the age of oil will be over. Will the human race then find itself using the fossil fuels that remain to stave off the cold of the next ice cycle? Icehouse conditions are expected to last a good while yet; Dr. Stow thinks even 50 million years into the future, and conjectures that a land connection between South America and Antarctica will have closed by then, further deepening the icehouse. Time will surely tell.

This is but one important story the author has to tell. He gives many details of the evidence he and others have gathered to piece together the story of how Pangaea was rearranged into a series of continents that coasted the great Tethys Ocean, then pressed it out of existence. The Mediterranean Sea is a re-emergence of a small portion of Tethys, and future oceans are being born as the Red Sea and the East African rift zones get organized as new spreading centers. They will probably become axial rifts of the next generation of oceans as the slow, persistent drama of plate tectonics continues.

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