A Brief History of Earth

Four Billion Years in Six Chapters

Andrew H. Knoll

• Publisher: Custom House
• Year: 2021
• ISBN:
  9780062853936
• Topic: Naturwissenschaften
• Subtopic: Geologie & Geowissenschaften

Harvard's acclaimed geologist "charts Earth's history in accessible style"(AP)

"A sublime chronicle of our planet."– Booklist, STARRED review

How well do you know the ground beneath your feet?

Odds are, where you're standing was once cooking under a roiling sea of lava, crushed by a towering sheet of ice, rocked by a nearby meteor strike, or perhaps choked by poison gases, drowned beneath ocean, perched atop a mountain range, or roamed by fearsome monsters. Probably most or even all of the above.

The story of our home planet and the organisms spread across its surface is far more spectacular than any Hollywood blockbuster, filled with enough plot twists to rival a bestselling thriller. But only recently have we begun to piece together the whole mystery into a coherent narrative. Drawing on his decades of field research and up-to-the-minute understanding of the latest science, renowned geologist Andrew H. Knoll delivers a rigorous yet accessible biography of Earth, charting our home planet's epic 4.6 billion-year story. Placing twenty first-century climate change in deep context, A Brief History of Earth is an indispensable look at where we've been and where we're going.

Features original illustrations depicting Earth history and nearly 50 figures (maps, tables, photographs, graphs).

1

Chemical Earth

MAKING A PLANET

Todd Marshall

IN THE BEGINNING WAS . . . well . . . a jot, a speck, a fleck at once incomprehensibly small but unimaginably dense. It wasn’t a localized concentration of stuff in the vast emptiness of the universe. It was the universe. How it got there, no one knows.

What, if anything, came before is equally mysterious, but about 13.8 billion years ago, this primordial kernel of universe began to expand rapidly—a “Big Bang” that unleashed an immense outward tide of energy and matter. Not the rocks and minerals of our daily existence; not even the atoms from which rocks, air, and water are built. At the dawn of the universe, matter consisted of quarks, leptons, and gluons, a curious cast of subatomic particles that would eventually coalesce into atoms.

Our understanding of the universe and its history comes largely from the most ephemeral of sources: light. The luminous pinpricks that give shape to the night sky may seem unlikely history books, but two properties of light help us to understand how the universe has evolved. First, the intensity of different wavelengths in incoming radiation points to the composition of its source. Our eyes can detect only a narrow range of wavelengths, but stars and other heavenly bodies emit or absorb a broad spectrum of radiation, from radio- and microwaves to x-rays and gamma rays, each with a story to tell. And, importantly, light obeys a strict speed limit: 299,792,458 meters per second, or 186,276 miles per second, in space. Sunlight is emitted eight minutes and twenty seconds before we see it, and for stars and other bodies farther away, the light we record emanated still earlier—much earlier for the most distant objects. That’s what makes our starry sky a celestial history book.

Microwaves distributed evenly across the sky speak of the Big Bang and its immediate aftermath, and radiation from the first generation of stars, formed a few hundred thousand years after time began, is just reaching us today. How did these early stars form? It all has to do with gravity, the architect of the universe. Gravity describes the attraction between different objects, with the strength of the attraction determined by the masses of the objects and the distance between them. As atoms formed within the early, expanding universe, gravity began to pull them together. Local aggregations grew, strengthening their gravitational pull, and eventually they collapsed into hot, dense balls, so hot and so dense that hydrogen nuclei fused to form helium, releasing light and heat. When that happens, a star is born. Large, hot, and short-lived, those primordial stars set the course of all that would come later, including us.

The matter generated by the Big Bang consisted mostly of hydrogen atoms, the simplest of elements, along with some deuterium (hydrogen with an added neutron) and helium. A tiny bit of lithium formed as well, along with still smaller amounts of other light elements, but there wasn’t much else. Actually, there was something else, but we don’t quite know what it is. In the 1950s, astronomers began to use the motions of stars and galaxies (a collection of stars, gas, and dust held together by, once again, gravity) to calculate gravitational attraction in deep space, but when they summed up the mass of all known objects in the sky they found it insufficient to account for their observations. There had to be something else out there, something that interacts with normal matter through gravity but doesn’t interact with light; astronomers dubbed it dark matter. Astronomers have thoughts about what dark matter might be, but no one is certain. Even more mysterious is dark energy, also deemed necessary to explain the workings of the universe. Together, dark matter and dark energy are thought to make up some 95 percent of all that exists, enigmatic constituents that we can’t detect but which are thought to have played a major role in shaping the universe. We still have a lot to learn.

Let’s get back to conventional matter. As the age of starlight began, the universe was a cold, diffuse cocktail of (mostly) hydrogen atoms. Early stars generated more helium, but there was nothing you could make into an Earth (see table). Where did the iron, silicon, and oxygen needed to build our planet come from? And what about the carbon, nitrogen, phosphorus, and other elements that make up your body? These and all other elements originated in succeeding generations of stars, foundries of the atoms that would one day form our planet. At the high temperatures and pressures within large stars, light elements fused to form carbon, oxygen, silicon, and calcium; iron, gold, uranium, and other heavy elements were forged in the giant stellar explosions called supernovae. The face you see in the mirror may be decades old, but it is made of elements formed billions of years ago in ancient stars.

Through the immensity of time, stars formed and died, each cycle adding to the inventory of the elements concentrated today in Earth and life. Galaxies merged and black holes (regions so dense that no light can escape) emerged, slowly shaping the universe we observe today.

We pick up the story about 4.6 billion years ago, focusing on an unassuming cloud of hydrogen atoms, along with small amounts of gas, ice, and mineral grains within the spiraling arm of a nondescript galaxy called the Milky Way. At first, the cloud was large, diffuse, and cold (really cold, with temperatures of 10–20 degrees Kelvin, or –460 to –420 degrees Fahrenheit). Probably nudged by a nearby supernova, this cloud began to collapse into a much smaller, denser, and hotter nebula. As had occurred billions of times elsewhere in the universe, gravity eventually drew most of the cloud into a hot, dense, central mass—our Sun. Most of the nebula’s hydrogen went into the Sun, but ice and mineral grains were partitioned into a disk that rotated around our fledgling star, broadly reminiscent of the rings of tiny particles that encircle Saturn today (Figure 1). At first, this disk was hot enough to vaporize the minerals and ices from which it formed. Over a few million years, however, it began to cool, faster in its outer reaches and slower close to the Sun’s heat.

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ELEMENTAL COMPOSITION OF THE EARTH AND LIFE

(percent, by weight)

Earth
Iron	33
Oxygen	31
Silicon	19
Magnesium	13
Nickel	1.9
Calcium	0.9
Aluminum	0.9
Everything else	0.3
Cells in the human body:
Oxygen	65
Carbon	18
Hydrogen	10
Nitrogen	3
Calcium	1.5
Phosphorous	1
Everything else	1.5

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FIGURE 1. This remarkable image, taken by the Atacama Large Millimeter Array, shows HL Tauri, a young Sun-like star, and its protoplanetary disk. The rings and gaps evident in the image record emerging planets as they sweep their orbits clear of dust and gas. Our own solar system may have looked much like this 4.54 billion years ago. ALMA (ESO/NAOJ/NRAO)/NASA/ESA

We know from our everyday experience that different substances melt or crystallize at distinct temperatures. At the Earth’s surface, for example, water will turn to ice at 0 °C (32 °F), but dry ice freezes from carbon dioxide at much lower temperatures (–78.5 °C). In much the same way, the minerals found in rocks crystallize from molten precursors at temperatures that range from hundreds to more than 1,000 °C. For this reason, as the planetary disk cooled, different materials crystallized into solids at different times and distinct places, all in relation to their respective distances from the Sun’s heat. Oxides of calcium, aluminum, and titanium formed first; then metallic iron, nickel and cobalt, and only later, beyond a distance from the Sun christened the frost line, ices of water, carbon dioxide, carbon monoxide, methane, and ammonia—the materials of oceans, air, and life. Bits of minerals and ice collided to form larger particles, and these coalesced into still bigger bodies. Within a few million years, only a handful of large spherical structures remained where the disk once rotated. The “third rock from the sun” was the Earth, a stony mass orbiting the Sun from a distance of about 93 million miles (150 million kilometers).

HOW, SPECIFICALLY, DID the Earth take shape, and what can we know about its infancy? If light chronicles the history of the universe, rocks tell our planet’s story. When you gaze into the Grand Canyon or marvel at the peaks framing Lake Louise, you’re viewing nature’s library, with volumes of Earth history on display, inscribed in stone. Sediments—cobbles, sands, or muds formed by erosion of earlier rocks, or limestones precipitated from water bodies—spread across floodplains and the seafloor, recording, layer upon layer, the physical, chemical, and biological features of our planet’s surface at the time and place they formed. Igneous rocks—formed from molten materials deep inside the Earth—tell us more about our planet’s dynamic interior, as do metamorphic rocks forged from sedimentary or igneous precursors at elevated temperature and pressure deep within the Earth. Collectively, these rocks offer a grand narrative of Earth’s development from youth to maturity, of life’s evolution from bacteria to you, and—perhaps the grandest narrative of all—of the ways that the physical and biological Earth have influenced each other through time. After forty years as a geologist, I’m still amazed that cliffs along the Dorset coast of southern England allow me to conjure up a picture of the Earth as it existed 180 million years ago. Still more remarkable, as we’ll see, are those rocks that tell of Earth and life billions of years ago.

If you look closely at imposing peaks in the Rocky Mountains or the Alps, another aspect of Earth history may snap into focus. Their tooth-like shapes don’t reflect deposition. On the contrary, they are being sculpted by erosion, physical and chemical processes that wear away rocks, eradicating their stories. Earth writes its history with one hand and erases it with the other, and as we go further back in time, erasure gains the upper hand. Our planet coalesced some 4.54 billion years ago, but Earth’s oldest known rocks date back only to about 4 billion years. Older rocks must have existed, but they’ve been eroded away or were buried and transformed through metamorphism into unrecognizable form. A few may still lie in some remote Canadian or Siberian hillside, waiting to be recognized, but largely, the first 600 million years of Earth history constitutes our planet’s Dark Age.

How can we reconstruct Earth’s infancy in the absence of historical records? It turns out we have backup copies, stored off-site, so to speak. The rocks in question are meteorites, stony survivors from the early solar system that fall to Earth from time to time. Our confidence that Earth and other planets took shape more than 4.5 billion years ago comes from geologic “clocks” trapped in the minerals that make up these special rocks. (More on dating Earth history in a bit.) Some meteorites, called chondrites, consist of rounded, millimeter-scale granules termed chondrules, thought to preserve those tiny particles that collided to form larger bodies during the earliest phases of planet formation (Figure 2). This view gains support from careful studies of chondrule composition, which includes the calcium, aluminum, and titanium minerals that were the first to condense as our solar disk began to cool, as well as rare grains ejected from a nearby supernova and later swept up as the solar system formed. Chondritic meteorites not only preserve a direct record of the early solar system, their chemical composition suggests that they are the principal materials from which Earth itself formed.

FIGURE 2. The Allende meteorite, a carbonaceous chondrite that fell to Earth in 1969. Rounded grains inside are chondrules, rocky spheroids that formed early in our solar system’s history and aggregated into larger bodies, eventually to form the inner planets of our solar system, including Earth. Carbonaceous chondrites contain both water and organic molecules, furnishing materials that would eventually end up in our atmosphere, o...