The World & I: Eternity in the Rocks

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By Marc J. Defant

Through reflection, we can better grasp the vast expanses of time embodied in swirling stars, colliding continents, sliding plates, and inexorably weathering landmasses.

The most distant object visible to the unaided eye is the Andromeda Galaxy, shown here with telescopic magnification. Although the human observer sees Andromeda as a faint, fuzzy patch, it comprises a few hundred billion stars whose light has been travelling for 2.1 million years.

he stars fascinate my six-year-old son. Last summer while on vacation, traveling at night across the uninhabited regions of southern Utah far from the interfering lights of the cities, I spied the swath of stars that makes up the plane of our disk-shaped galaxy, the Milky Way. It was worth the time to pull over and wake the children so they could witness the spectacle. But even with my son's enthusiasm, it was hard to convey that Earth is part of a medium-sized solar system on the fringe of a vast galaxy. Containing more than 200 billion stars, it spans some 100,000 light-years (a light-year is the distance traveled by light in one year).
        In many ways, the aeons of time elapsed in Earth's history are similar to the unimaginable distances of space. As we looked out on the night sky, I remembered that we were looking back in time, and the farther out we looked, the farther back in time we saw. The light coming from the Sun takes 8 minutes to travel to Earth. The next closest star, Alpha Centauri, is 4.3 light-years from Earth.         I directed my son's attention to the constellation known as Andromeda, appearing close to the Milky Way. Nestled within this cluster is the Andromeda Galaxy; the most distant object visible to the human eye, it is 2.1 million light-years away. The barely visible fuzzy patch of light did seem distant. The entire galaxy, roughly as big as our own, is so far away that it looks about the size of a large star.
        Space. Time. Their vastness is unimaginable even for a professional geologist. From a geologic perspective, the light traveling 2.1 million years from Andromeda to Earth represents merely a drop in the bucket of Earth's span of history. Two million years ago, the major landscaping events of the southwestern United States were complete, with the Sierra Nevada and Rocky Mountains having reached maturity more than seven million years earlier. Two million years ago, the Southwest was a generally arid region and the last of the spectacular dinosaurs preserved in Dinosaur National Monument in northeast Utah and northwest Colorado had gone extinct 63 million years earlier. The famous Canyonlands National Park, close to where we were that night, had formed millions of years earlier, when the gigantic Colorado Plateau was pushed upward by great geologic forces.
        If we equate the time lapsed since the Earth formed to a 100-yard football field--that is, if we place the present at our goal line and 4.556 billion years ago at our opponent's goal line--2.1 million years ago is only 1.5 inches from our chalk line. While the light of Andromeda we saw that night traversed the trillions of miles to Earth, Utah's major geological features changed little, even as local surface features changed dramatically. Certainly a great deal has happened in Utah over the last 2 million years: from the advance and recession of huge glacial ice sheets; to the sudden draining of vast Lake Bonneville, leaving as its remnant today's Great Salt Lake; and finally, the arrival of humans 10,000 years ago and the extinction of camels, mammoths, and horses.

Inklings of time's immensity

ritish naturalist James Hutton was one of the first to recognize that Earth was much older than anyone had imagined. In 1795, he proposed that understanding the processes of the present was a key to understanding Earth's history. Hutton noticed that the old Roman roads crossing his Scottish homeland were in remarkable condition, considering their age. If the forces

In Italy, British geologist Charles Lyell found strong evidence that Earth's history must be extended back into time. Prime sources were mud flats surrounding the ancient Roman port of Classis and two volcanoes, one in the Bay of Naples and the second, Mt. Etna, on Sicily. Each red dot represents one of Italy's volcanoes.

of erosion had had little effect on roads more than 1,500 years old, much more time must have been needed to form the massive sedimentary rocks he detected throughout Great Britain. He reasoned that they must have been formed by a sequence of processes that led from the eroding of older rocks to the depositing of their debris as sediments subsequently transformed into the younger rocks.
        In that same decade, a British surveyor named William "Strata" Smith made a major contribution toward visualizing Earth's history. He became quite good at recognizing similar rock formations or strata by the fossils he found in them. In the deeply buried older rocks he often found fossils with no living equivalents. He also learned that younger formations near the surface were much more likely to contain fossils resembling presently living species.
        At the time, most Europeans accepted Archbishop Usher's declaration, made in 1658, that Earth was created on Sunday, October 23, 4004 b.c. at precisely 6:00 a.m.--a date extracted from the Bible by back-calculating begats and exercising a bit of imagination. British geologist Charles Lyell challenged this orthodox worldview when he published the first volume of his Principles of Geology in 1830. Based on a plethora of evidence collected primarily from his work in Italy, he was able to show that a mere 6,000 years could not possibly have allowed enough time for the great geologic processes displayed in the rock record.
        One critical component of the rock record was sedimentary rocks. By Lyell's time, the concept of sedimentary rocks was already well established: sediments laid down by erosional processes somehow, over time, solidified into rocks. Time was one of the unknowns of the process, but in Italy Lyell found a clear time reference. He noted that the ruins of the old Roman port of Classis were separated from the coast by five miles of mudflats, sediments laid down over 1,800 years. But the sediment remained loose, unlike the large formations of solidified sedimentary rock common everywhere. How long did it take for sediments to be turned into rock? Much longer, certainly, than the time since the Roman Empire.
        Thinking of time and geological processes, Lyell noticed other disturbing evidence. Sedimentary rocks lying hundreds of feet above sea level on the sides of a volcano in the Bay of Naples particularly caught his attention. These rocks contained fossils that resembled living species. To Lyell, it could only mean that the large volcano formed recently in geologic time if the sedimentary rocks resting on its side had no fossils of extinct species.
        Later, on the side of Mount Etna, a volcano on the island of Sicily, he discovered small cones formed during ash eruptions. These cones had existed as long as anyone in the region could remember. If no one had witnessed the formation of the small cones, then how long ago had Etna formed? He found the answer in layer after layer of lava flows, ash deposits, and buried cones in valleys exposed by water erosion along the sides of the mountain. He realized that the 10,000-foot volcano had grown from hundreds or perhaps thousands of small eruptions that must have occurred over millennia.
        But what really astounded Lyell was the limestone layer that clearly ran underneath Etna and across the plains surrounding the volcano. The limestone contained fossils of animals that were exactly the same as those living in the Mediterranean of his day. The limestone, in turn, was deposited on deeper sedimentary formations, layers that must have existed before the limestone was deposited on top. These older layers contained fossils of extinct animals. Mount Etna, ancient and enduring though it is in human terms, must be a recent geologic event, built on top of young limestone.
        These were the earliest inklings of evidence that geological processes take great expanses of time to run their course. Today we know that Mount Etna formed during the past 500,000 years through periodic and often violent eruptions. If we average out its growth to a towering 10,875 feet above the foundation limestone, Etna has grown at a rate of about 2.5 inches per 1,000 years. After slowly precipitating from seawater along the ocean floor over thousands of years, the underlying limestone was eventually pushed above sea level by tectonic forces.

Dating my volcano

s a professional volcanologist, I sometimes forget how long it takes giant volcanoes to build. On the top of El Valle volcano in Panama is a caldera, or bowl-shaped structure, with a plane at the base of the bowl that stretches nearly three miles from one edge of its periphery to the other. A caldera is formed when a magma chamber below the volcano empties through eruptions, leaving a void underneath that often initiates collapse of the entire top of the volcano. Geologists witnessed the formation of such a volcanic depression during the eruption of Mount Pinatubo in the Philippines in 1990--the second largest eruption of the past century.
        Today, the floor of the El Valle caldera is speckled with homes, mostly owned by Americans working in nearby Panama City. The cool temperatures and drying breezes at this elevation make it a great reprieve from the hot and humid climate at sea level. In the center of the depression, three large domes rise from the otherwise flat terrain. They were formed from viscous lavas, squeezed out, like toothpaste, onto the caldera's floor. Unlike Lyell, we were able to obtain radiometric dates of the rocks to approximate the sequence of events. (Radiometric dating is based on the principle that radioactive elements decay at a constant rate.)
        The age obtained for the domes is 150,000 years. Since they were extruded onto the caldera's floor, it is clear that they must be younger than the caldera itself. The youngest rocks along the depression's sides were over one million years old. Logic told us that El Valle erupted and collapsed at some point between approximately 150,000 and 1 million years ago.
        Outside the caldera is a large ash deposit that may have formed during the collapse. Ash deposits consist of tiny frozen droplets of liquid rock (magma) blown out of the volcano as gas violently escapes the magma chamber. Radiometric dates of the deposit showed the ash cooled 500,000 years ago, making it likely that the depression formed during this eruption. But still, all these events at El Valle were within .4 inches of the goal line, an increment that could easily be obscured by a slight irregularity in drawing the white chalk mark of the goal line. The record of El Valle's foundations lies mostly hidden as huge ash flows that are immediately beneath it. These flows are about 15 million years old.

Mid-Pacific time steps

here is a special place in the middle of the Pacific Ocean where we can step along volcanoes back into time. The Pacific Plate, which underlies most of the Pacific, moves a few centimeters a year over a gargantuan magma chamber, a hot spot, that sits below the largest volcanoes on Earth: Mauna Loa and currently erupting Kilauea on the island of Hawaii. They rise 5.5 miles above the ocean floor. The constant motion of the plate over the hot spot of stationary magma has formed a chain of volcanoes that constitute great steps back into geological time as we move toward the northwest from the main island.
The island of Hawaii, which rests approximately above the chamber today, includes the only active volcanoes along the entire 4,000-mile chain. The Hawaiian island farthest to the northwest is Kauai, an extinct volcanic island that last erupted more than 5

Roughly 500,000 years ago, geologists estimate, a great outburst of ash from El Valle volcano in Panama produced this depression, called a caldera. Later, about 150,000 years ago, the domes in the center formed from viscous lavas squeezed out onto the caldera's floor.

million years ago. The chain continues to the northwest as a series primarily of seamounts and a few extinct volcanic islands. Many of the seamounts were once majestic volcanoes that towered above the sea but eventually were eroded below sea level by pounding surf.
Midway Island was located over the hot spot 20 million years ago. Since then, the motion of the Pacific Plate has carried it to its present position, 1,500 miles from actively erupting Kilauea. The chain of seamounts continues past Midway to where the Pacific Plate dives below the Asian continent. The oldest of the seamounts went extinct 70 million years ago. The trip that takes us across half the Pacific Ocean and 70 million years back in time only brings the ball to the 1.5-yard line.

So old and worn

f volcanoes take such a long time to build but are only a small part of the building of great mountain ranges, how long does it take for Earth's storied ranges to form? Take the Appalachians, for example. They are so old and worn that we don't known how high they once stood. Nonetheless we can learn a great deal about the range because its exterior is exposed. Geologists estimate that about 200 million years ago the Appalachians were as big as the Rockies or perhaps even the Himalayas. The massive sediments and sedimentary rocks that cover the southeastern United States and lie at the bottom of the Gulf of Mexico are a testimony not only to how large the mountains once were but to the power of erosion over great lengths of time. Erosion whittled the ancient Appalachians down to the relative nubs that exist today over a period of a little more than 200 million years. But on the scale of the football field, 200 million years is still only 4.5 yards from our goal line.
You almost have to feel the sediments to appreciate the span of time it took to erode the Appalachians. The next time you're on the beach along the Atlantic coast, grab a handful of sand. It consists of one of the hardest known common minerals, quartz, which forms in granite

Marking time steps across the Pacific, Hawaii's islands were formed by volcanoes rising from a magmatic hot spot as the great Pacific Plate moved across it. Midway Island arose from the hot spot roughly 20 million years ago. Each red dot represents one of Hawaii's volcanoes.

as magma cools many kilometers below the surface. As the quartz sifts through your fingers, imagine the processes that brought it to the beach. The rise of the mighty Appalachians made the rocks susceptible to erosion by glaciers and water. The Appalachians actually float in the mantle, achieving the same kind of isostatic equilibrium achieved by a toy boat floating in a tub of water. Push down on the boat and it sinks. But as you release the applied pressure, the boat rises in the water.
        The same is true of the Appalachians. As the rocks were eroded from the tops of the mountains, the mountains floated higher in the mantle. The process of continued erosion eventually brought the innermost sections or core of the Appalachians--the deeply formed granites, the womb of quartz--to the surface. As streams and rivers carried the minerals from the exposed granite toward the sea, the minerals were continually rounded and abraded. Only the hardest minerals, primarily quartz, survived the voyage. A casual drive across the Appalachians allows one to see the inner core of the mountain chain--one of the few exposed in the world. All those ancient granites were once 12 to 18 miles below the surface in the deep interior of the mountain belt where they formed.
        The story of the Appalachians is complex. About 600 million years ago, precursors of North America and Africa split apart, creating an ancestral Atlantic Ocean. About 500 million years ago, the continents reversed their course and began heading toward one another, closing up the ancestral Atlantic. Some 375 million years ago (about 8 yards from our chalk goal line), the ancestral ocean was lost as North America collided with Africa and Europe. Between 375 and 200 million years ago, the Appalachians reached their highest elevations, but the exact heights are unknown.
        Continents moving at only centimeters per year, much slower than turtles crossing a riverbank, were the architects of the once great Appalachian chain; its mountains were built from the collision of the ancestral continental masses. When the continents parted 200 million years ago, initiating the Atlantic Ocean, the North American continent carried with it a chunk of the African ancestral landmass. Today that chunk lies under eroded sediments from the Appalachians. While drilling for oil, geologists have penetrated rock below Florida's mile-thick sediment cover, finding fossils indigenous only to Africa.

Endless repetition

ost of Earth's history has been one of continental motion giving rise to an almost endless repetition of the same processes: continents coming together in collisions, mountains being thrust to lofty levels, continents rifting apart, erosion tearing down the mountains, and the processes starting over somewhere else. The rock remnants of former mountain chains occur in hundreds and perhaps thousands of places over all the continents of Earth. Adventurers who reach the bottom of the Grand Canyon have descended through 350 million years of sedimentary layers (about 7.5 yards on the football field) only to find themselves standing on metamorphic rocks. Contorted and deformed, these 1.5-billion-year-old rocks are the remnants of sedimentary units recrystallized under great heat and pressure.
        Think for a moment about the processes that formed the Grand Canyon's basement rocks. There is only one process we know of that can fold, crush, and recrystallize rock in this manner--mountain building during the collision of great landmasses. After those ancient mountains were eroded away, thousands of feet of sedimentary rocks were deposited on their remains and the land was uplifted, allowing the Colorado River to cut through the deposits to expose the rocks. All of this was done in less than 33 yards of the football field (1.5 billion years).
        Mountain building appears to have been an ever-present process since the earliest times of Earth. Few of the geological features on Earth's surface today will take us more than 33 yards toward the goal line at 4.556 billion years ago, and most of them date from our first 10 yards. Extremely rare old rocks have revealed some important secrets. The oldest and simplest forms of life on Earth, fossil bacteria, date back to 3.5 billion years ago (77 yards from our goal line). The oldest rock ever discovered is 3.9 billion years old (85.5 yards away).
        Geologic processes reflect variations in the scale of time. Volcanoes form over periods of less than a few million years, mountain ranges are built and destroyed in hundreds of millions of years, but Earth's interior has been cooling since our planet formed 4.556 billion years ago. Once Earth cools completely in the next few billion years, the mantle will cease to convect and the continental motion, driven by convection, will end. No more mountains or volcanoes--only erosion. After that, the continents will be eroded below sea level in a few hundred million years and the oceans will swallow the last parcels of land. In the end, ours will be a completely blue planet.

Through a time portal

hen I got the children back in the car after viewing the Milky Way over Utah, I noticed a cliff face exposed during construction of the road. I was surrounded by the immensity of time. Looking out at the stars, I was seeing the past. Along the road, where layers of rock were exposed, I could touch the distant past. As I ran my hand along the rock, I was touching Jurassic sedimentary layers--rocks that saw the light of day more than 200 million years ago. The creatures preserved in the layer lived in water tens of meters deep. It was almost like reaching through a portal in time. Who says we can't time-travel?

Additional Reading
An overview of Earth history, including the contributions of Lyle and Hutton: Preston Cloud, Oasis in Space: Earth History From the Beginning, W.W. Norton, New York, 1988.
Radiometric dating: Cherry Lewis, The Dating Game: One Man's Search for the Age of the Earth, Cambridge University Press, London, 2000.

Marc J. Defant is professor of geology at the University of South Florida. He specializes in the study of volcanoes and is the author of Voyage of Discovery: From the Big Bang to the Ice Ages, a panoramic history of the universe, including our galaxy, solar system, and planet.