Timefulness

by Marcia Bjornerud

  pect of a still- earlier crust that had formed, cooled, and then

  remelted within the first 150 million years of the planet’s ori-

  gin. Equally surprising, the ratios of different oxygen isotopes

  in the old zircons suggest that the magma from which they

  crystallized had interacted with relatively cool surface water.

  An atlas of time 61

  Abandoning customary scientific restraint in their conclud-

  ing paragraph, the authors of the 2001 Nature paper boldly

  suggested— on the basis of a few crystals smaller than fleas—

  that not only continents and oceans existed on Earth 4.4 bil-

  lion years ago but that if surface water was around, perhaps

  there was even life.

  T H I N K I N G L I K E A P L A N E T

  The Jack Hills zircon paper, one of the most cited in all the

  geologic literature, was a virtuosic culmination of a century

  of isotope geochemistry and required the most advanced ana-

  lytical methods available. Yet, in its audacious inductive spec-

  ulations and strong predilection for uniformitarianism, it is

  remarkably similar to the very first work of modern geology:

  Hutton’s Theory of the Earth. Whether, in fact, the early Earth

  should be viewed through strictly uniformitarian spectacles is

  currently a matter of lively debate among geologists. There are

  compelling reasons to suggest that Earth’s habits were different

  in its first 2 billion years.

  But the story of how the still- unfinished Atlas of Deep Time

  has evolved, from Siccar Point to Chicxulub to Jack Hills, makes

  it clear that mapping time has been a very human endeavor that

  requires just this kind of give- and- take. It has involved a great

  variety of minds— visionary thinkers like Hutton and Lyell not

  too obsessed with details; attentive fossil- hunters like William

  Smith who are; polymaths like Darwin and Holmes who see

  connections across disciplines; fastidious instrumentalists like

  Nier and Patterson; bureaucracies like the International Com-

  mission on Stratigraphy; and legions of hardy, anonymous field

  mappers (including a few jocks) who understand both chronos

  and kairos, and how to turn rocks into verbs.

  C H A P T E R 3

  T H E PAC E O F

  T H E E A R T H

  How many years can a mountain exist

  before it is washed to the sea?

  — B O B DY L A N , 1 9 6 3

  E P H E M E R A L G E O G R A P H I E S

  One of my earliest school memories is of watching a film about

  the emergence of Surtsey, a volcanic island off the coast of Ice-

  land that began to rise from the Atlantic late in 1963. The black-

  and- white footage showed explosive spires of steam and ash

  creating a blank new world of coal- dark cinder not yet on any

  map. A ship captain had been the first to notice the eruption

  and had initially thought it was another vessel on fire. To my

  impressionable young mind, the idea of new land forming was

  thrilling; it suggested a secret life- force inside the impassive,

  stony- faced Earth. Between 1963 and 1967, Surtsey built itself

  up from a submarine ridge 130 m (425 ft) below sea level to a

  small cone more than 170 m (570 ft) above it. At its maximum

  extent, Surtsey had an area of about a square mile. But after the

  eruptions ceased, its destruction by erosion, settling, and sub-

  sidence was nearly as rapid. Today, it has been reduced to about

  half its 1967 size and is expected to disappear entirely by 2100

  (or sooner, depending on the rate of sea level rise). To my still

  impressionable middle- aged mind, it is somehow unsettling to

  The pace of the earth 63

  have seen the arc of Surtsey’s life— the birth, youth, brief prime,

  and inexorable demise of a landmass.

  To Hutton, Lyell, and Darwin, most geologic processes

  seemed imperceptibly slow, and for decades, geologists

  drummed this idea into the public consciousness. But today,

  thanks to high- precision geochronology, direct satellite obser-

  vation of Earth processes from space, and a century of moni-

  toring the planet’s vital signs— temperature, precipitation, river

  flow, glacier behavior, groundwater reserves, sea level, seismic

  activity— many of the geologic processes that once seemed be-

  yond the reach of direct human observation can now be clocked

  in real time. And we are finding that the pace of the planet is

  neither as slow nor as constant as was previously thought.

  B A S A LT O F T H E E A R T H

  Hutton’s original epiphany that the age of the earth was effec-

  tively infinite compared with human lifespans arose from his

  recognition that the unconformity at Siccar Point represented

  the time needed for a mountain belt to form and be beveled

  again to a flat plain. So how long, exactly, would this take? The

  forces behind mountain building were not known until about

  175 years after Hutton’s death— in fact, around the time of

  Surtsey’s birth in the 1960s, when plate tectonic theory finally

  explained how the solid Earth works. Today we realize that the

  tempo of mountain growth is ultimately set by the formation

  and destruction of ocean basins.

  Unlike the continents, which are a messy amalgam of many

  different rock types of a wide range of ages and individual histo-

  ries, the ocean crust is simple and homogeneous. It’s all basalt—

  the black volcanic rock of Surtsey— and it’s all produced in the

  same way: by partial melting of the Earth’s mantle beneath

  64 Ch a pter 3

  submarine volcanic rifts, marked by high- standing midocean

  ridges. Counter to fanciful depictions in fiction and film, the

  mantle (which constitutes more than 80% of Earth’s volume)

  is not a vat of seething magma but solid rock— though it flows

  over geologic timescales. Every few hundred million years, the

  mantle overturns itself in the manner of a gargantuan lava lamp,

  through the process of thermal convection: hotter, buoyant

  rock from depth rises while cooler, denser rock sinks. Mantle

  convection is Earth’s main heat- loss mechanism (contrary to

  Lord Kelvin’s erroneous assumption that the mantle was static

  and Earth had cooled over its lifespan through conduction).

  Arthur Holmes was among the first to suggest, in the 1930s,

  that the mantle convects; today, high- pressure experiments

  simulating the behavior of minerals at mantle depths show that

  convection of rock in Earth’s interior is inevitable.

  Midocean ridges are thought to coincide with areas of con-

  vective upwelling, where the Earth’s crust is forced to stretch

  and thin above the rising plume of hot rock. Paradoxically,

  however, no melt forms until the ascending rock has lost

  much of it heat. So what makes still- solid mantle rock melt as

  it nears the surface? The mechanism is counterintuitive— not

  an input of heat but a decrease in pressure. Unlike water, a

  completely abnormal compound from which most of us derive

  our under standing of phase changes, rock behaves the way a

 
proper substance should: it expands on melting and contracts

  on freezing. This means that if a rock is close to its melting

  temperature at some depth in the Earth and is depressurized

  (e.g., by rising closer to the surface), the lower- density phase—

  melt— becomes favored, and magma is formed. This phenome-

  non is called decompression melting and can happen even if the

  rock is actually cooling, as long the pressure is decreasing faster

  than the temperature is. (Decompression melting is especially

  The pace of the earth 65

  hard for skiers and skaters to understand, since the opposite

  be havior by water— melting under higher pressure— is the very

  basis for winter sports that involve slippery surfaces).

  On Earth today, after 4.5 billion years of cooling through

  mantle convection, upwelling mantle rock does not carry

  enough thermal clout to undergo wholesale melting. Instead,

  magmas at ocean ridges represent the components in mantle

  rock that melt at the lowest temperature. This partial, or frac-

  tional, melting is what generates basalt, which has a different

  composition— higher in silica, aluminum, and calcium, and

  lower in magnesium— than its parent, the mantle.

  As each new pulse of basaltic melt rises and fills the central

  axis of an oceanic rift, the previous batches, now frozen into

  rock again, are displaced symmetrically outward in the pro-

  cess called seafloor spreading (see figure 7). The most recently erupted basalt is warmer and less dense than the slightly older

  rock it has pushed aside, and each generation cools progres-

  sively as it moves away from its birthplace at the rift. This is the

  reason that the midocean ridges stand high, like a soufflé fresh

  from the oven. In fact, one of the clues that led to the epiphany

  of plate tectonic theory in the early 1960s, when deep- seafloor

  maps first became available, was that the cross- sectional form

  of the ocean ridges is essentially a pair of mirror- image cooling

  curves— the shape of two skis placed on the floor tip to tip.

  A L L O V E R T H E M A P

  Let us pause to contemplate how incredible it is that most of the

  Earth’s surface— the deep- ocean floor— had not been mapped

  until the middle of the twentieth century. Even today, the

  topography of much of the seafloor is known to a resolution of

  only about 3 miles; bathymetric charts of the ocean are about

  66 Ch a pter 3

  F I G U R E 7. Midocean ridge, seafloor spreading, and magnetic reversals

  100 times “blurrier” than current maps of the surface of Venus

  and Mars.1 Still more incredible is the fact that one person al-

  most single- handedly created the first maps of two- thirds of

  the planet yet is unknown to the average citizen of Earth (while

  Amerigo Vespucci, whose cartographic credentials are suspect,

  has two continents named for him). The unsung mapmaker was

  The pace of the earth 67

  Marie Tharp, who earned a master’s degree in geology from

  the University of Michigan, worked briefly for an oil company,

  and then in 1948 became a drafter for a new oceanographic

  project led by Maurice Ewing at Columbia University.2 For

  years, Ewing’s all- male team of graduate students collected

  sonar soundings of the ocean floor while Tharp laboriously

  transformed the linear strings of depth readings into three-

  dimensional topography.

  Tharp’s exquisite shaded relief maps, painstakingly drawn in

  pen and ink, revealed that the seabed— previously thought flat

  and featureless— had a rugged, globe- encircling range of ridges

  and terrifyingly deep trenches. By 1953, she had noticed that

  the high ridges all had central down- dropped valleys and spec-

  ulated that this might be evidence for crustal stretching. She

  shared her idea with another member of Ewing’s group, Bruce

  Heezen, who infamously dismissed it as “girl talk.” But Heezen

  and Tharp became close collaborators at Columbia, producing

  a series of seafloor maps that revolutionized geologists’ view of

  the Earth. In 1963, when two British geologists first articulated

  the concept of seafloor spreading in a paper in Nature 3 (and

  Surtsey was demonstrating the process), Heezen— and much

  later, the rest of the geologic community— acknowledged that

  Tharp had been right.

  The authors of the 1963 paper, Fredrick Vine and Drum-

  mond Matthews, proposed seafloor spreading on the basis of a

  perceptive geometric argument rather than firsthand geologic

  observation (the ridges would not be directly seen or sampled

  for another decade). Vine and Matthews had access not only

  to Tharp’s maps but also to data from the U.S. and Royal Navies

  on the magnetic signatures of rocks at the bottom of the ocean.

  They noted that both the ridge topography and the magnetic

  intensity readings had mirror symmetry moving outward from

  68 Ch a pter 3

  the ridge crest— that is, bands of similarly magnetized rocks ran

  in parallel stripes on either side of the ridge (see figure 7). The ridge heights fell off with distance in just the way one would

  expect for deflating soufflés or cooling and contracting rocks.

  The symmetrical pattern of magnetic stripes suggested that

  successive generations of ocean crust had formed at the ridge,

  cooled enough for their iron- bearing minerals to align with

  the ambient magnetic field, and then been cleaved in half and

  displaced outward in a great conveyor system. Meanwhile, the

  polarity of Earth’s magnetic field had repeatedly reversed itself,

  the north and south geomagnetic poles switching places on an

  erratic schedule (a second revolutionary inference in a paper

  that is barely three pages long).

  By the early 1970s, age determinations for seafloor samples

  from deep- ocean drilling, as well as correlation of the marine

  magnetic record with magnetic reversals in well- dated volca-

  nic sequences on land, had created a new way to demarcate

  geologic time, and the geomagnetic timescale was grafted

  onto the biostratigraphic (fossil- based) and geochronologic

  (radioisotope- calibrated) timescales. Today, with the date of

  each magnetic field reversal well constrained, it is possible to

  determine the age of a rock anywhere on the seafloor without

  even getting a physical sample— simply by counting how many

  magnetic stripes it is away from the ridge.

  On a map showing the ages of seafloor in the world’s oceans,

  the most striking pattern is that the swaths of rock of any given

  age are much wider in the Pacific Ocean than in the Atlantic.

  Since the start of the Cenozoic Era 65 million years ago (i.e.,

  since the demise of the dinosaurs), seafloor spreading rates in

  the Atlantic have averaged about 1 cm (ca. 1/2 inch) per year,

  which is on the order of the rate at which one’s fingernails

  grow. It’s fast enough that at Thingvellir in Iceland, one of the

  The pace of the earth 69

  few places where an ocean ridge stands above sea level— and

  the site the Vi
kings chose in AD 930 for their annual parlia-

  ment meeting, the Althing— the visitor center was built to be

  as wide as the amount by which the crust has stretched since

  Viking times.

  On the other hand, the rate of spreading in the Atlantic is

  slow enough that a species of green sea turtle ( Chelonia mydas)

  from Brazil that has made an annual swim to a high spot on the

  Mid- Atlantic Ridge to breed and nest since the time of the dino-

  saurs hasn’t seemed to notice that the ridge is now nearly 1100

  km (700 mi) farther distant. Luckily the turtle’s natal beaches

  weren’t in the Pacific, where spreading rates are almost an order

  of magnitude faster, at close to 10 cm (4 in.) per year (a little

  slower than the “velocity” of hair growth). If these rates simply

  reflected the pace of mantle convection, why would that pace

  be more vigorous beneath one ocean than another?

  P L AT E S P U L L T H E I R W E I G H T

  Marie Tharp’s marvelous maps hold clues to the disparity in

  rates of plate motion in the two oceans. In particular, they show

  profound differences between the edges of the Pacific and At-

  lantic basins: the margins of the Atlantic Ocean are mainly shal-

  low continental shelves, like the area off the coast of the eastern

  United States, where water depths are less than about 200 m

  (660 ft), and submerged crust gives way gradually to emergent

  land. The margins of the Pacific Ocean, in contrast, are delin-

  eated by vertiginous chasms, like the one off the west coast

  of South America, whose deepest points lie more than 8000

  m (24,000 ft) below sea level. These trenches mark the sites

  of subduction, where old, cold, ocean crust—with the same

  instinct as the Brazilian turtles—returns to its place of origin.

  70 Ch a pter 3

  F I G U R E 8 . Subduction zones and volcanic arcs

  When sea floor basalt is around 150 million years old, and

  hundreds of miles from its natal ridge, it has become about as

  dense as the underlying mantle and sinks back into Earth’s inte-

  rior at a slant, pulling the rest of the plate behind it, like a blan-

  ket sliding off a bed (figure 8). This “slab pull” force is almost certainly what sets the tempo for the rapid seafloor spreading in the Pacific— its rifts are simply keeping up with rates