r/K selection theory
A North Atlantic right whale with solitary calf. Whale reproduction follows a K-selection strategy, with few offspring, long gestation, long parental care, and a long period until sexual maturity.
In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus upon either increased quantity of offspring at the expense of individual parental investment of r-strategists, or reduced quantity of offspring with a corresponding increased parental investment of K-strategists, varies widely, seemingly to promote success in particular environments.
The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson based on their work on island biogeography; although the concept of the evolution of life history strategies has a longer history.
The theory was popular in the 1970s and 1980s, when it was used as a heuristic device, but lost importance in the early 1990s, when it was criticized by several empirical studies. A life-history paradigm has replaced the r/K selection paradigm but continues to incorporate many of its important themes.
A litter of mice with their mother. The reproduction of mice follows an r-selection strategy, with many offspring, short gestation, less parental care, and a short time until sexual maturity.
In r/K selection theory, selective pressures are hypothesised to drive evolution in one of two generalized directions: r- or K-selection. These terms, r and K, are drawn from standard ecological algebra as illustrated in the simplified Verhulst model of population dynamics:
where r is the maximum growth rate of the population (N), K is the carrying capacity of its local environmental setting, and the notation dN/dt stands for the derivative of N with respect to t (time). Thus, the equation relates the rate of change of the population N to the current population size and expresses the effect of the two parameters.
In the etymology of the Verhulst equation, r comes from rate while K comes from carrying capacity. In German, the word for capacity is Kapazität and K stands for the "Kapazitätsgrenze" (capacity limit).
As the name implies, r-selected species are those that place an emphasis on a high growth rate, and, typically exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood (i.e., high r, low K). A typical r species is the dandelion Taraxacum genus.
In unstable or unpredictable environments, r-selection predominates due to the ability to reproduce quickly. There is little advantage in adaptations that permit successful competition with other organisms, because the environment is likely to change again. Among the traits that are thought to characterize r-selection are high fecundity, small body size, early maturity onset, short generation time, and the ability to disperse offspring widely.
Organisms whose life history is subject to r-selection are often referred to as r-strategists or r-selected. Organisms that exhibit r-selected traits can range from bacteria and diatoms, to insects and grasses, to various semelparous cephalopods and small mammals, particularly rodents.
By contrast, K-selected species display traits associated with living at densities close to carrying capacity, and typically are strong competitors in such crowded niches that invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood (i.e., low r, high K). In scientific literature, r-selected species are occasionally referred to as "opportunistic" whereas K-selected species are described as "equilibrium".
In stable or predictable environments, K-selection predominates as the ability to compete successfully for limited resources is crucial and populations of K-selected organisms typically are very constant in number and close to the maximum that the environment can bear (unlike r-selected populations, where population sizes can change much more rapidly).
Traits that are thought to be characteristic of K-selection include large body size, long life expectancy, and the production of fewer offspring, which often require extensive parental care until they mature. Organisms whose life history is subject to K-selection are often referred to as K-strategists or K-selected. Organisms with K-selected traits include large organisms such as elephants, humans and whales, but also smaller, long-lived organisms such as Arctic terns.
The pericycle is a cylinder of parenchyma or sclerenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants.
The endodermis is the central, innermost layer of cortex in some land plants. It is made of compact living cells surrounded by an outer ring of endodermal cells that are impregnated with hydrophobic substances (Casparian Strip) to restrict apoplastic flow of water to the inside. The endodermis is the boundary between the cortex and the stele.
In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis, which is the innermost cell layer of the cortex.
are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor or changes in growth (therefore K+ ion concentration usually controls such movement in plants). Nastic movements differ from tropic movements in that the direction of tropic responses depends on the direction of the stimulus, whereas the direction of nastic movements is independent of the stimulus's position.The tropic movement is growth movement but nastic movement maybe or may not be growth movement. The rate or frequency of these responses increases as intensity of the stimulus increases. An example of such a response is the opening and closing of flowers (photonastic response). They are named with the suffix "-nasty" and have prefixes that depend on the stimuli:
Abby was satisfied with Kristen’s reaction.Kristen looked surprised and started praising how good she looked in thesweater. “Love you,” she flew into Kristen’s arms and kissed on her rosy cheek.Actually, the sweater she wore belonged to Kristen. She needed it because shedidn’t have a white sweater for the chorus.
Kristen was her BFF (best friend forever),her soulmate, and her everything. Abby never thought it was inappropriate touse such sweet and romantic words to describe Kristen. She had so many friends,but Kristen was different.
Kristen was charming. She had an hourglassbody type with adorable wavy hair and the leadership ability that made hershiny in the chaos. There were some boys chasing Kristen, and they asked Abbyfor help. Abby gave them such suggestions as what was Kristen’s favorite restaurant,what did Kristen actually thought when she frowned, and how to console Kristenwhen she failed in different situations and pretended to mention their namesunintentionally in the conversation with Kristen. However, sometimes Abby feltawful when she thought one day Kristen would fall in love with one of thoseguys, silly, stubborn, selfish, hard, and inconsiderate. Kristen deserved someonebetter.
Kristen promised her to come to theauditorium afterschool, although no one expect for members of chorus could staythere when the rehearsal began. As Abby got on the stage from the backstage,she was delighted to find Kristen sitting under the stage and staring at her.She made a gesture of heart and started making a bigger one with her arms whenKristen gave a smile in return. Kristen left several minutes after and shepromised Abby she would wait outside at the school gate when rehearsal wasfinished.
During the break, Abby usually didn’t feeltired. Especially today, she felt super excited for no reason.
“OMG, I am addicted with Kristen’s smell onher sweater, and I am so excited about the field trip, Kristen and I plan toput our beds together and watch horror movie for all night. Do you notice herdress just now? She looks gorgeous in that, and that is me who help her pickthe dress. I need to ask the name of her perfume. It just smells so good. ”
“Yeah, it’s good, but don’t you think it’sa little bit strange?” Someone stood beside Abby asked her, “Sorry, I mean, Ishould not say that.”
Abby realized what she meant immediately.She kept silent for the rest of the rehearsal, on her way home, and even whenher mother tried to draw her attention during the dinner. She felt herselfacted weirdly these days. She felt anxious when Kristen didn’t reply hermessage, felt angry when Kristen had lunch with one of her wooers, and evenfelt betrayed when Kristen chatted with others happily.
There must be something wrong.
Abby decided to keep away from Kristen,because she knew she cannot stop kissing, hugging, or doing any weird, maybedisgusting close things when she stayed with her. It would be a torture for herto reject Kristen’s invites for lunch. But it was okay, she thought, everythingchanged with the time passed by.
Abby put that sweater into one of herfavorite paper bag and gave it back to Kristen the next day.
Kristen was not in a good mood recentlybecause she seemed to lose Abby, one of her best friend. She tried to persuadeherself it was normal and common to lose some friends in life, but when she sawAbby chatting with others she was confused. She did not think she did anythingwrong or argued with Abby. It was just one day she realized that maybe theywere not BFF anymore. She had to confess that she felt jealous when she sawAbby sitting in the middle of crowds and sharing the funny things when she hangout with some guys on the weekend. I supposed to be the one, she thought. Yetshe never considered talking to Abby, it was not her style.
They still talked to each other, but not asclose as before. They smiled and waved to each other at the graduationceremony, without a hug or a kiss. That night, Kristen had a date with a guywho had chased her for three years rather than went to the graduation party.She never met Abby afterwards.
Kristen brought nearly all of the clotheswith her to university, except for the white sweater. Even her mother advisedher to buy something new in the new school, she insisted and claimed that itwas a kind of unforgettable memory.
“Dear, do you still need this sweater?Maybe we can sell it or give it to your younger sister. She always asks you toborrow it, you know,” her mother asked her.
“Sorry mom, can I just keep it in my closet?”
C3 plants are those which fix and reduce inorganic CO2 into organic compounds using only the C3 pathway in photosynthesis while C4 and CAM plants employ both C3 and C4 cycles. In other words, the first classification refers to those plants having C3 photosynthesis, C4 plants employ the C4 photosynthesis, and CAM plants the CAM photosynthesis.
Plants utilizing only the C3 cycle are most common in the Plant kingdom. They comprise about 85% of all plant species (Moore et al. 2003). In contrast, only about 3% are C4 plants while about 8% were identified as CAM plants as of 2010 (Simpson 2010).
Table C-1. Some characteristics and the general distribution of the three plant types grouped according to their mechanisms of photosynthesis in the Dark reactions.
FEATURE/ CHARACTERISTIC C3 Plants C4 Plants CAM Plants
Distribution in the plant kingdom (% of plant species) ~85% (Moore et al. 2003) ~3% (Simpson 2010), all angiospermous including most troublesome weeds; mostly monocots (C4 grasses and sedges about 79% of all C4 plants) ~8% (Simpson 2010), mostly succulent plants but not all succulents are CAM plants
Type of photosynthesis C3 photosynthesis C4 photosynthesis CAM photosynthesis
CO2 fixation pathway via C3 cycle only via C3 and C4 cycles, spatially (C4 in the mesophyll cell then C3 in the bundle sheath cell) via C3 and C4 cycles, both spatially (in different parts of same cell) and temporally (C4 at night, C3 at day time)
Leaf anatomy Large air spaces bordered by loosely arranged spongy mesophyll cells; mesophyll cells but not bundle sheath cells (BSC) contain chloroplasts Generally thinner leaves, closer arrangement of vascular bundles, smaller air spaces than C3; veins sorrounded by thick-walled BSC further sorrounded by thin-walled mesophyll cells (wreath-like arrangement of BSC is called Kranz anatomy); mesophyll cells and BSC contain chloroplasts, those of the BSC much larger Thick and fleshy leaves, mesophyll cells having large, water-filled vacuoles
Stomatal movement Stomata open at daytime, close at night Stomata open at daytime, close at night Inverted stomatal cycle (open at night, close in the day)
Typical Environmental / Geographical adaptation (where most common) Temperate Tropical or semi-tropical, high light intensity, high temperature, drought conditions Desert or arid (xeric) habitats
Examples of C3 plants:
- most small seeded cereal crops such as rice (Oryza sativa), wheat (Triticum spp.), barley (Hordeum vulgare), rye (Secale cereale), and oat (Avena sativa); soybean (Gycine max), peanut (Arachis hypogaea), cotton (Gossypium spp.), sugar beets (Beta vulgaris), tobacco (Nicotiana tabacum), spinach (Spinacea oleracea), potato (Solanum tuberosum); most trees and lawn grasses such as rye, fescue, and Kentucky bluegrass.
Also includes evergreen trees and shrubs of the tropics, subtropics, and the Mediterranean; temperate evergreen conifers like the Scotch pine (Pinus sylvestris); deciduous trees and shrubs of the temperate regions, e.g. European beech (Fagus sylvatica) (Moore et al. 2003), as well as weedy plants like the water hyacinth (Eichornia crassipes), lambsquarters (Chenopodium album), bindweed (Convolvolus arvensis), and wild oat (Avena fatua) (Llewellyn 2000).
(Ben G. Bareja Aug 2013)
IN THE SUMMER of 1971, a young geologist named Mike Voorhies was scouting around on
some grassy farmland in eastern Nebraska, not far from the little town of Orchard, where he
had grown up. Passing through a steep-sided gully, he spotted a curious glint in the brush
above and clambered up to have a look. What he had seen was the perfectly preserved skull of
a young rhinoceros, which had been washed out by recent heavy rains.
A few yards beyond, it turned out, was one of the most extraordinary fossil beds ever
discovered in North America, a dried-up water holethat had served as a mass grave for scores
of animals—rhinoceroses, zebra-like horses, saber-toothed deer, camels, turtles. All had died
from some mysterious cataclysm just under twelve million years ago in the time known to
geology as the Miocene. In those days Nebraska stood on a vast, hot plain very like the
Serengeti of Africa today. The animals had been found buried under volcanic ash up to ten
feet deep. The puzzle of it was that there were not, and never had been, any volcanoes in
Today, the site of Voorhies’s discovery iscalled Ashfall Fossil Beds State Park, and it has a
stylish new visitors’ center and museum, with thoughtful displays on the geology of Nebraska
and the history of the fossil beds. The center incorporates a lab with a glass wall through
which visitors can watch paleontologists cleaning bones. Working alone in the lab on the
morning I passed through was a cheerfully grizzled-looking fellow in a blue work shirt whom
I recognized as Mike Voorhies from a BBC television documentary in which he featured.
They don’t get a huge number of visitors to Ashfall Fossil Beds State Park—it’s slightly in
the middle of nowhere—and Voorhies seemed pleased to show me around. He took me to the
spot atop a twenty-foot ravinewhere he had made his find.
“It was a dumb place to look for bones,” he said happily. “But I wasn’t looking for bones. I
was thinking of making a geological map of easternNebraska at the time, and really just kind
of poking around. If I hadn’t gone up this ravine or the rains hadn’t just washed out that skull,
I’d have walked on by and this would never have been found.” Heindicated a roofed
enclosure nearby, which had become the mainexcavation site. Some two hundred animals
had been found lying together in a jumble.
I asked him in what way it was a dumb place to hunt for bones. “Well, if you’re looking for
bones, you really need exposed rock. That’s why most paleontology is done in hot, dry places.
It’s not that there are more bones there. It’s just that you have some chance of spotting them.
In a setting like this”—he made a sweeping gesture across the vast and unvarying prairie—
“you wouldn’t know where to begin. There could be really magnificent stuff out there, but
there’s no surface clues to show you where to start looking.”
At first they thought the animals were buried alive, and Voorhies stated as much in a
National Geographicarticle in 1981. “The article called the site a ‘Pompeii of prehistoric
animals,’ ” he told me, “which was unfortunate because just afterward we realized that the
animals hadn’t died suddenly at all. Theywere all suffering from something called
hypertrophic pulmonary osteodystrophy, which is what you would get if you were breathing a
lot of abrasive ash—and they must have been breathing a lot of it because the ash was feet
thick for hundreds of miles.” He picked up a chunk of grayish, claylike dirt and crumbled it
into my hand. It was powdery but slightly gritty. “Nasty stuff to have to breathe,” he went on,
“because it’s very fine but also quite sharp. So anyway they came here to this watering hole,
presumably seeking relief, and died in some misery. The ash would have ruined everything. It
would have buried all the grass and coated every leaf and turned the water into an undrinkable
gray sludge. It couldn’t have been very agreeable at all.”
The BBC documentary had suggested that the existence of so much ash in Nebraska was a
surprise. In fact, Nebraska’shuge ash deposits had been known about for a long time. For
almost a century they had been mined to make household cleaning powders like Comet and
Ajax. But curiously no one had ever thought to wonder where all the ash came from.
“I’m a little embarrassed to tell you,” Voorhies said, smiling briefly, “that the first I thought
about it was when an editor at the National Geographicasked me the source of all the ash and
I had to confess that I didn’t know. Nobody knew.”
Voorhies sent samples to colleagues all over the western United States asking if there was
anything about it that they recognized. Several months later a geologist named Bill
Bonnichsen from the Idaho Geological Survey got in touch and told him that the ash matched
a volcanic deposit from a place called Bruneau-Jarbidge in southwest Idaho. The event that
killed the plains animals of Nebraska was a volcanic explosion ona scale previously
unimagined—but big enough to leave an ash layer ten feet deep almost a thousand miles away
in eastern Nebraska. It turned out that under the western United States there was a huge
cauldron of magma, a colossal volcanic hotspot, which erupted cataclysmically every
600,000 years or so. The last such eruption was just over 600,000 years ago. The hot spot is
still there. These days we call it Yellowstone National Park.
We know amazingly little about what happens beneath our feet. It is fairly remarkable to
think that Ford has been building cars and baseball has been playing World Series for longer
than we have known that the Earth has a core. And of course the idea thatthe continents move
about on the surface like lily pads has been common wisdom for much less than a generation.
“Strange as it may seem,” wrote Richard Feynman, “we understand the distribution of matter
in the interior of the Sun far better thanwe understand the interior of the Earth.”
The distance from the surfaceof Earth to the center is 3,959 miles, which isn’t so very far.
It has been calculated that if you sunk a well to the center and dropped a brick into it, it would
take only forty-five minutes for it to hit the bottom (though at thatpoint it would be
weightless since all the Earth’s gravity would be above and around it rather than beneath it).
Our own attempts to penetrate toward the middle have been modest indeed. One or two South
African gold mines reach to a depth of two miles, but most mines on Earth go no more than
about a quarter of a mile beneath the surface. Ifthe planet were an apple, we wouldn’t yet
have broken through the skin. Indeed, we haven’t even come close.
Until slightly under a century ago, what the best-informed scientific minds knew about
Earth’s interior was not much more than whata coal miner knew—namely, that you could dig
down through soil for a distance and then you’d hit rock and that was about it. Then in 1906,
an Irish geologist named R. D. Oldham, while examining some seismograph readings from an
earthquake in Guatemala, noticed that certain shock waves had penetrated to a point deep
within the Earth and then bounced off at an angle, as if they had encountered some kind of
barrier. From this he deduced that the Earth has a core. Three years later a Croatian
seismologist named Andrija Mohoroviči´c was studying graphs from an earthquake in Zagreb
when he noticed a similar odd deflection, but ata shallower level. He had discovered the
boundary between the crust and the layer immediately below, the mantle; this zone has been
known ever since as the Mohoroviči´c discontinuity, or Moho for short.
We were beginning to get a vague idea ofthe Earth’s layered interior—though it really was
only vague. Not until 1936 did a Danish scientist named Inge Lehmann, studying
seismographs of earthquakes inNew Zealand, discover that there were two cores—an inner
one that we now believe to be solid and an outer one(the one that Oldham had detected) that
is thought to be liquid and the seat of magnetism.
At just about the time that Lehmann was refining our basic understanding of the Earth’s
interior by studying the seismic waves of earthquakes, two geologists at Caltech in California
were devising a way to make comparisons between one earthquake and the next. They were
Charles Richter and Beno Gutenberg, though for reasons that have nothing to do with fairness
the scale became known almost at once as Richter’s alone. (It has nothing to do with Richter
either. A modest fellow, he never referred to the scale by his own name, but always called it
“the Magnitude Scale.”)
The Richter scale has always been widely misunderstood by nonscientists, though perhaps
a little less so now than in its early days when visitors to Richter’s office often asked to see
his celebrated scale, thinking itwas some kind of machine. The scale is of course more an
idea than an object, an arbitrary measure of the Earth’s tremblings based on surface
measurements. It rises exponentially, so that a 7.3 quake is fifty times more powerful than a
6.3 earthquake and 2,500 times more powerful than a 5.3 earthquake.
At least theoretically, there is no upper limit for an earthquake—nor, come to that, a lower
limit. The scale is a simple measure of force,but says nothing about damage. A magnitude 7
quake happening deep in the mantle—say, four hundred miles down—might cause no surface
damage at all, while a significantly smaller one happening just four miles under the surface
could wreak widespread devastation. Much, too, depends on the nature of the subsoil, the
quake’s duration, the frequency and severity of aftershocks, and the physical setting of the
affected area. All this means that the most fearsome quakes are not necessarily the most
forceful, though force obviously counts for a lot.
The largest earthquake since the scale’s invention was (depending on which source you
credit) either one centered on Prince William Sound in Alaska in March 1964, which
measured 9.2 on the Richter scale, or one in the Pacific Ocean off the coast of Chile in 1960,
which was initially logged at 8.6 magnitude but later revised upward by some authorities
(including the United States Geological Survey) to a truly grand-scale 9.5. As you will gather
from this, measuring earthquakes is not always an exact science, particularly when
interpreting readings from remote locations. At all events, both quakes were whopping. The
1960 quake not only caused widespread damage across coastal South America, but also set off
a giant tsunami that rolled six thousand miles across the Pacific and slapped away much of
downtown Hilo, Hawaii, destroying five hundred buildings and killing sixty people. Similar
wave surges claimed yet more victims as far away as Japan and the Philippines.
For pure, focused, devastation, however, probably the most intense earthquake in recorded
history was one that struck—and essentially shook to pieces—Lisbon, Portugal, on All Saints
Day (November 1), 1755. Just before ten inthe morning, the city was hit by a sudden
sideways lurch now estimated at magnitude 9.0 and shaken ferociously for seven full minutes.
The convulsive force was so great that the water rushed out of the city’s harbor and returned
in a wave fifty feet high, adding to the destruction. When at last the motion ceased, survivors
enjoyed just three minutes of calm before a second shock came, only slightly less severe than
the first. A third and final shock followed two hours later. At the end of it all, sixty thousand
people were dead and virtually every building for miles reduced to rubble. The San Francisco
earthquake of 1906, for comparison, measured an estimated 7.8 on the Richter scale and
lasted less than thirty seconds.
Earthquakes are fairly common. Every day on average somewhere inthe world there are
two of magnitude 2.0 or greater—that’s enough to give anyonenearby a pretty good jolt.
Although they tend to cluster in certain places—notably around the rim of the Pacific—they
can occur almost anywhere. In the United States, only Florida, eastern Texas, and the upper
Midwest seem—so far—to be almost entirely immune. New England has had two quakes of
magnitude 6.0 or greater in the last two hundred years. In April 2002, the region experienced
a 5.1 magnitude shaking in a quake near LakeChamplain on the New York–Vermont border,
causing extensive local damage and (I can attest) knocking pictures from walls and children
from beds as far away as New Hampshire.
The most common types of earthquakes are those where two plates meet, as in California
along the San Andreas Fault. As the plates push against each other, pressures build up until
one or the other gives way. In general, the longer the interval between quakes, the greater the
pent-up pressure and thus the greater the scope for a really big jolt. This is a particular worry
for Tokyo, which Bill McGuire, a hazards specialist at University College London, describes
as “the city waiting to die” (not a motto you will find on many tourism leaflets). Tokyo stands
on the boundary of three tectonic plates in a country already well known for its seismic
instability. In 1995, as you will remember, the city of Kobe, three hundred miles to the west,
was struck by a magnitude 7.2 quake, which killed 6,394 people. The damage was estimated
at $99 billion. But that was asnothing—well, as comparatively little—compared with what
may await Tokyo.
Tokyo has already suffered one of the mostdevastating earthquakes in modern times. On
September 1, 1923, just before noon, the city was hit by what is known as the Great Kanto
quake—an event more than ten times more powerful than Kobe’s earthquake. Two hundred
thousand people were killed. Since that time,Tokyo has been eerily quiet, so the strain
beneath the surface has been building for eighty years. Eventually it is bound to snap. In 1923,
Tokyo had a population of about three million. Today it is approaching thirty million. Nobody
cares to guess how many people might die, but the potential economic cost has been put as
high as $7 trillion.
Even more unnerving, because they are less well understood and capable of occurring
anywhere at any time, are the rarer type of shakings known as intraplate quakes. These
happen away from plate boundaries, which makes them wholly unpredictable. And because
they come from a much greater depth, they tend to propagate over much wider areas. The
most notorious such quakes ever to hit the United States were a series of three in New
Madrid, Missouri, in the winter of 1811–12. The adventure started just after midnight on
December 16 when people were awakened first by the noise of panicking farm animals (the
restiveness of animals before quakes is not an old wives’ tale, but is in fact well established,
though not at all understood)and then by an almighty rupturing noise from deep within the
Earth. Emerging from their houses, locals found the land rolling in waves up to three feet high
and opening up in fissures several feet deep. A strong smell of sulfur filled the air. The
shaking lasted for four minutes with the usual devastating effects to property. Among the
witnesses was the artist John James Audubon, who happened to be in the area. The quake
radiated outward with such force that it knocked down chimneys in Cincinnati four hundred
miles away and, according to at least one account,“wrecked boats in East Coast harbors and .
. . even collapsed scaffolding erected around the Capitol Building in Washington, D.C.” On
January 23 and February 4 further quakes of similar magnitude followed. New Madrid has
been silent ever since—but notsurprisingly, since such episodes have never been known to
happen in the same place twice. As far as we know, they are as random as lightning. The next
one could be under Chicago or Paris or Kinshasa. No one can even begin to guess. And what
causes these massive intraplate rupturings? Something deep within the Earth. More than that
we don’t know.
By the 1960s scientists had grown sufficiently frustrated by how little they understood of
the Earth’s interior that they decided to try to do something aboutit. Specifically, they got the
idea to drill through the ocean floor (the continental crust was too thick) to the Moho
discontinuity and to extract a piece of the Earth’s mantle for examination at leisure. The
thinking was that if they could understand the nature of the rocks inside the Earth, they might
begin to understand how they interacted, and thus possibly be able to predict earthquakes and
other unwelcome events.
The project became known, all but inevitably, as the Mohole and it was pretty well
disastrous. The hope was to lower a drill through 14,000 feet ofPacific Ocean water off the
coast of Mexico and drill some 17,000 feet through relatively thin crustal rock. Drilling from
a ship in open waters is, in the words of one oceanographer, “like trying to drill a hole in the
sidewalks of New York from atop the Empire State Building using a strand of spaghetti.”
Every attempt ended in failure. The deepest they penetratedwas only about 600 feet. The
Mohole became known as the No Hole. In 1966, exasperated with ever-rising costs and no
results, Congress killed the project.
Four years later, Soviet scientists decided to try their luckon dry land. They chose a spot on
Russia’s Kola Peninsula, near the Finnish border,and set to work with the hope of drilling to
a depth of fifteen kilometers. The work proved harder than expected, but the Soviets were
commendably persistent. When at last they gave up, nineteen years later, they had drilled to a
depth of 12,262 meters, or about 7.6 miles. Bearing in mind that the crust of the Earth
represents only about 0.3 percent of the planet’s volume and that the Kola hole had not cut
even one-third of the way through the crust, we can hardly claim to have conquered the
Interestingly, even though the hole was modest, nearly everything about it was surprising.
Seismic wave studies had led the scientists to predict, and pretty confidently, that they would
encounter sedimentary rock to a depth of 4,700 meters, followed by granite for the next 2,300
meters and basalt from there on down. In the event, the sedimentary layer was 50 percent
deeper than expected and the basaltic layer was never found at all. Moreover,the world down
there was far warmer than anyone had expected, with a temperature at 10,000 meters of 180
degrees centigrade, nearly twice the forecasted level. Most surprising of all was that the rock
at that depth was saturated with water—something that had not been thought possible.
Because we can’t see into the Earth, we have to use other techniques, which mostly involve
reading waves as they travel through the interior. We also know a little bit about the mantle
from what are known as kimberlite pipes, where diamonds are formed. What happens is that
deep in the Earth there is an explosion thatfires, in effect, a cannonball of magma to the
surface at supersonic speeds. It is a totally random event. A kimberlite pipe could explode in
your backyard as you read this. Because theycome up from such depths—up to 120 miles
down—kimberlite pipes bring up all kinds of things not normally found on or near the
surface: a rock called peridotite, crystals of olivine, and—just occasionally, in about one pipe
in a hundred—diamonds. Lots of carbon comes up with kimberlite ejecta, but most is
vaporized or turns to graphite. Only occasionally does a hunk of it shoot up at just the right
speed and cool down with the necessary swiftness to become a diamond. It was such a pipe
that made Johannesburg the most productive diamond mining city in the world, but there may
be others even bigger that we don’t know about. Geologists know that somewhere in the
vicinity of northeastern Indiana there is evidence of a pipe or group of pipes that may be truly
colossal. Diamonds up to twenty carats or morehave been found at scattered sites throughout
the region. But no one has ever found the source. As John McPhee notes, it may be buried
under glacially deposited soil, like the Manson crater in Iowa, or under the Great Lakes.
So how much do we know about what’s inside the Earth? Very little. Scientists are
generally agreed that the world beneath us is composed of four layers—rocky outer crust, a
mantle of hot, viscous rock, a liquid outer core, and a solid inner core.
We know that the
surface is dominated by silicates, which are relatively light and not heavy enough to account
for the planet’s overall density. Therefore there must be heavier stuff inside. We know that to
generate our magnetic field somewhere in the interior there must be a concentrated belt of
metallic elements in a liquid state. That much is universally agreed upon. Almost everything
beyond that—how the layers interact, what causes them to behave in the way they do, what
they will do at any time in the future—is a matter of at least some uncertainty, and generally
quite a lot of uncertainty.
Even the one part of it we can see, the crust, is a matter of some fairly strident debate.
Nearly all geology texts tell you that continental crust is three to six miles thick under the
oceans, about twenty-five miles thick under the continents, and forty to sixty miles thick
under big mountain chains, but there are many puzzling variabilities within these
generalizations. The crust beneath the Sierra Nevada Mountains, for instance, is only about
nineteen to twenty-five miles thick, and no one knows why. By all the laws of geophysics the
Sierra Nevadas should be sinking, as if intoquicksand. (Some people think they may be.)
For those who crave a more detailed picture of the Earth's interior, here are the dimensions of the various
layers, using average figures: From 0 to 40 km (25 mi) is the crust. From 40 to 400 km (25 to 250 mi) is the
upper mantle. From 400 to 650 km (250 to 400 mi) isa transition zone between the upper and lower mantle.
From 650 to 2,700 km (400 to 1,700 mi) is the lower mantle. From 2,700 to 2,890 km (1,700 to 1,900 mi) is the
"D" layer. From 2,890 to 5,150 km (1,900 to 3,200 mi) is the outer core, and from 5,150 to 6,378 km (3,200 to
3,967 mi) is the inner core.
How and when the Earth got its crust are questions that divide geologists into two broad
camps—those who think it happened abruptly early in the Earth’s history and those who think
it happened gradually and rather later. Strength of feeling runs deep on such matters. Richard
Armstrong of Yale proposed an early-burst theory in the 1960s, then spent the rest of his
career fighting those who did not agree with him. He died of cancer in 1991, but shortly
before his death he “lashed out athis critics in a polemic in an Australian earth science journal
that charged them with perpetuating myths,” according to a report inEarthmagazine in 1998.
“He died a bitter man,” reported a colleague.
The crust and part of the outer mantle together are called the lithosphere (from the Greek
lithos, meaning “stone”), which in turn floats on top of a layer of softer rock called the
asthenosphere (from Greek words meaning “without strength”), but such terms are never
entirely satisfactory. To say that the lithospherefloats on top of the asthenosphere suggests a
degree of easy buoyancy that isn’t quite right. Similarly it is misleading to think of the rocks
as flowing in anything like the way we think of materials flowing on the surface. The rocks
are viscous, but only in the same way that glass is. It may not look it, but all the glass on Earth
is flowing downward under the relentless drag of gravity. Removea pane of really old glass
from the window of a European cathedral and it will be noticeably thicker at the bottom than
at the top. That is the sort of “flow” we are talking about. The hour hand on a clock moves
about ten thousand times faster than the “flowing” rocks of the mantle.
The movements occur not just laterally as the Earth’s plates move across the surface, but up
and down as well, as rocks rise and fall under the churning process known as convection.
Convection as a process was first deduced by the eccentric Count von Rumford at the end of
the eighteenth century. Sixty years later an English vicar named Osmond Fisher presciently
suggested that the Earth’s interior might well be fluid enough for the contents to move about,
but that idea took a very long time to gain support.
In about 1970, when geophysicists realized just how much turmoil was going on down
there, it came as a considerable shock. As Shawna Vogel put it in the book Naked Earth: The
New Geophysics: “It was as if scientists had spent decades figuring out the layers of the
Earth’s atmosphere—troposphere, stratosphere, and so forth—and then had suddenly found
out about wind.”
How deep the convection process goes has been a matter of controversy ever since. Some
say it begins four hundred miles down, others two thousand miles below us. The problem, as
Donald Trefil has observed, is that“there are two sets of data, from two different disciplines,
that cannot be reconciled.” Geochemists say that certain elements on Earth’s surface cannot
have come from the upper mantle, but must have come from deeper within the Earth.
Therefore the materials in the upper and lower mantle must at least occasionally mix.
Seismologists insist that there is no evidence to support such a thesis.
So all that can be said is that at some slightly indeterminate point as we head toward the
center of Earth we leave the asthenosphere and plunge into pure mantle. Considering that it
accounts for 82 percent of the Earth’s volume and 65 percent of its mass, the mantle doesn’t
attract a great deal of attention, largely because the things that interest Earth scientists and
general readers alike happen either deeper down (as with magnetism) ornearer the surface (as
with earthquakes). We know thatto a depth of about a hundred miles the mantle consists
predominantly of a type of rock known as peridotite, but what fills the space beyond is
uncertain. According to a Naturereport, it seems not to be peridotite. More than this we do
Beneath the mantle are the two cores—a solid inner core and a liquid outer one. Needless to
say, our understanding of the nature of these cores is indirect, but scientists can make some
reasonable assumptions. They know that the pressures at the center of the Earth are
sufficiently high—something over three million times those found at the surface—to turn any
rock there solid. They also know from Earth’s history (among other clues)that the inner core
is very good at retaining its heat. Although it islittle more than a guess,it is thought that in
over four billion years the temperature at the core has fallen by no more than 200°F. No one
knows exactly how hot the Earth’s core is, but estimates range from something over 7,000°F
to 13,000°F—about as hot as the surface of the Sun.
The outer core is in many ways even less well understood, though everyone is in agreement
that it is fluid and that it is the seat of magnetism. The theory was put forward by E. C.
Bullard of Cambridge University in 1949 that this fluid part of the Earth’s core revolves in a
way that makes it, in effect, an electrical motor, creating the Earth’s magnetic field. The
assumption is that the convecting fluids in the Earth act somehow like the currents in wires.
Exactly what happens isn’t known, but it is felt pretty certain that it is connected with the core
spinning and with its being liquid. Bodies that don’t have a liquid core—the Moon and Mars,
for instance—don’t have magnetism.
We know that Earth’s magnetic field changes in power from time to time: during the age of
the dinosaurs, it was up to three times as strong as now. We also know that it reverses itself
every 500,000 years or so on average, though that average hides a huge degree of
unpredictability. The last reversal was about 750,000 years ago. Sometimes it stays put for
millions of years—37 million years appears to bethe longest stretch—and at other times it has
reversed after as little as 20,000 years. Altogether in the last 100 million years it has reversed
itself about two hundred times, and we don’t have any real idea why. It has been called “the
greatest unanswered question inthe geological sciences.”
We may be going through a reversal now. The Earth’s magnetic field has diminished by
perhaps as much as 6 percent in the last century alone. Any diminution in magnetism is likely
to be bad news, because magnetism, apart from holding notes to refrigerators and keeping our
compasses pointing the right way, plays a vital role in keeping us alive. Space is full of
dangerous cosmic rays that in the absence of magnetic protection would tear through our
bodies, leaving much of our DNA in useless tatters. When the magnetic field is working,
these rays are safely herded away from the Earth’s surface and into two zones in near space
called the Van Allen belts. They also interact with particles in the upper atmosphere to create
the bewitching veils of light known as the auroras.
A big part of the reason for our ignorance,interestingly enough, is that traditionally there
has been little effort to coordinate what’s happening on top of the Earth with what’s going on
inside. According to Shawna Vogel: “Geologists and geophysicists rarely go to the same
meetings or collaborate on the same problems.”
Perhaps nothing better demonstrates our inadequate grasp of the dynamics of the Earth’s
interior than how badly we are caught out whenit acts up, and it would be hard to come up
with a more salutary reminder of the limitations of our understanding than the eruption of
Mount St. Helens in Washington in 1980.
At that time, the lower forty-eight United States had not seen a volcanic eruption for over
sixty-five years. Therefore the government volcanologists called in to monitor and forecast St.
Helens’s behavior primarily had seen only Hawaiian volcanoes in action, and they, it turned
out, were not the same thing at all.
St. Helens started its ominous rumblings on March 20. Within a week it was erupting
magma, albeit in modest amounts, up to a hundredtimes a day, and being constantly shaken
with earthquakes. People were evacuated to whatwas assumed to be a safe distance of eight
miles. As the mountain’s rumblings grew St. Helens became a tourist attraction for the world.
Newspapers gave daily reports on the best places to get a view. Television crews repeatedly
flew in helicopters to the summit, and people were even seen climbing over the mountain. On
one day, more than seventy copters and light aircraft circled the summit. But as the days
passed and the rumblings failed to develop intoanything dramatic, people grew restless, and
the view became general that the volcano wasn’t going to blow after all.
On April 19 the northern flank of the mountain began to bulge conspicuously. Remarkably,
no one in a position of responsibility saw that this strongly signaled a lateral blast. The
seismologists resolutely based their conclusions on the behavior ofHawaiian volcanoes,
which don’t blow out sideways. Almost the onlyperson who believed that something really
bad might happen was Jack Hyde, a geology professor at a community college in Tacoma. He
pointed out that St. Helens didn’t have an open vent, as Hawaiian volcanoes have, so any
pressure building up inside was bound to be released dramatically and probably
catastrophically. However, Hyde was not part of the official team and his observations
attracted little notice.
We all know what happened next. At 8:32 A.M. on a Sunday morning, May 18, the north
side of the volcano collapsed, sending an enormous avalanche of dirt and rock rushing down
the mountain slope at 150 miles an hour. It was the biggest landslide in human history and
carried enough material to bury the whole of Manhattan to a depth of four hundred feet. A
minute later, its flank severelyweakened, St. Helens exploded with the force of five hundred
Hiroshima-sized atomic bombs, shooting out a murderous hot cloud at up to 650 miles an
hour—much too fast, clearly, for anyone nearby to outrace. Many people who were thought to
be in safe areas, often far out of sight of the volcano, were overtaken. Fifty-seven people were
killed. Twenty-three of the bodies were never found. The toll would have been much higher
except that it was a Sunday. Had it been a weekday many lumber workers would have been
working within the death zone. As it was,people were killed eighteen miles away.
The luckiest person on that day was a graduate student named Harry Glicken. He had been
manning an observation post 5.7 miles from the mountain, but he had a college placement
interview on May 18 in California, and so had left the site the day before the eruption. His
place was taken by David Johnston. Johnston was the first to report the volcano exploding;
moments later he was dead. His body was never found. Glicken’s luck, alas, was temporary.
Eleven years later he was one offorty-three scientists and journalists fatally caught up in a
lethal outpouring of superheated ash, gases, and molten rock—what is known as a pyroclastic
flow—at Mount Unzen in Japan when yet another volcano was catastrophically misread.
Volcanologists may or may not be the worst scientists in the world at making predictions,
but they are without question the worst in the world at realizing how bad their predictions are.
Less than two years after the Unzen catastrophe another group of volcano watchers, led by
Stanley Williams of the University of Arizona, descended into the rim of an active volcano
called Galeras in Colombia. Despite the deathsof recent years, only two of the sixteen
members of Williams’s party wore safety helmets or other protective gear. The volcano
erupted, killing six of the scientists, along with three tourists who had followed them, and
seriously injuring several others, including Williams himself.
In an extraordinarily unself-critical book called Surviving Galeras, Williams said he could
“only shake my head in wonder” when he learned afterward that his colleagues in the world
of volcanology had suggested that he had overlooked or disregarded important seismic signals
and behaved recklessly. “How easy it is to snipe after the fact, to apply the knowledge we
have now to the events of 1993,” he wrote. He was guilty of nothing worse, he believed, than
unlucky timing when Galeras “behaved capriciously, as natural forces are wont to do. I was
fooled, and for that I will take responsibility. But I do not feelguilty about the deaths of my
colleagues. There is no guilt. There was only an eruption.”
But to return to Washington. Mount St. Helens lost thirteen hundred feet of peak, and 230
square miles of forest were devastated. Enough trees to build 150,000 homes (or 300,000 in
some reports) were blown away. The damage was placed at $2.7 billion. A giant column of
smoke and ash rose to a height of sixty thousand feet in less than ten minutes. An airliner
some thirty miles away reported being pelted with rocks.
Ninety minutes after the blast, ash began to rain down on Yakima, Washington, a
community of fifty thousand people about eighty miles away. As you would expect, the ash
turned day to night and got into everything, clogging motors, generators, and electrical
switching equipment, choking pedestrians, blocking filtration systems, and generally bringing
things to a halt. The airport shut down and highways in and out of the city were closed.
All this was happening, you will note, just downwind of a volcano that had been rumbling
menacingly for two months. Yet Yakima had no volcano emergency procedures. The city’s
emergency broadcast system, which was supposed to swing into action during a crisis, did not
go on the air because “the Sunday-morning staff did not know how to operate the equipment.”
For three days, Yakima was paralyzed and cut off from the world, its airport closed, its
approach roads impassable. Altogether the city received just five-eighths of an inch of ash
after the eruption of Mount St. Helens. Now bear that in mind, please, as we consider what a
Yellowstone blast would do.