Timefulness

by Marcia Bjornerud

of sub duction on its edges. In contrast, the Atlantic spreading

  rate probably reflects something close to the mantle’s natural,

  The pace of the earth 71

  stately pace. Earth’s convective behavior should therefore be

  considered an “active lid” system, in that the plates do not

  merely dance to the mantle metronome but in some cases set

  their own meter, and this ultimately dictates how fast moun-

  tains grow. To build mountains, however, we first need to cook

  up some continental crust, and the recipe takes us back to the

  midocean ridges.

  WAT E R W O R K S

  Vine and Matthews correctly interpreted the morphology of

  ocean ridges as a record of the cooling of successive batches of

  basalt. But fresh ocean basalt does not give up its heat passively,

  like our soufflé cooling quietly in the kitchen. Instead, heat is

  robbed from it by frigid ocean water, which streams through

  cracks and pores, jealously steals joules, then makes a high-

  speed getaway through chimneylike underwater geysers called

  black smokers. The water also pilfers elements like calcium from

  the young rocks, and leaves behind sodium, mediating the

  salinity of the oceans. (This was unknown to John Joly when

  he tried to estimate the age of the Earth based on the salinity

  of the sea. His value of 100 million years was not meaningless—

  but it represented the typical residence time of sodium in the

  sea, not the time since Earth’s formation). It is estimated that

  the entire volume of the world’s oceans flushes through rocks

  of the midocean ridges in about 8 million years.4

  Not all the infiltrating water escapes, however. Having en-

  tered into labyrinthine passages and forged chemical bonds

  with minerals in the basalt, some is now locked into the ocean

  crust for the long term. As it happens, this accidental entrap-

  ment of water is one of the most essential components of Earth’s

  tectonic system. A subducting slab carries the stowaway water

  72 Ch a pter 3

  from its youth as it descends into the mantle. The cold slab

  slowly warms, and when it reaches depths of about 30 miles, it

  finally sweats out this ancient seawater. We tend to think of the

  water cycle as a relatively short- term phenomenon; the average

  molecule of water stays in the atmosphere for about nine days;

  the residence time of water even in the largest lakes, like Supe-

  rior, is a century or two; deep groundwater may be stored for

  a millennium. But there is a 100 million- year water cycle that

  involves the interior of the Earth, and adding water to the man-

  tle is in fact the critical step in the recipe for continental crust.

  In the presence of water, the otherwise solid rock in the

  wedge of mantle above a subducting slab will melt at significantly

  lower temperatures than it normally would, in the same way that

  salt lowers the melting temperature of ice on a sidewalk. This

  “water- assisted” melting is both creative and destructive: it ulti-

  mately forges new continental crust but does so via some of the

  deadliest volcanoes on Earth, which form on the overriding plate

  in the subduction zone, directly above the spot where the down-

  going slab gives up its long- sequestered water. The volcanoes

  typically form an arcuate chain— a broad C shape that reflects the

  curvature of a subduction trench on a spherical Earth, like the

  crescentic shape of a dent in a ping- pong ball. Where the upper

  plate is also basaltic ocean crust, the volcanic chain is called an

  island arc. Examples include Japan, Indonesia, the Philippines,

  the Aleutians, and the north half of New Zealand. If a subducting

  plate dives beneath a continent, the resulting volcanoes form a

  continental arc, like the Cascades and Andes (see figure 8).

  In both arc settings, the water- generated mantle melt must

  make its way through the upper plate en route to the surface.

  The magma may be stalled by the rigidity of this lid of obstruct-

  ing crust and, while ponded there, partially melt it. Just as at

  midocean ridges, the low- melting temperature components

  The pace of the earth 73

  will be most readily extracted, giving rise to new magmas that

  are even richer in silica and less like the mantle than basalt

  is. Through many such cycles of such smelting, crust that is

  progressively more “evolved” is generated, ultimately yielding

  granite, the lightweight raw material for the continents. Plate

  tectonics on the modern Earth is an extraordinary system. The

  creation, maturation, and eventual destruction of ocean crust

  are all necessary for the genesis of continental crust— a perfect

  samsara cycle of birth, death, and reincarnation.

  M O U N TA I N T I M E

  An oceanic subduction zone will function smoothly (though

  not necessarily aseismically) as long as the crust entering the

  trench is thin and dense enough to slide into the mantle. But

  if the slab pulls in “undigestible” things like ocean crust that is

  too hot or too thick, or a lumpy old island arc— or an unsink-

  able continent— traffic comes to a halt. And if the upper plate

  is a continent, a major pile- up is unavoidable, and a mountain

  belt begins to grow. The loftiest mountains on Earth, like the

  Himalaya now, and the Alps, Appalachians, and Caledonides

  in their day, form when a long- lived subduction zone has swal-

  lowed an entire ocean basin, and two continents are set on a

  collision course.

  How long does it take to raise a mountain belt? In the case

  of the Himalaya, the seafloor spreading history recorded by

  marine magnetic anomalies makes it possible to track India’s

  headlong rush from its place in the ancient southern continent

  of Gondwandaland in the late Cretaceous to its current position

  as part of Asia.5 Pulled northward by subducting ocean crust,

  India traversed about 2500 km (1500 mi) in 30 million years

  (an impressive average pace for such a marathon, at more than

  74 Ch a pter 3

  8 cm (3 in.) a year), before first running into Asia about 55 mil-

  lion years ago. Since then, the Himalayan range has risen as the

  northern part of India has wedged itself beneath Asia, and the

  crust of both continents has also thickened vertically through

  faulting and folding. As the convergence continues, a wave of

  deformation is propagating outward from the original point of

  contact, both north and south, progressively widening the welt

  of uplifted and contorted crust.

  Before the emergence of plate tectonic theory in the 1960s,

  mountain belts were difficult to explain. Many geologists rec-

  ognized that the buckled, crumpled strata typical of montane

  regions required horizontal compression, but the motive force

  behind this was difficult to understand under the prevailing

  assumption that continents were fixed in place. A nineteenth-

  century Austrian geologist, Eduard Suess (grandfather of Hans,

  who would document the dilution of atmospheric 14C by “dead”

  carbon from fossil
fuel burning), recognized that many of the

  rocks in the Alps had formed on the seafloor and had somehow

  been elevated to their present positions. He postulated that

  Earth’s mountains were akin to the wrinkles on a raisin, ridges

  formed by shrinkage as the result of steady cooling and con-

  traction of the planet— a notion consistent with Lord Kelvin’s

  views of the thermal evolution of the interior.

  The art critic, intellectual polymath, and alpine enthusiast

  John Ruskin, a contemporary of Suess’s, also had an intuitive

  sense that mountains are not static, eternal monuments but

  records of dynamic events. For Ruskin, however, the morphol-

  ogy of the Alps evoked liquid fluidity rather than desiccated

  fruit: “There is an appearance of action and united movement

  in these crested masses, nearly resembling that of sea waves . . .

  fantastic yet harmonious curves, governed by some grand

  under- sweep like that of a tide running through the whole body

  The pace of the earth 75

  of the mountain chain.”6 He also recognized that that these

  “harmonious” shapes represented the countervailing effects

  of the “elevatory force in the mountain” and “sculptural force

  of water upon the mountain.” But how efficiently do these op-

  posing forces act?

  The highest Himalayan peaks, at elevations of 9000 m

  (29,000 ft), lie where the coast once was. So it might seem log-

  ical to estimate their growth rate simply by dividing their height

  by 55 million years, which yields a positively underwhelming

  uplift rate of 0.015 cm (0.006 in.) per year. But this calcula-

  tion is a gross underestimate of the actual rate of mountain

  building— because as soon as tectonic forces start constructing

  mountains, the highly efficient erosion crew arrives to begin

  demolition. So we need to find ways to measure these opposing

  processes independently.

  Today, surface uplift can be measured in nearly real time

  thanks to high- precision global positioning system (GPS) satel-

  lites. In the highest part of the Himalaya, the Tibetan Plateau,

  GPS- based uplift rates averaged over a decade are in the range

  of 2 mm (0.1 in.) per year. This is about an order of magnitude

  slower than the tectonic convergence rate (around 2 cm, or

  1 in., per year)7 and reflects a fairly typical ratio of vertical to

  horizontal deformation in the crust. But the instrumentally

  measured uplift is more than 100 times faster than a long- term

  estimate that ignores the effects of erosion. How can we know

  whether modern satellite- based estimates are representative of

  uplift rates over longer periods of geologic time? As the “roof

  of the world” is rising, its top stories are constantly being re-

  moved, in a process geologists call exhumation. What were

  once the subterranean levels now have high penthouse views.

  To reconstruct long- term uplift rates, we need to know how

  many floors have been dismantled, and how quickly.

  76 Ch a pter 3

  There are several ways to calculate how much additional rock

  once existed in the airspace above a mountain belt. One is to ask

  the rocks now at the surface how deep they used to be at a certain

  time in the past. This can be done using a technique called fission

  track dating, developed largely by oil companies to reconstruct

  the thermal histories of sedimentary rocks to predict whether

  they are likely to produce petroleum or natural gas (sediments

  need to have been warm enough that their organic matter got

  properly “cooked,” but never so hot that it all burned off).

  Fission track dating makes use of the fact that the more abun-

  dant isotope of uranium, 238U, is not just radioactive but also

  has an unstable nucleus that splits itself in spontaneous fission

  events at a known rate. Under high magnification, uranium-

  bearing minerals including zircon (the darling of geochronol-

  ogy) and apatite (the mineral in teeth and bones) retain a vis-

  ible record of these high- energy events in the form of damage

  zones or “fission tracks.” Each uranium- bearing mineral has

  a particular temperature above which the crystal lattice can

  heal itself and erase these scars, like an Etch- a- Sketch that has

  been well shaken. Below this temperature, however, the tracks

  will remain etched in the crystal. So, by counting the density

  of fission tracks in a given volume of a mineral, it is possible to

  determine how long it has been since it cooled through a certain

  temperature (and depth) in the crust. Fission track thermo-

  chronology for Himalayan rocks shows that modern uplift rates

  based on a few decades of satellite data are in fact consistent

  with uplift over geologic timescales.8

  R E M A I N S O F T H E D AY

  Another approach to estimating how much has been trimmed

  from mountains by erosion is to look at the volumes of sediment

  The pace of the earth 77

  that have accumulated at their feet, like snippets of hair on a

  barbershop floor. In the Himalaya, most of the erosional detritus

  has accumulated in two gigantic sandpiles on the seafloor: the

  Indus and Bengal submarine “fans,” where the Indus, Ganges,

  and Brahmaputra Rivers have dumped sediment for the past

  50 million years. On Marie Tharp’s maps, the Indus and Bengal

  fans are long tongues lolling far out onto the floor of the In-

  dian Ocean. The Bengal fan is the largest in the world. From the

  mouth of the combined Ganges- Brahmaputra on the coast of

  Bangladesh— which itself is made entirely of sediment shed from

  the mountains— the fan extends 3000 km (1800 mi) southward.

  If superimposed on the continental United States, the Bengal

  fan would stretch from the Canadian border to Mexico, and for

  almost half that distance, it is more than 6.5 km (4 mi) thick.

  Drilling and geophysical exploration of the Indus9 and Ben-

  gal10 fans have revealed an upside- down, impressionistic re-

  cord of the unroofing of the Himalaya, with the disaggregated

  remains of rocks that were at the top of the mountains in their

  infancy now forming the lowest layers in the immense mass of

  deep- sea sediment. The total volume of the Bengal fan alone,

  an estimated 12.5 million km3 (3 million mi3),11 is greater than

  the present- day volume of the crust of the Tibetan Plateau

  above sea level.12 That is, more rock has been removed from the

  Himalaya by erosion than forms the towering modern range.

  This fact makes the seemingly simple question posed by Hut-

  ton (and Dylan)— How long does it take to wear mountains

  down?— more difficult to answer. Which mountains are we

  talking about? The Himalaya have existed for 55 million years,

  but today’s mountains are not the same as the mountains whose

  ruins lie on the floor of the Indian Ocean.

  The ephemeral nature of mountains— or any landscape— is

  one reason that unconformities in the rock record, like Hutton’s

  78 Ch a pter 3

  famous outcrop at Siccar Point, hold such fascination
. Un-

  conformities preserve buried topography and thus provide

  glimpses of the long- vanished terrain of earlier eons. Wiscon-

  sin’s Baraboo Hills region, a mecca for geology field trips (and

  home of the late Ringling Bros. and Barnum & Bailey Circus,

  the “Greatest Show on Earth”), represents one of the most

  remarkable examples of paleotopographic preservation any-

  where in the world. This Precambrian mountain belt, formed

  about 1.6 billion years ago, was buried by hundreds of feet

  of marine sediments when early Paleozoic seas washed over

  what is now the Great Lakes region. Today, erosion of these

  Paleozoic rocks has reached a stage in which the unconformity

  between the Precambrian and Paleozoic worlds is exposed in

  many places. The long- hidden mountains are being unburied,

  or exhumed, and the modern land surface closely approximates

  that of the late Proterozoic. Interestingly, this ancient landscape

  was the inspiration for two great environmental thinkers: John

  Muir, whose family immigrated to the area from Scotland when

  he was a young boy, and Aldo Leopold, whose Sand County

  Almanac is set in the shadow of the primordial Baraboo Hills.

  There are much older rocks, and deeply eroded roots of older

  mountain belts in other places (even Wisconsin), but the Bara-

  boo Range represents some of the oldest preserved topography

  on Earth— a Great Show indeed.

  T H E H I L L S A R E A L I V E

  The sediments shed from the Himalaya tel us that while there has

  been some variation in uplift and exhumation rates over time,

  on average these rates fall within the range of estimates from

  both GPS observations and thermochronologic approaches

  like fission track dating. This is a comfortingly uniformitarian

  The pace of the earth 79

  result; Lyell would be pleased. The great heaps of sediment also

  underscore an amazing fact about the Earth: that the speeds of

  tectonic processes, driven by the internal radioactive heat of

  the Earth are, by happy coincidence, about evenly matched13

  by the tempo of external agents of erosion— wind, rain, rivers,

  glaciers— powered by gravity and solar energy. In the barber-

  shop analogy, it is as if the hair on a customer’s head keeps

  growing as fast as the barber can cut it. And while the tectonic

  growth and erosional trimming of mountains both proceed at