The Face of the Earth

Landscapes of Internal Fire

On the Spot: Over a River of Lava

SueEllen Campbell

We park the cars where lava obliterates the road, then set out across the wavy, corrugated terrain of Kilauea's lower edge. The view is sweeping, simple, and subtle: to our right the clean line of the Pacific, to our left the deceptively gentle slopes of Kilauea and Mauna Loa, the most massive mountain on earth, and underfoot a wondrous textured sheen of iridescent blacks.

Despite the warm, muggy air, everyone in the group of volcanologists who have allowed me to tag along is booted, gloved, blue-jeaned, and hard-hatted, and so am I. Given all the warnings I've read, I feel a bit uneasy, so I'm carefully following the route of my most seasoned guide and watching my step on the lava. Thank goodness we aren't crossing the jagged rubble of 'a'a, so sharp and chaotic that every step might bring an ankle-breaking, skin-splitting fall. This pāhoehoe is much more level, its ropy coils most often sprawling like pools of stirred taffy or dropped and tangled skeins of yarn, but even it can be tricky when the skeins tip sideways into sinuous ravines. Now and then someone steps on a thin-skinned bubble and produces the sound of breaking glass, a reminder that these black coils might hide lava still hot enough to burn both boot and foot. A step on an especially fragile bit of crust that just happens to cover an emptied lava tube will mean bruises, scrapes, and broken bones-or immediate death if the tube is still full of molten magma. Stray too close to the shore and we might fall with a big chunk of the rock shelf in a spectacular and likely fatal splash.

Right now none of these hazards is visible. But I was also out here last night, and that was a different story. Along with dozens of others, my husband and I cautiously picked our way by flashlight far enough out over the black to see just the outside edge of one spot where a river of orange lava hit the ocean and shattered into black sand and a gaudy burst of steam. We spent a long time watching these fireworks and studying the scattered flickerings on the slopes above, spots where fresh, bright lava was advancing toward the sea, expanding the land itself-reminding us, showing us, how all such oceanic islands have been built.

Somewhere up to our left, we knew, was Pu'u 'Ō'ō, the side vent of Kilauea that has been erupting pretty much continuously since 1983. Everywhere these lava flows have left kipukas, islands within islands of untouched vegetation, shady refuges for many of Hawaii's endemic species. Where we were, on the wet, windward side of the Big Island, lava is covered in mere decades with a tangled scrub of ferns and other hardy plants, though on the dry, leeward side, the lava flows stay bare for a very long time.

Today we are aiming for a hazard: a skylight, a hole through the top of an active lava tube.

Now the leaders of our group are slowing down and gathering in our stragglers. "We're here," one says, points to our right, and leads us in a wide circle until we are facing back toward the cars. We inch forward. The air grows warmer, then hot. Something sharp and acrid hits the back of my throat and I make my breath shallow. My eyes begin to water. Wind rushes across my ears. My heart pounds. I stop moving and look-ahead a few feet, and then down.

How can I describe this?

I see a river of fire, but fire without flames, without those pale yellows, blues, and greens of a campfire, just a stunning intensity of reddish orange that says nothing so urgently as hot. I feel it scorching my skin. Thicker than water or syrup, it's the color of an electric stove burner on high. It pulses as it flows, the pulsing of hot coals, of blood in arteries. It's very fast and mostly silent-a quiet hiss, an occasional soft gurgle. It's maybe ten feet across, no telling how deep. Patches of darkness appear and disappear on the surface, orange cooling to black, folding under, returning to liquid. It looks like the films I've seen of liquid steel flowing out of huge vats into the forms of I beams, the outpouring of some enormous underground foundry.

This is not the familiar stuff of Earth's surface, this beautiful, terrifying river of fire. It is the material of the mantle channeled upward through magma chambers and plumes, melting as the pressure on it lessens closer to the surface. I have learned this. But something closer to instinct tells me what it is, too, something beneath or maybe before words, some recognition hard-wired into my species by our long association with volcanic landscapes. I know that just here and just now I'm seeing the inside of the planet, the world turned inside out.

Prologue

All Earth's landscapes are shaped in large part by structures and forces that lie below our feet, beyond the reach of our senses, and usually far from our minds. We live on our planet's surface among our human kin, things we ourselves have built, plants and other animals; time moves for us at a rate set by our own heartbeats. We can't see the spinning of the earth's core, the drifting of continents, the rising and falling of mountain ranges, and usually it takes an educated eye even to recognize the traces of such slow and mighty happenings. Sometimes, though, the ground buckles, mountains explode, water boils into the air, and our curiosity and imagination snap awake. We glimpse the inside of our globe, maybe also a different kind of time, and we begin to wonder just what it is that we stand on.

It may be that this is one of the oldest human questions. We developed as a species at least partly in Africa's Rift Valley, where the earth's surface is tearing apart and underground forces are especially visible, and many of our earliest ancestors lived among hot springs, geysers, volcanoes, and boiling lakes of lava. We know this because aridity and volcanic ash are both excellent preservatives that have helped keep for us such evidence as the footsteps of two or perhaps three walkers captured about 3.6 million years ago between two layers of ash in Laetoli, Tanzania. These hominids were probably members of the species Australopithecus afarensis, as was the woman now called Lucy, who lived some four hundred thousand years later in Ethiopia's Afar Depression, an even more active tectonic region farther north along the rift. Like the walkers' footprints, Lucy's bones were first on the surface, then swallowed into the earth's near depths. Brought back up into the light, such traces of our ancestors' moving bodies raise questions about both our planet and our own nature. Their brains were much smaller than ours and their thought processes different (they did not, for instance, know how to make stone tools), but they walked the way we do, upright, all ten toes facing forward, with a spring in their step from their arched feet. It is hard to imagine that they did not feel some of the things we do about the ground we share. At what point in our evolution did we begin to experience not just fear at such sights-mountains turning into giant clouds of ash-but also awe and curiosity about our world? And when did we begin to imagine things we could neither see nor touch?

Not just coincidentally, many of our earliest civilizations appeared in tectonically active areas, where volcanoes created fertile soils and where the uplift caused by colliding plates led to varied habitats and a mixture of fresh and salt water. In Indonesia and Central America, in the Andes and around the Mediterranean, in the sorts of places that most vividly allow us to glimpse and then create stories about the hidden inner nature of Earth, landscape features like volcanoes have been key shapers of human myths, religions, natural philosophies, and sciences.

Geological violence, we have often thought, must come from the actions of powerful beings who live inside or beneath volcanoes-gods, monsters, spirits. If so, we need to propitiate them, offer them sacrifices, or call to our defense other beings who are equally powerful but more benign. There are many such figures. The fire goddess Pele lives in Hawaii's volcanoes, the monster Oni in Japan's. Papua New Guinea's are home to the Kaia spirits. The Titan Typhon is imprisoned under Etna, along with his jailkeeper, the metalsmith Hephaestus (called Vulcan by the Romans), and the one-eyed giants, the Cyclops, who help Hephaestus at his forge. (The Titans were the first Greek gods, later overthrown by the Olympians. Their mother was Gaea, Earth; their father was Ouranos, Sky. Their names show up often in the context of geology.) Those who live near Etna and Vesuvius are protected by the Christian saints Gennaro and Agatha. Satan is ruler of a volcanic hell, and the lion-headed horses described in the Bible's book of Revelation breathe fire, smoke, and brimstone, which is volcanic sulfur.

Stories about characters like these recognize the real mystery and raw power of Earth's interior. They show us the contrast between small humans and mighty natural forces. Lava streaming down a volcano's slopes does look like long strands of hair, as it becomes in images of Pele. When they erupt, volcanoes do rumble, bang, throw out rocks, and shake the land, as stories about angry Oni and Typhon suggest. Fresh lava does resemble the melted metal of forges. And the heat and sulfuric vapors of active tectonic features do torment the human senses. We watch the world, and then we think about it with the same kinds of plots, images, and characters we use to think about ourselves; we apply our understanding of human motives, emotions, and actions to the world around us; we personify. The stories that result are the ones we usually call mythological and religious.

For a long time, some of us have also studied the natural world by looking for laws, processes, essences, and principles-often retaining personification for its metaphorical and rhetorical strengths but trying to set it aside as a source of literal explanations. This is an effort with a complicated history, one that we'll glance at in this chapter as we begin to explore some of the ways we have seen landscapes shaped by forces from inside our planet. We'll focus on the European tradition that is most often called natural philosophy. This tradition, generally speaking, developed (roughly between 600 B.C.E. and 1900 C.E.) into physical sciences like geology, chemistry, physics, and astronomy, the fields that we might say are most concerned with general laws and those that have most to do with landscapes shaped by tectonic forces. (The companion effort, natural history, which typically paid more attention to specific and often anomalous natural details, developed into biology and other life sciences. Only in the past two centuries has the word science taken on its current professional and disciplinary meaning.) We'll also look at current scientific ideas about tectonic landscapes. Along the way, we'll consider such questions as how our understanding of nature involves cultural and subjective forces-imagination, curiosity, wonder, and storytelling-as much as it does careful observations, logical deductions, and classificatory schemes. And, as we'll do throughout this book, we'll keep an eye out for places where cultural and scientific frameworks (these labels, of course, oversimplify for convenience) come together to shape our knowledge of and responses to particular kinds of landscapes.

Imagining the Interior

Let's look at some of the ways this long tradition has found to envision the physical and especially the hidden nature of our planet. What did the natural philosophers observe, think, understand, imagine, propose?

We must begin with the world's basic elements. For many early European theorists, these were fire, air, water, and earth, along with their corresponding qualities of heat, cold, wetness, and dryness. For such a simplification to work, of course, each of these four elements and qualities has to be understood broadly. So fire encompasses all sources of heat and light, including the sun, lightning, stars, and fresh lava. Air includes all gases, not just those we breathe. Water includes other liquids. And earth includes solids of all kinds. Some theorists added to their list of basics a mysterious substance they called ether, something others saw as a purer form of air or of fire. Ether-a word whose root means "to kindle, burn, or shine"-fills the atmosphere beyond the reach of air, past the clouds or the moon, and it might also be a key constituent of the soul. (This has always been the most poetic element, probably because of its intangibility. Consider Alexander Pope's wonderful line, "All the unmeasured aether flames with light.") Other thinkers added a similarly intangible "quintessence" (literally, the fifth essence) to describe the substance of which heavenly bodies are made; for alchemists, the quintessence was a substance latent in all things, one that might, ideally, be abstracted. And many insisted that change is essential as well-a force that affects everything else, the only thing, paradoxically, that endures. Variations on these ingredients and their interactions dominated all kinds of theories from the Greeks and Romans through the Renaissance and even into the nineteenth century, a testament to their flexibility and comprehensiveness as explanatory and organizing categories.

What made earth rise above water into hills and mountains? Do seafloor and dry land regularly change places, or has the process of drying out been continuous since the biblical Flood? What accounts for the sequence of rock layers? What created precious metals and gems, and how are they distributed? Is Earth losing heat, and what caused its heat in the first place? How might such oddities as corals and fossils be explained? And, more specifically, the questions examined in this chapter: What is inside the earth? Why do volcanoes erupt? How does rock melt? What creates earthquakes? Why are hot springs hot, and where is their source of water?

To answer all such questions, earth, air, fire, and water have sufficed in some combination. Consider, for instance, the question of what lies far beneath our feet. The planet is full of air and windy caverns: this is what Lucretius, Aristotle, Seneca, and Pliny thought. Or it is filled with water, great lakes, watery abysses, rivers, and moist vapors, said Plato, Virgil, St. Isidore of Seville, and Paracelsus-a theory that lasted at least nineteen centuries. Descartes declared it to be full of exhalations. John Leslie, in the nineteenth century, imagined it filled with light. And many have envisioned Earth as a globe of incandescent caverns and passageways of fire.

And the very center of the earth, the core? It is fire itself, said Pythagoras (500 B.C.E.), Athanasius Kircher (1665), and Christopher Polhem (1731). Something molten, imagined Empedocles, the philosopher we credit with insisting (about 450 B.C.E.) that everything changes constantly. Perhaps it is an empty space to which the rays of stars penetrated, then began growing toward the surface as metals, proposed Johann Glauber, 1661. Benjamin Franklin thought the core was hot gas, or maybe metal-rich fluid. It must be magnetic material (William Gilbert, 1600; Thomas Cooper, 1813), or heavy metals that sank to the center of an original ball of mud (Amos Eaton, 1818), or something solid that had absorbed heat as the planet passed through a hot region in space (Simon Poisson, 1835). Or maybe it was a great empty cavern (Mary Somerville, 1840); a dense cube or spherical tetrahedron (Richard Owen, 1857); fluid (James Dana, 1879); supercritical gas (Siegmund Guenther, 1884). Many of these theories involve change, too. Cores expand, collapse, or trade places with the crust. Heavy metals sink and precious ores grow outward.

Inside the planet, interactions among the basic elements cause earthquakes and volcanoes. From the time of the early Greeks into the nineteenth century, many natural philosophers attributed these events to the motion of winds through caverns and tunnels, the heating of these winds by friction and sometimes compression, and then their release. Vapors are often involved, they thought, caused by heat (both interior and solar) acting on rain, or by the "fermentation" (that is, chemical reactions) or burning of coal, bitumen, sulfur, and other minerals; sometimes the winds are simply "exhalations." More winds mean more pressure, perhaps explosions, and stronger earthquakes; ventilation shafts might be dug to help release that pressure. Winds that catch fire melt stone into magma and then propel gases, rocks, and ash out of craters and vents. Or explosive quakes and eruptions happen when underground fires encounter underground water. For some, volcanoes are safety valves that help reduce earthquakes. Both are more common near seas and oceans, where wave and storm pressures force salt water into the earth through fissures, and where the phases of the moon and tides affect the timing of tremors. Occasionally, it was thought, some surface event set off these forces: several days of lightning, a passing comet. Or perhaps internal exhalations release electricity, which in turn creates both earthquakes and lightning.

These are relatively sober theories. Many of them-even some of those that now sound especially unrealistic-were serious attempts to revise earlier ideas to account for new observations. Other theories have been more imaginative and sometimes much more idiosyncratic, though even the strangest typically have found more than one believer. More than a few natural philosophers have wondered about the nature of minerals and stones, whether they might be somehow organic. The thirteenth-century scholastic theologian Johannes Duns Scotus believed that stones and metals are alive. An Italian contemporary, Ristoro d'Arezzo, believed that distant stars sometimes pull earth from water to create mountains and valleys. Dante Alighieri agreed (1320), and added that the mechanism includes interior generating vapors caused by those stars. Pietro d'Anghiera wrote in 1516 that a tree of gold grows out from the earth's interior, and others of his era thought base metals had been turned into noble ones, including gold, by celestial rays. For Edward Jorden (1631), minerals grow from seed, and the heat released by their fermentation creates hot springs. For John Josselyn (1674), trees of metal grow upward in mountain hollows. For others, mountains grow like plants, heated by the earth.

That Dante Alighieri took an interest in such questions reminds us that literature, religion, and science have frequently not been clearly separate endeavors. But it is hard for us now to see what we'd call science in the model in his Divine Comedy. This complicated and inventive vision includes an icy center of the globe in which Satan is imprisoned, fiery layers farther from the center for those whose sins are of passion, a massive funnel-shaped hole from surface to core that was created when God hurled Lucifer out of Heaven, a Mount Purgatory on whose summit perches the Garden of Eden, and other elaborate mixtures of conventional Christian allegory, a poet's wild imagination, and a highly original mixture of physical and metaphysical speculations. Many scholarly hours have been spent trying to explain such cosmographies in terms that make sense to modern readers.

John Milton's Paradise Lost, written some three and a half centuries later, offers a similarly intricate mixture of classical and biblical theories, with occasional traces of the newer theories of the Renaissance. Milton imagines a "wild Abyss" of Chaos, the blinding light of Heaven above it, the created universe hanging from Heaven by a golden chain, and a volcanic Hell at the bottom, one whose vivid imagery may owe something to his visit some decades earlier to Naples, where Vesuvius was erupting, and the nearby Phlegraean Fields with their smaller volcanic features. Satan and his companion angels fall from Heaven into a

dismal Situation waste and wild,

A Dungeon horrible, on all sides round

As one great Furnace flam'd, yet from those flames

No light, but rather darkness visible

... and a fiery Deluge, fed

With ever-burning Sulphur unconsum'd.

They lie at first on a burning lake, but soon enough Satan gathers his energies to move:

Forthwith upright he rears from off the Pool

His mighty Stature; on each hand the flames

Driv'n backward slope thir pointing spires, and roll'd

In billows, leave i' th' midst a horrid Vale.

Then with expanded wings he steers his flight

Aloft, incumbent on the dusky Air

That felt unusual weight, till on dry Land

He lights, if it were Land that ever burn'd

With solid, as the Lake with liquid fire.

When Satan flies toward our universe to exact his revenge through us on God, it is through an anarchic, confused Chaos where "hot, cold, moist, and dry, four Champions fierce/Strive here for Maistry, and to Battle bring/Thir embryon Atoms"- the four classical qualities, which have not yet even crystallized into "Sea, nor Shore, nor Air, nor Fire." Although he wrote at a time when modern science was beginning to emerge and he was familiar with recent theories (such as those of Galileo, whom he met), Milton still drew on the mythic power of these ancient ideas and images to create his own immensely influential drama.

Combinations of theology, developing science, and imagination also appear in many theories based on attempts to read Genesis as a geological textbook. In these cases, the Bible provides a fixed theoretical framework but little help incorporating detailed field observations. Although specifics vary from one version to another, the story of the Flood has been a particularly durable source of ideas about mountains (raised up by the water's turbulence), valleys and ocean basins (created to contain the waters), and fossil fish and shells found inland (left by receding waters). It has been less successful at explaining such things as volcanoes, although Mount Ararat, where Noah's ark rested while the waters abated, is itself a very high volcano, far higher than its surroundings. One influential book from 1684, Thomas Burnet's Sacred Theory of the Earth, offered a more complicated story that assumed the historical occurrence of a global flood and put it in a scientific framework. The initially fluid planet, Burnet explained, settled into a solid core wrapped in layers of water and crust. Baked by the sun, the outer crust cracked, creating mountains, valleys, and ocean basins and releasing the interior waters, which rushed out as the Flood. Finally, the waters either drained back into the interior or settled into the Pacific, Atlantic, and Mediterranean basins. By the end of the nineteenth century, at least for the respected geologist, linguist, politician, and paleontologist Eduard Suess, the Flood had become a local event with local causes: an earthquake in the Persian Gulf, possibly joined by a cyclone, led to a flood in the lower Euphrates valley. Similar theories continue to be formulated today, involving such events as global or regional climate change or tsunamis set off by earthquakes or volcanoes.

Other strikingly imaginative visions of the world appeared during the same centuries, visions whose ties to the emerging earth sciences are sometimes quite tenuous. One Benoît de Maillet argued in 1720 that our world arose entirely from a diminishing sea, with mermen and mermaids becoming men and women. A prominent Frenchman, Comte Georges Louis Leclerc de Buffon, wrote in 1778 that the Atlantic Ocean was formed when Atlantis sank-more than two thousand years after Plato described the disappearance of that island. The even more prominent Englishman Charles Darwin proposed that the Pacific formed when the moon was torn from Earth. George Catlin, who is better known for his drawings and paintings of the American West and its native inhabitants than for his work on geology, thought that two currents underlay the Americas: one runs south under the Rocky Mountains, then excavates the Gulf of Mexico; the other runs north under the Andes, where volcanoes warm the Gulf Stream.

Among the more durable of these proposals, that Earth is hollow, had an influential early appearance in 1692, in an essay by Edmond Halley (of comet fame). He was wondering what makes Earth's magnetic poles move. Parts of his answer are surprisingly close to current beliefs, while other parts led to several centuries of wild speculation. Halley proposed that the planet's wandering magnetic field is caused by the rotation at different speeds of a crust and a core separated by some fluid medium ... or perhaps by three rotating concentric spheres, each illuminated by some unknown source and capable of supporting life. In 1721, a French engineer named Henri Gautier argued that gravity should decrease with depth and become negative, and that therefore (though the logic here is hard to recapture) the earth must contain a mirror image of its surface, an interior ocean connected with the exterior one at the poles, and a hollow center. A hundred years later, partly through the unlikely conduit of Puritan clergyman Cotton Mather, whose 1721 book The Christian Philosopher endorsed Halley's ideas, this theory found an American adherent in the eccentric John Cleves Symmes Jr. and his promoter, J.N. Reynolds. Symmes lobbied for funding to locate and explore those polar holes, and after his death, Reynolds worked to fund a voyage to the South Seas and on to Antarctica, where he expected to find an open sea beyond a barrier of ice. This expedition did take place, with Reynolds aboard until mutinous sailors put him off the ship in Chile; one result of his travels was a story later read by Herman Melville called "Mocha Dick, or the White Whale of the Pacific."

Edgar Allan Poe made use of Symmes's and Reynolds's ideas in a couple of short stories and in his 1838 novel, The Narrative of Arthur Gordon Pym, which ends with notorious abruptness as its narrator-protagonist is drawn down into a stormy white chasm near the South Pole. That Symmes, Reynolds, and their hollow-earth idea were familiar to Americans caught up in the romance and heroism of exploration is also evident in the use to which Henry David Thoreau put it in the final chapter of Walden (1854). "What was the meaning of that South-Sea Exploring Expedition, with all its parade and expense," he asks,

but an indirect recognition of the fact that there are continents and seas in the moral world, to which every man is an isthmus or an inlet, yet unexplored by him, but that it is easier to sail many thousand miles ... than it is to explore the private sea, the Atlantic and Pacific Ocean of one's being alone.... It is not worth the while to go round the world to count the cats in Zanzibar. Yet do this even till you can do better, and you may perhaps find some "Symmes' Hole" by which to get at the inside at last.

Probably neither Poe nor Thoreau believed this polar hole to be real; it was, after all, a possibility that diminished as polar exploration advanced during the nineteenth century. But it's typical of the two writers that Poe uses the idea as an occasion for an overwrought dramatic moment while Thoreau (who was quite interested in science) includes it as an ironic reminder that we ought first to know ourselves.

Perhaps the best-known literary exploration of the hollow-earth idea is Jules Verne's Journey to the Center of the Earth. As scholars of such matters point out, hollow can mean many things, from one enormous interior space large enough to hold a second sun to a maze of smaller caverns and passageways. Dozens of writers and dreamers in the nineteenth and early twentieth centuries (including L. Frank Baum, creator of Oz, and Edgar Rice Burroughs, creator of Tarzan) were entranced not so much by the scientific theorizing as by the opportunities such schemes offered for romance, utopian speculation, satire, and cultural critique. In Verne's case, they allowed him to tell a lively adventure story while playing with scientific ideas, or, in other words, to help create the genre of science fiction. Thanks to translations by the poet Charles Baudelaire, Verne was an admirer of Poe and even wrote a sequel to The Narrative of Arthur Gordon Pym (titled The Sphinx of the Ice Fields), which leaves Pym dead at the South Pole. In Journey to the Center of the Earth (1864), Verne takes his own characters-a scientist, his nephew, and a local guide-into the interior not through a polar hole (though he knew about Symmes's theory) but through volcanoes. They enter via Iceland's Snæfells and exit via Italy's Stromboli. As his characters travel, debating such questions as how much hotter it will be as they descend, they find emptied lava tunnels, fossil-laden sedimentary strata, a mighty cavern with walls of coal, colorful threads of precious metals, a subterranean river, a vast central sea, light like that of the aurora borealis, a forest of giant mushrooms, a battle between an ichthyosaur and a plesiosaur, a geyser, a field of bones from all the animals that have ever lived, and more-in an amusing and wildly unlikely mishmash of modern and obsolete science and pure fantasy.

The story's playful spirit-and the shift of the hollow-earth idea away from science and into popular culture and fiction-is even more evident in the two film versions of this novel. One, made in 1959 and starring James Mason, Pat Boone, and Arlene Dahl, added not just a female character and mild romantic interest but also a few musical numbers, the ruins of Atlantis, and a memorable scene in which the explorers accidentally release a torrent of water when one of them chips a large crystal off a wall. The second, made in 2008 (with Brendan Fraser in the lead), adds not just the female character-now a feminist adventure guide-but also such elements as a passage across stones kept floating in the air by magnetism and some very large diamonds the explorers carry back to the surface. In this version, they read and carry with them Verne's novel. Neither film, fittingly, shows any interest in scientific believability. Another recent movie, The Core (2003), retains some older hollow-earth elements inside a framework more closely related to modern geological theories and technology. Encased in a ship made of the wondrously hard element "unobtainium," the heroic crew descends into Earth's core to try to restore its proper rotation inside the mantle and thus fix the planet's malfunctioning magnetic field and shield. Along the way, they pass through the inside of an enormous geode; at one point, the navigator calls out, "Guys, we're dodging diamonds the size of Cape Cod." (There are such things as underground crystal caves, but they are in the crust, not farther down as these films suggest.) Up-to-date trappings aside, the makers of these movies-like Verne himself-know they are not scientists, or even natural philosophers. They are entertainers who want to tap into our curiosity about what lies below.

Certainly all these now-abandoned theories about the structure of the earth are fun to read. But reading them with any seriousness can also be very disconcerting. What begins by seeming like a very odd idea indeed may gradually come to seem quite reasonable, and as a result, what we think we know to be true may sound as fantastical as the most startling ideas ever proposed. It is more, not less, unsettling to read a bare summary of them in chronological order, such as the one offered by Susan J. Thompson's A Chronology of Geological Thinking from Antiquity to 1899, a fascinating catalog of ideas that defy easy organizing or simple story lines. What emerge most strongly in such a summary are the energy and inventiveness with which we have tried to make sense of the physical world by combining observation, deduction, and imagination-and how myth, religion, and other kinds of assumptions and beliefs help shape what we see, how we think, and what we imagine.

Yet of course we want to see some kind of reasonably linear plot in this history. We hope for some gradual replacement of mistakes with truths, or even some clear sequence of major theories, perhaps with faster progress during the Renaissance or the Enlightenment, when the scientific method should come clearly into play. Alas, no such storylines are obvious. And so we need to do some heavy editing to detect the narrative lines buried among miscellaneous theories. We can see how some topics emerge as shared concerns, then fade away. Some clusters of similar beliefs last for centuries, consolidate into fairly clear alternatives, and finally settle into broad agreement. With a strong enough desire to read the past in terms of the present-as we usually do-we can even see our current understandings slowly emerging.

For instance, although new ideas based on new evidence rarely take hold as quickly as one might think, we can find some advances that are clearly driven by new discoveries and improved instruments. The eighteenth-century discovery of inactive volcanic cones (the Puys) in the center of France is a relatively early example, and developments in nineteenth-century chemistry helped determine that columnar or prismatic basalt was not a precipitate from water but a form of cooled lava. Together, these two insights helped settle a lively debate between the Neptunists, who saw geological history in terms of water, and the Plutonists, who focused on volcanism. Technological advances, not surprisingly, accelerate closer to the present. Early in the twentieth century, the development of radioactive dating methods led to much stronger work on the age of the earth and its rocks. And in the 1960s, more knowledge of the planet's past magnetic reversals combined with the technology for seafloor exploration to help solidify the theory of plate tectonics.

Other stories illuminate some of the ways science and social history are tied together. For instance, we can see the extent to which geological theories have depended on where their inventors lived and traveled, how their work was influenced by local landscapes, intellectual and religious traditions, and economic and political conditions. While the Greeks and Italians saw volcanism as central, for instance, the English long regarded it as exceptional and focused instead on sedimentary layers. One of the first Englishmen to take volcanoes seriously was William Hamilton, envoy to Naples from 1764 to 1800, years in which Vesuvius was often active. And such important geologists as Charles Lyell took advantage of renewed access to the Continent following the French Revolution and the Napoleonic wars. Improved communications helped broaden scientific perspectives, but actual field experience has always been crucial. Several Scots, who gradually came to recognize the many volcanic features in their country, combined theories of sedimentation and volcanism and markedly increased the supposed age of the earth. American scientists resisted revisions to the age suggested by Genesis longer than Europeans did, and in the early nineteenth century, the relatively secular geologists from southern states accepted longer ages than did their more religious northern colleagues.

As we try to understand this history of ideas, we should also remember that how we think is always tangled up with how we classify things, how we organize and separate them into categories, how we draw lines between one kind of thing and the next-and according to what principles, either unconscious or deliberate. In a history like Thompson's Chronology, attempts to order the parts of the world are everywhere, as natural philosophers ask such questions as, Are there four elements, five, a hundred? Are lightning and lava like cooking fire? Are corals stone, dead creatures, or alive? What kind of thing is a fossil, and are all rocks shaped like plant or animal parts the same kind of thing? How many kinds of rock are there; how many minerals? By what criteria should we categorize rock strata? Is basalt a precipitate? If not, why does it often appear in large, flat sheets layered with sandstones and limestones?

From today's perspective, many earlier attempts to classify the world's furnishings sound very much like the inventions of Argentine short-story writer Jorge Luis Borges. In "The Analytical Language of John Wilkins," Borges describes a Chinese encyclopedia (Celestial Emporium of Benevolent Knowledge) that classifies animals into categories including those "that belong to the Emperor," "those that are trained," "mermaids," "those that tremble as if they were mad," "stray dogs," "those that are included in this classification," and "those that resemble flies from a distance." Borges also describes a classification of stones into groups of ordinary, intermediate, precious, transparent, and insoluble-a list not so different from some that have been suggested in all seriousness. Scientists would certainly say that their current classifications are better than discarded ones-more accurate, more useful, more complete, based on better information. But it's also hard to imagine that we'll ever be fully and permanently satisfied that we've finished this task. As Borges notes, "Obviously there is no classification of the universe that is not arbitrary and conjectural. The reason is very simple: we do not know what the universe is."