OGG member Paul Goodrich writes…
When astronomers talk of distances, they talk in AU’s ; Astronomical units, or Parsecs. The AU is the mean distance to our star, Sol, the Sun. It is 149,597,870,691 km, or thereabouts. A Parsec on the other hand is; 3.26 light years or 31,000,000,000,000 or 1.91735116 × 1013 miles. Even a parsec with all those noughts is still only three quarters of the way to our nearest intra galactic neighbour, Proxima Centuri. And most of the stars you can see at night unaided, are a thousand times more distant than that. Particle and nuclear Physicists on the other hand, go in the other direction. They go to microns, nanometres and Angstroms. More noughts with a decimal point way over to the left this time.
Numbers with a lot of zeros behind them begin to lose meaning, so terms like Parsec and Angstrom are used – to give a clearer concept of scale and for easier handling of equations. Well, at least for Astrophysicists. Conceptualising scale is important. I mean, when asked, how big is that water melon, one wouldn’t suggest that its about forty seven squillion bajillion oxygen atoms across. Thirty centimetres would be more appropriate, or one foot in old money. Geologists have similar difficulty in calculating quantities. Amounts of recoverable reserves in hydrocarbon reservoirs for example, or the volume and mass of the Earth. We use powers or orders of magnitude.
So when asked by a student, how heavy is the Earth? the long answer is 6,000,000, 000,000,000,000,000,000 kilograms. The short answer is, it has an approximate mass 5.975e24 kg. Neither makes much sense, so analogies are often used, like piling up bags of sugar all the way to the moon. Then there is pressure. If I dig a hole 35 km deep by one metre square and placed a hand specimen of mudstone and a pressure gauge at the bottom, back filled and tamped it neatly down, what would the pressure be down there with all that backfill over it? Excavate the hole for a second time and our gauge will have recorded about 30 MPa/km or 1050 MPa, or something over a billion tonnes per square metre. Actually, our pressure gauge would not have survived the experiment and the lump of dull mudstone would now be a nice shimmering piece of slightly smaller slightly denser – schist.
But the most perplexing concept of all, to Geologists, Physicists and Philosophers, is that of time. We all experience it, seconds and hours – months and years. A decade is a long time. A century is barely beyond the experience of everyman and a millennia is utterly unknowable. In the 17th Century, Bishop Ussher calculated that the Earth and all things in existence came into being on October 23, 4004 BC. Which would make the Earth, its vast oceans, it deserts and mountains, flora and fauna, just over 6000 years old. A reasonably comfortable number. Still big, but agreeably comprehensible. So when occasional heretics came along and questioned this concept of time, or at least – that there could have been so little of it, they were met with scepticism, derision and often outright hostility.
Geology as a science was in its infancy, it was lumped in with ‘Natural Philosophy’ practiced by gentlemen of means, aesthetes and ironically, some notable clergymen. When they pointed at rocks with their great bands of strata piled on top of each other, they wondered out loud, how it could be that it took so little time for grains of detritus to gather together and create such immense edifices as observed by Hutton, Lyle, Sedgwick and Murchison? And so the concept of Deep Time was postulated. To stand on a beach and see the waves slowly wear away the rocky cliffs and beach outcrops, 6000 years could not seem enough, not nearly enough. Rivers, winds and ocean currents carrying sand and clay minerals must surely need vast tracts of time to deposit enough material to create rocks thousands of miles square and thousands of metres deep.
Only for those rocks to be worn away, tilted on their side and the whole process repeated in great successions above an unconformable division or contact between the two rock type of different color and composition. Yes, Siccar Point! Tens or hundreds of thousands of years – perhaps, blasphemously, even millions of years were required to deposit, sculpt and shape the lands these pioneers of reason, science and art walked upon in search of answers and enlightenment. Over the next two and a half centuries, the Science of Geology and its Earth Science siblings of Palaeontology, Geophysics, Geochemistry and Geomorphology were providing answers, missing pieces of an enormous puzzle.
By the time of Lord Kelvin, an age of 100 million years was – while remarkable, acceptable.
His calculations based on heat reached the not unreasonable conclusion that a molten globe the size of the Earth and the dissipation of heat therein would require that amount of time to cool to its present level. Kelvin as an old man, was present at a lecture given by Ernest Rutherford, a luminary of the University of Manchester – wherein Rutherford spoke of radioactive decay and the energy/heat it produces. The products of that decay, the parent and daughter isotopes could be established, Kelvin heard of Uranium transmuted to Lead, of unstable and stable elements, and lastly – how this process dates the Earth. For the last century, accurate dating has made sense of the great piles of strata, ocean floors, mountains and the fossilised flora and fauna therein.
Radiometric dating is now a commonplace concept. Modern geologists often find it difficult to grasp just how the early pioneers worked, severely disadvantaged as they were by the lack of accurate chronology. Relative dating produced the now familiar periods from the Cambrian to the Cainozoic. These were accurate only in their division of time, of periods, eras and epochs when sands, muds and extinct biota were deposited, eroded and redeposited. The Ordovician for example was known for its Graptolites, Trilobites and other marine fauna in the rocks of the ancient Avalonian North Wales Basin. But over how long these rocks were formed and for how long the benthic creatures endured, was unknown for over a century.
Now, a visit to the labs at the Williamson Building in Manchester with a small sliver of Graptolite bearing slate will give an absolute date with a narrow margin of error. We can say that the Ordovician was a period of about 45 million years, book ended by mass extinctions at 488.3 mya and 443.7 mya. My small fragment of it was from the Caradoc, Soudleyan age, about 462 million years old. Give or take.
Finding Zircons in the Canadian Shield or the Cratons of South Africa and Australia, give reasonably accurate dates back to the Archean at 3850By to 4.400By. A mind boggling temporal distance from – for example, the Cambrian Explosion – the great diversification of life still over three billion years in the future.
Fragments of meteorites push the Earths formation further still to 4.54By. Or 4.54 × 109 years. More incomprehensible numbers. We can’t see these great periods of time, touch them or feel them. Its not something easily visualised or conjured up in the minds eye. So we go back to analogies and thousand word pictures. There have been have been the familiar clocks where the human race stands at a few seconds before midnight, and Sir David Attenborough’s moving band of time, diminishing to a vanishing point behind us.
I am a Geologist with an appreciation of the concept of Deep Time, but I value these depictions of past eons. To times when there was a Heavy Bombardment , the softened craters of which exist only in the Cratons, the remainder wiped clean by erosion and tectonics.
To the time of great oxidation events, of Snowball Earth, of Large Igneous Provinces, the first Terrestrial Plants, the great Mass Extinctions, to the closing of the Tethys and the opening of the North Atlantic, to the country where I live, only very recently sculpted by glaciers and planed by mile high Ice Sheets, ending a very short 8,000 years ago.
A comprehensible number indeed.
All Images, Open source Creative Commons.