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Reading the Rocks: The Autobiography of the Earth |
Posted by MIchael Cohen, Feb 02, 2006
Marcia Bjornerud, Reading the Rocks: The Autobiography of the Earth (Westview, 2005). 237pp. $26.00
Although this is a book that might appeal more to the amateur geologist than to the amateur astronomer, Reading the Rocks has a lot of well-presented information about the solar system, the formation of the Earth, its subsequent history, and its uniqueness among the planets. Marcia Bjornerud, a geologist teaching in a small Wisconsin college town, points out in “Prologue: Stone Crazy” that even a little backwater like Lake Winnebago has had its dramatic history: she points to the outcrop of pink rhyolite nearby as evidence of volcanoes in the area, where there were also “mammoth hunts, cataclysmic flooding, continental glaciation . . . marine inundation, luxuriant coral reefs . . . and . . . a major mountain range.” The rocks and the landscape record this history, which we ignore at our peril.
In “The Tao of Earth,” she details how Earth’s systems have worked to keep it alive and interesting while other planets (Venus, Mars, Mercury) and the moon have turned to boiling methane cauldrons or dead rocks. Lord Kelvin was fooled by Earth’s cooling rate to imagine it much younger (he thought twenty-four million years, even though Darwin knew it had to be longer and estimated at least three hundred million, an estimate still ten times too short) because radioactive isotopes in the interior (unknowable to Kelvin) generate heat that drives plate tectonics. Meanwhile, carbonate rocks on earth lock up carbon dioxide and help prevent the runaway greenhouse heating Venus has experienced. She mentions the Gaia hypothesis that looks at Earth as a self-regulating superorganism, a theory attacked by scientists because it seems teleological. But even taking the known four mass extinctions of life forms (in the Ordovician, the Devonian, the Permian, and the Cretaceous Periods) into consideration, the Earth bounces back, recycling its crust through subduction and vulcanism, avoiding runaway climate shifts through glaciation and inundation by tropical seas, providing a viable biosphere.
The third chapter is “Reading Rocks: A Primer.” She describes the replacement of a catastrophic view of the earth’s history by the uniformitarianism first argued by James Hutton (1788) and espoused by Charles Lyell in his Principles of Geology (1830-33). She presents the basic igneous, sedimentary, and metamorphic categories of rocks, introducing terms that have to do with splitting or cleaving or splintering (shale, slate, schist) as opposed to clumping, clodding or glomming together (clay). She divides sedimentary rocks into clastic (deposited as physical particles by water, wind or ice) and chemical (deposited by precipitation). Naming of clastic sedimentary rocks depends on grain size: shale or mudstone (less than 1/16 millimeter); sandstone (usually quartz grains 1/16 to 2 mm); and conglomerate, which contains chunks or pebbles of older rock or minerals. Igneous rocks are either volcanic (extrusive), formed at the surface, or plutonic (intrusive), formed below the surface in the earth’s mantle. The rate of cooling of the molten magma governs the extent of crystallization. Obsidian (volcanic glass) is noncrystalline, still structurally a liquid, while pumice is volcanic foam. But slower cooling means crystalline forms of minerals. Fractional melting brings silicon-, aluminum-, calcium-, potassium-, phosphorus-and sodium-rich but magnesium-poor (felsic) magma toward the crust from the main body of magnesium-rich (mafic) and silicon-etcetera-poor melt. The process leads to a continental crust that is largely granite (three quarters silicon dioxide, lots of aluminum, little iron), with an ocean crust that is basaltic, with more magnesium and iron and about the same amount of aluminum. Metamorphic rocks are formed from recrystallization of other rocks through heat and pressure. Sedimentary rocks tell us a lot about marine environments of the past but not much about land, which erodes. Sediments preserve hard portions of animals—fossils—but not usually the soft parts (except for the rare lagerstätten or “lode places” like the Burgess Shale and the Nebraska Ashfall). Using “index fossils” of organisms alive for only comparatively brief times, geologists began to construct a calendar or timescale. With radioactive dating using the half-lives of known isotopes, the scale was refined. She talks about the timescale at different places in the book, and a full example is printed before the first chapter.
In “The Great and the Small,” Bjornerud explains that when you plot elevation against percentage of Earth’s surface at a given height, you get two distinct high points, one at a half mile above sea level, another at two and a half miles below it, suggesting a crust built of two materials with contrasting buoyancy relative to the substrate.
But the Earth has other dimensions, and is “hydrologically, tectonically, thermally, and magnetically active.” Water flows along surface currents like the Gulf Stream (10-100 million cubic meters per second), groundwater flows through the subsurface, rivers flow into the sea (the Mississippi rarely exceeds .7 million cubic feet per second). The magnetic field of the Earth has only a twentieth the force of a refrigerator magnet, but it protects the earth from high-energy solar particles and cosmic rays. The field has declined 10% over the past 150 years, a possible sign that it’s about to reverse, a process that probably takes a millennium and has happened hundreds of times.
Water, glacial ice, and even rock are all in motion on the Earth’s surface, sometimes very quickly during earthquakes. Darwin recognized after looking at the results of a Chilean quake that earthquakes must build mountains.
Polar ice temperatures in the past can be inferred from comparing concentrations of oxygen isotopes in recent ice and they tell us “that global temperatures are strongly correlated with greenhouse gas concentrations.” But measuring Earth’s systems proves to be an “elusive goal.”
Entropy is the rule, Bjornerud points out in “Mixing and Sorting,” so it is “surprising then, that Earth is such an organized planet.” The periodic table’s organization belies the fact that 99 percent of the cosmos is hydrogen and helium. But Earth ended up with an anomalously rich mixture of heavier elements because of a supernova—our solar system mixed “spray” from a supernova with older interstellar material that we know about from chondrites, meteorites that predate the Earth. The sun formed and ignited. Then planets formed from temperature-sorted materials: heavier refractory elements and compounds near the Sun; light and volatile elements farther out. Earth formed and then was smashed with a Mars-sized planet whose core was subsumed into the Earth while the impactor’s mantle, along with some of Earth’s mantle, formed the Moon. The presence of massive Jupiter probably prevented further violent encounters with asteroids and other bodies. Earth somehow acquired more water than other planets, possibly from comets. Water acts as a flux at subduction zones to create the continental crust, which is granite, more buoyant than the basalt ocean floor and rich in rare elements. The plate tectonics which recycle ocean floor and expose continental crust to weathering were only discovered after the middle of the twentieth century.
In the past, Bjornerud writes, “Innovation and Conservation” have alternated in Earth’s rock record and biosphere. The transition from a carbon dioxide/water vapor atmosphere to one with free oxygen two billion years ago left massive layers of rust at the ocean bottoms and red beds (“in rivers or fans of sediment at the feet of ancient mountains”) where iron was oxidized and precipitated out of solution. This caused many of the predominantly anaerobic prokaryotes—the nucleus-lacking one-celled organisms that were the only form of life at the time—to die, but others assimilated mitochondria, smaller cellules that could use oxygen. Cell nuclei began to appear. The ozone layer formed in the stratosphere, protecting shallow water creatures and later land forms from deadly ultraviolet rays. Sexual reproduction seems to have originated soon after (the rock record is silent about the exact time it happened). Then an ice age intervened before multicelled creatures began to gain a foothold. Predation was invented. The Proterozoic or first-life-form period ended, and there ensued a period of five hundred million years of evolution and proliferation punctuated by mass extinctions every hundred million years or so. Around the time of the last one in the Cretaceous, which included the dinosaurs, flowering plants and insects coevolved and both exploded into hundreds of thousands of species.
Bjornerud traces, in “Strength and Weakness,” the history of views of the Earth. In Thomas Burnet’s Sacred Theory of the Earth (1680), the Earth supposedly showed the scars and ravages of the Edenic fall from grace. In Locke’s view, the Earth was as benevolent and valuable as man could make it through cultivation and use. In the eighteenth century, Carolus Linnaeus, a Swedish biologist, created a naming system that looked toward an organized and tamed nature. James Hutton’s uniformitarianism, work by chemists such as Lavoisier, the geological surveys of Meriwether Lewis and William Clark (1803-6), Charles Lyell’s Principles of Geology (1830) and James Dana’s 1869 Manual of Mineralogy (still in use) moved toward a view of the Earth as knowable mechanism. An early voice warning of limited resources was Thomas Robert Malthus’s Essay of the Principle of Population in 1798, but for most of the nineteenth century the Earth seemed to be a machine if not infinite, at least inexhaustible. Then the dust bowl of the 1930s and voices such as Rachel Carson’s (Silent Spring, 1962) and Paul Ehrlich’s (The Population Bomb, 1968) changed the view to that of a fragile, finite organism.
In “Epilogue: The Once and Future Earth,” Bjornerud argues that though “some aspects of the Earth will always remain unknown,” we must try to learn from the rock record what we can and use the knowledge wisely. She does her part by translating what a geologist can read from the rocks into language the ordinary reader can understand.
