Stories in Stone (13 page)

Qwest Building, originally Northwestern Bell Telephone building,
built 1930–1932, Minneapolis, Minnesota.

Although Beltrami recognized the potential of the Morton rocks, he was not a geologist.
William Keating, also on Major Long’s
1823 exploration, was the first geologist to see the Morton rock along the Minnesota River.
“The character of these rocks
was examined with care, and found very curious,” he wrote in the official trip narrative.
“It seemed as if four simple minerals,
quartz, feldspar, mica, and amphibole, had united here to produce almost all the varieties of the combination which can arise
from the association of two or more of these minerals.”
⁶

Keating encountered the Morton rocks at an interesting turning point, particularly on the subject of geologic time.
For over
a thousand years, biblical begats, begets, and begots had provided the means by which people had derived the age of Earth.
By Keating’s time, the best-known and most widely accepted day one of Earth was Sunday, October 23, 4004 BCE, according to
Irish Archbishop James Ussher.
⁷
The biblical trap, however, was beginning to loosen in the early 1800s, because of the historic boat ride of James Hutton,
John Playfair, and Sir James Hall.

In describing their momentous day, Playfair had written,“The palpable evidence presented to us, of one of the most extraordinary
and important facts in the natural history of the earth, gave a reality and substance to those theoretical speculations, which,
however probable, had never till now been directly authenticated by the testimony of the senses .
.
.
What clearer evidence
could we have had of the different formation of these rocks, and of the long interval which separated their formation, had
we actually seen them emerging from the bosom of the deep?
.
.
.
The mind seemed to grow giddy by looking so far into the
abyss of time.”
⁸

Geologists now know that Hutton, Playfair, and Hall were looking back 425 million years in time, to the age of deposition
of the graywacke.
The unconformity they saw represented a gap of 80 million years, which from a geologic point of view is
a blink, but it was enough time for four-legged amphibians to have moved onto land, for forests to have spread widely across
the globe, and for sharks to diversify and dominate the seas.

Hutton fleshed out the ideas the three men had discussed at Siccar Point in his book
Theory of the Earth; or an Investigation into the Laws Observable
in the Composition, Dissolution, and Restoration of Land upon the
Globe
.
Playfair followed several years later in 1802 with
Illustrations of
the Huttonian Theory of the Earth
.
He had been forced to write his book, which covers the same material, because Hutton’s dense and obtuse prose had prevented
most readers from understanding his revolutionary geologic ideas.

As he noted in his title, Hutton based his ideas on what he could see out in the field.
This in itself was a new beginning
for most scientists, but he also observed that the planet was in a constant state of change and ever recycling itself.
Dead
organisms accumulated in the sea and made limestone.
Mountains eroded to a “confused mass of stones, gravel, and sand,” which
consolidated into puddingstone.
Rivers carried sands that became sandstones.
Hutton’s geologic reasoning has been summed up
with a very Hillel-like statement, “The present is the key to the past.”

One man who did penetrate Hutton’s dense language was Charles Lyell, a twenty-seven-year-old lawyer and amateur geologist.
In the year following Keating’s visit to the Minnesota River valley, Lyell visited Sic-car Point with James Hall.
Lyell recognized
the significance of the great unconformity between the schistus and the sandstone.
In 1830 he wrote
Principles of Geology, Being an Attempt to Explain the Former Changes of the
Earth’s Surface, by Reference to Causes Now in Operation
.

Principles
took Hutton’s great statement and backed it up with numerous examples.
Lyell also made two additional points: that natural
laws did not change over time and that change occurred slowly and gradually and not catastrophically.
Together these three
principles form the core of uniformitarianism, one of the central tenets of geology.
In addition, Lyell’s book further pushed
geologists to start pondering an Earth much older than the one described in the Bible.
With
Principles
geologists now had their own scripture.

Although
Principles
pointed the way, acceptance of an ancient Earth came slowly.
When Charles Darwin wrote in the first edition of
On the
Origin of Species
(1859) that the valley of the Weald in southern England took 306,662,400 years to erode, critics lambasted him and he pulled
the number in the third edition.
Yet in 1862 one of Darwin’s detractors, William Thomson—later Lord Kelvin—proposed an age
for Earth as great as 400 million years.

Kelvin did not base his calculation on uniformitarianism but on the second law of thermodynamics, that a hot mass will cool
over time.
Since Earth had started out hot, as indicated by rocks such as the gneiss of the Minnesota River valley, it must
be cooling.
Few could dispute Kelvin’s observation.
As he gathered more data, however, he dropped his number to 100 million
years and finally to 24 million years.

Not all geologists agreed with the physicist Kelvin.
What about the slow, steady rates of geological and biological processes,
which required more time than Kelvin proposed?
By looking at sedimentary processes, such as deposition and erosion, geologists
derived numbers between 3 million and 15 billion with the most popularly accepted age of Earth being around 100 million years.
⁹

Compared to what we now know the age of the earth to be—4.5 billion years—a range of 24 to 100 million years seems to be puny.
But consider the relative change between six thousand years and 100 million years.
To make that leap requires four orders
of magnitude compared with just one order of magnitude between 100 million and 4.5 billion.
Hutton and Lyell had helped to
push geologists across their biggest relative chasm of time.
To get across the bigger, absolute chasm would require another
radical shift in our understanding of the planet.

Geologists had long known that the Morton Gneiss was very old but not until 1956 when four University of Minnesota researchers
published a crystallization date of 2.4 billion years did they learn how old.
¹⁰
The quartet based their age on a fundamental property of many elements, radioactivity, which simply means that an element
is naturally unstable and constantly decaying.
The best-known element for radiometric dating, as this technique is called,
is carbon-14, which works for ages less than sixty thousand years.
For ages of millions or billions of years, geologists turn
to potassium, argon, lead, and, the most useful for the Morton, uranium.

When an element decays, or breaks down, it changes from an unstable isotope, called the parent, to a stable isotope, called
the daughter, sort of like a 1960s flower child giving birth to an accountant.
Isotopes refer to different forms of an element
that have the same number of protons in their nucleus but a different number of neutrons.
For uranium, which has several isotopes,
the most important for geologists studying Morton Gneiss are uranium-238 (written 238U), which decays to form the lead isotope
206Pb, and 235U, which decays to 207Pb.
¹¹

What makes radioactive decay of uranium, and other elements in radiometric dating, useful to geologists is that the change
from parent to daughter occurs at a measurable rate, called the half-life.
The half-life of 238U is 4.47 billion years, which
means that 4.47 billion years after crystallization, half of the parent 238U will have become the daughter, 206Pb.
(Carbon-14
has a half-life of 5,700 years.) To figure out the age of formation of the uranium, and hence the age of formation of its
surrounding rock, all geologists have to do is count the number of parent uranium isotopes and number of daughter lead isotopes.

Chemist Ernest Rutherford was the first to calculate the amount of the parent and daughter elements in a mineral.
In 1904
he obtained an age of 497 million years.
Three years later, an American chemist, Bertram Bolt-wood, showed that a uranium-bearing
mineral (uraninite-UO2) from Canada had formed 2.2 billion years ago.

Rutherford and Boltwood weren’t geologists, so radioactivity for them was more of a technical challenge than a way toward
understanding geologic time.
Geologists quickly recognized the insights to be gained with radiometric dating, and by the 1930s,
it had become the accepted form of ascertaining the age of rocks.
Researchers have continued to perfect their techniques both
through improved technology and improved understanding of radioactivity.
They have continued to refine the geological timescale,
precisely dating eons, eras, periods, epochs, and ages.
They have also continued to seek out and identify older and older
rocks, including a beautiful building stone from Minnesota.

To analyze uranium in the Morton Gneiss, geologists studied zircon,
¹²
an extremely resistant mineral because of its high melting temperature and extreme hardness.
In addition to zircon grains’
long life, they have an unusual crystalline structure, which facilitates the incorporation of uranium from a magma, usually
in concentrations of a few hundred parts per million.
¹³
(Zircon isn’t radioactive because it contains so little uranium.)

Zircon’s heat resistance also aids geologists in age dating.
During deep burial or when intruded by magma, many minerals cannot
withstand the heat these processes generate and end up melting.
In contrast a crystal of zircon resists melting and instead
attracts any uncrystallized zircon, and uranium, that might be in the introduced liquid.
The new zircon forms a layer or rind
on the original.
Each subsequent high-temperature geologic event produces additional layers and each rind creates a time stamp
as the uranium begins to decay, allowing geologists to date when that event occurred.

Adding new layers, like a tree adding rings, creates a technological problem.
Although zircons occasionally grow to five-eighths
of an inch long, the ones used to date the Morton Gneiss, and most other rocks, are about as wide as a two human hairs.
The
rinds are more miniscule, on the order of spider silk.
When geologists first started to study the Morton they did not have
the technology to date separately the rinds and the core.
Instead they analyzed the entire crystal and came up with what Pat
Bickford, professor emeritus of petrology at Syracuse University, called “precise but inaccurate” ages for zircons.
¹⁴

As their dating tools improved, geologists pushed the date of the Morton back.
By 1963 it was 3.2 billion years old,
¹⁵
and in 1974 two researchers reported an age for the Morton of 3.8 billion years old, the oldest rock on Earth.
¹⁶
As one can imagine, there was much rejoicing.
Fame was fleeting though.
By 1980 the commonly accepted age of the Morton had
dropped to 3.5 billion years.
At that number, it was still the oldest, most commonly used building stone in the world, but
the title of Earth’s oldest rock belongs to gneiss found near Slave Lake in the Northwest Territories of Canada.
Its age is
4.03 billion years.

In 2006 Pat Bickford led a team of researchers who obtained the Morton’s most up-to-date age of 3,524 million years ago.
¹⁷
His date will probably be the one that sticks, mostly because he was able to take advantage of technology developed in the
1990s to analyze individually the core and rinds of a zircon crystal.