Stories in Stone (14 page)

“If you want to know the answer badly enough, you will do tedious and persnickety work,” said Bickford.
To obtain a radiometric
date, he starts by breaking rocks into plum-sized pieces, grinds them into powder with a jaw crusher, and separates out denser
material (“like panning for gold”).
He further isolates the zircons by floating the residue in a heavy liquid, which suspends
the lighter particles, and by running the material through a magnetic separator.
Once he obtains a good supply of zircons,
he places them under a microscope and uses tweezers to handpick pure, crack-free crystals, each of which is half the size
of a grain of salt.
Finally, he mounts the zircons in epoxy and polishes them to half their original thickness.
A typical
puck of epoxy contains as many as a thousand zircons.
Bickford does all this work at his lab in Syracuse before shipping the
mounts out to Stanford.

There he uses the Sensitive High-Resolution Ion Microprobe, or SHRIMP, one of the tools that has enabled geologists to tease
out the rind’s geologic information.
To date a zircon, the SHRIMP bombards the crystal’s surface with a tightly focused beam
of oxygen ions, which excavates micrometer-sized pits from the zircon crystal and strips uranium and lead atoms of some of
their electrons.
This gives the uranium and lead atoms, now called ions, an electrical charge.

Once ionized, the uranium and lead are accelerated through a strong magnetic field, which separates the ions on the basis
of their mass.
By changing the pull of the magnets, Bickford can precisely focus and measure specific uranium and lead isotopes.
He usually counts 238U and 206Pb, as well as 235U and its daughter 207Pb, which have a half-life of 747 million years.
The
second set of isotopes provides an independent, corroborating age date.

“The SHRIMP has completely revolutionized geochronology.
It means we can pinpoint specific regions of a zircon and get specific
dates for specific events,” said Bickford.
With the SHRIMP uranium-lead method, Bickford can narrow down his dates to the
point where the range of uncertainty is small enough that the number can be counted as accurate.
For example, one zircon crystal
from Morton Gneiss recorded three dates, which reflect initial formation and two subsequent geologic events.
The average margin
of error for each date is .005 of a percent.

Time is one of the hallmarks and central challenges of geology.
How does one relate to billions and millions, or even tens
of thousands, numbers not typically bandied about in daily conversation?
To do so requires an openness and respect for the
possibilities of time.
For example, I can’t see back in time but I can look at a sandstone and easily grasp that it was once
sand that washed out of an eroding mountain chain, settled in a dune, disappeared under more sand, and finally got converted
to rock.

I also trust the numbers used by geologists.
The numbers are based on the laws of science and in the realms of observation,
experimentation, and hypothesis.
Scientists have tested and retested these numbers, not just on the same rocks or in the same
laboratory or under the same conditions, but on different rocks, in different labs, and under different conditions around
the world.
The numbers have withstood the intellectual challenges of other scientists, scientists who had no interest, either
financially, intellectually, or emotionally, in the truth of the numbers.

John McPhee coined the term “deep time” to describe the great abyss that made John Playfair so giddy.
Deep time is what makes
possible the almost imperceptible spreading of two plates to become the Atlantic Ocean.
It is what allows a single finch on
a small group of volcanic islands in the Pacific Ocean to evolve into thirteen species of finches, each adapted to a specific
niche.
Deep time is what helps with understanding the billions of years necessary for the formation of a rock unit in the
Minnesota River valley that looks like a mixture of bubble gum and fudge.

According to our Morton amanuenses, Pat Bickford’s zircons, the story of the Morton Gneiss began 3,524,000,000 years ago.
Bickford found that the oldest zircons originated in the gray layers of the Morton.
Because the gray layers have the chemistry
of a type of rock known as tonalite, Bickford and many other geologists who have studied the Morton believe that the original
source for the gneiss must have been a tonalite.
Tonalites (named for rocks found near Tonale, Italy) are similar in composition
to granite but a bit darker, thus in the beginning, the Morton was probably a drab, gray igneous rock with lighter speckles
of biotite and chocolate chips of hornblende.

Bickford does not know where the Morton originated.
He can, however, theorize about how it formed by looking at more modern
tonalites.
For example, 40 million years ago, during the collision of Africa with Europe, oceanic crust subducted continental
rock and generated a magma that later solidified into the Italian tonalite.
Colliding plates probably generated the Morton’s
tonalite, but geologists face a problem with making an absolute statement about the collision.
Planet Earth 3.5 billion years
ago did not look anything like the green and blue planet we now inhabit.

First, the color green probably didn’t exist, or at least green life did not exist.
The oldest fossil evidence for life are
microbes found in 3.45-billion-year-old rock in western Australia, which means that when you see the Morton, you are seeing
a rock that first cooled and solidified on a planet devoid of life.
Second, what we think of as continents may not have existed
either or they were a much less significant feature of our budding planet.

I use the word “may” because one of the great questions in geology is “When did plate tectonics begin?” At its most basic,
plate tectonics describes the interaction of the dozen large and several smaller plates, consisting of continental or oceanic
crust, which constitute Earth’s outer layer.
The word “tectonic” comes from the Latin
tectonicus
, pertaining to building.
New crust forms at spreading centers in the oceans and moves away from its point of origin.
Plates
disappear when they dive beneath other plates at subduction zones but at the same time subduction generates continental crust.
Although plate tectonics is one of the best known, most widely accepted, and easily explainable founding concepts of any field
of science, a controversy centers on when plates began to form, interact, and disappear, or die, at least in the mode seen
on modern Earth.

“All geologists were suckled on plate tectonics and they never accept that something other than plate tectonics existed,”
says Robert Stern of the University of Texas at Dallas.
¹⁸
He is one of the few who believes plate tectonics began as recently as only 1 billion years ago.
Stern holds that early Earth
was much hotter than it is today and crustal plates may have been too buoyant and too thick to subduct each other.
If subduction
was occurring on a young Earth, it should have generated three types of rock: ophiolites, blueschists, and ultrahigh-pressure
(UHP) metamorphic terranes.
Stern sees little evidence for these rocks prior to a billion years ago and without them, no plate
tectonics.

Furthermore, Stern argues that a shift to an Earth dominated by plate tectonics must have had a profound affect on global
environments.
An increase in subduction-generated volcanism would have shot gases and fine particles into the atmosphere,
cooling the planet and leading to a climatic condition that geologists call “snowball Earth,” one of which froze the entire
planet around 710 million years ago.
“All in all, the arguments for early plate tectonics are fairly unconvincing,” Stern
concludes.

Kent Condie of New Mexico Tech counters that plate tectonics has operated since at least 2.7 billion years ago and possibly
as far back as 4.4 billion years, the age of the oldest known mineral, a zircon from Australia.
He challenges Stern’s concerns
about lack of subduction-related rocks by observing that a hotter young Earth altered how plates sub-ducted, and that resulted
in fewer ophiolites, blueschists, and UHPs, although all three rock types do exist as far back as perhaps 3.8 billion years
ago.
Condie also cites rocks such as the Morton’s original tonalite, which are widespread in the Archean era (3.8 to 2.5 billion
years ago) and require oceanic crust and subduction for formation.
“It may be that plate tectonics did not begin globally
all at once,” says Condie.
“It may have been episodic but by 2.1 billion years it was continuous.”
¹⁹

Condie makes an additional point.
He observes that no other planets in our solar system have higher life forms, mostly because
plate movement fostered our oxygen-bearing atmosphere by creating continents.
Continents allowed for the evolution of photosynthetic
plants, the source of oxygen.
Furthermore, the shallow marine shelves that develop on the edges of continents are an ideal
place for carbon dioxide to be deposited, in the form of calcite.
If such deposition did not occur, carbon dioxide would accumulate
to dangerously high levels in our atmosphere.
“Without plate tectonics, humans wouldn’t exist,” says Condie.

No matter when and where the Morton formed, its birth as an igneous rock raises another question.
What was the source of the
magma?
Magma doesn’t just appear magically, it has to have a parent rock that melts to form a liquid.
The process begins in
the asthenosphere, the partially molten layer of the mantle made primarily of peridotite, a greenish rock rich in the mineral
olivine.
The asthenosphere starts about 45 miles below the surface and extends to about 150 miles deep.

When asthenospheric peridotite partially melts, it produces a magma that solidifies to basalt, similar to what erupts in Hawaii.
A good analogy is the partial melting of a Popsicle as you eat it outside on a hot day.
As you hold your frozen treat, it
invariably melts, sending a stream of sugary syrup down your arm.
When you take the next bite of your icy confection, you
will notice it has a slightly different taste and texture.
The Popsicle is still solid but less sweet and more granular with
icy particles.
When a peridotite partially melts, it loses various elements such as iron, magnesium, and calcium to magma,
but still remains a peridotite.

In the geologic process, less than 3 percent of the total peridotite changes to a basaltic magma and the rest remains as peridotite.
The most common place to find basalt is at spreading centers where plates begin to pull apart from each other.
Most of these
spots are what geologists call a midocean ridge, such as occurs in the middle of the Atlantic Ocean or off the coast of Washington
State where the Juan de Fuca Plate moves away from the Pacific Plate toward the North American Plate.

When basalt partially melts, the resulting magma can generate a tonalite.
This process occurs most often at a subduction zone,
where the basalt dives into the planet, warming roughly twenty-four to thirty-two degrees Celsius for every mile it descends.
When it reaches temperatures of seven hundred to eight hundred degrees Celsius it partially melts.
Because the melt is only
partial, on the order of 10 to 15 percent, it is not unusual to find basalt inclusions in a tonalite, sort of as if chunks
of Popsicle also dropped onto your arm at the same time the Popsicle melted.

In the Morton Gneiss, basalt appears as solid black rafts floating in the gray and pink swirls.
The rafts range in size from
a few inches to sixty-five feet across.
Geologists refer to these anomalous blocks as enclaves or xenoliths, meaning foreign
rock.
Mark Gross, who has worked the Morton quarries for Cold Spring Granite for twenty-eight years, called the smaller of
these variously shaped blobs “knots” or “cigars” (pronounced
see
-gars).
²⁰
Quarrymen don’t like the knots and cigars because buyers don’t want such imperfections marring their stone.

Unfortunately it is impossible to date the Morton’s basalt rafts because they lack zircons.
Bickford believes there is a strong
possibility that the basalt originated more that 3,524 million years ago but the rafts could also have formed during a later
geologic event, which melted the basalt only enough to generate more basalt that injected itself as dikes into the surrounding
rock.
Such dikes are not unusual in other rocks that formed from basalt.

After recording the first crystallization of the Morton tonalite, the zircons logged geologic events at 3.42, 3.385, 3.14,
and 3.08 billion years ago.
Little field evidence for these events remains.
The only feature that records one of them may
be veins of light-colored rock peppered with black minerals that shoot randomly across the rock.
Gross called the veins “big,
ugly ropes,” and like knots they lower the value of the stone.