The Rocks Know

Questions:

What was the weather like millions of years ago?

What did dinosaurs eat for breakfast?

How did people on Easter Island build those statues?

Answer: Ask a rock.

Rocks are some of the most basic features of any landscape. They can be found in cities, deserts, forests, and they can be sculpted, rough, shiny or drab. Most of our natural�resources�are pulled from rocks, from the petroleum in our gas tank to the mica flakes in our toothpaste (it’s those little sparkly bits). They also hold vast amounts of information. They can tell us in what direction glaciers flowed, or what comprised the trade patterns and societal structures of past civilizations. Rocks from outer space can tell us about the very essence of the universe. Got a question about the natural world past or present? It’s a pretty safe bet that the rocks around you know at the very least a part of the puzzle.

Sometimes the most mundane things in the world around us can hold the most fascinating information. This blog will focus on cool tidbits in earth science and archaeology, with fun (and sometimes unrelated) things thrown in along the way. Use the Glossary to look up the words/people/places in bold, and let me know if there are things I mention that I need to define.

It seems appropriate to start the story at the beginning, with one of the first great stories that rocks ever told; how old is the earth we walk on?

Archbishop James Ussher- Painting by Sir Peter Lely

Once upon a time there was a Calvinist Archbishop in Ireland named James Ussher (the extra s was just for fun). He was a renowned scholar who calculated the age of the earth using biblical dates, and figured out that creation began sometime on the night before October 23, 4004 BC. Everyone at the time thought that this was awesome, so much so that they began printing his chronology in the front of family bibles. This was 1654, the earth was 5,658 years old, and all was well.

James Hutton- Father of Modern Geology- Painting by Sir Henry Raeburn

Then the Enlightenment began, and science entered the scene. It got to be the late 1700′s and another James rose to challenge Ussher’s chronology. James Hutton was a Scot, and a guy that really REALLY liked rocks. He studied them for fun, because what else would a gentleman of leisure do in the 1700′s? He was a little bothered by Ussher’s chronology though. If Ussher was right, then how on earth did something like sandstone exist? It seemed to have been deposited in rivers and on beaches over time, but the time it took to turn sand into stone would end up being far longer than the time it took for the entire earth to be created.

Whew. That’s a puzzle. The prevailing theory at the time was that ‘it was just different in the past’, and that modern observations of the time it took to erode and deposit a rock couldn’t apply. Still, Hutton was bothered with this idea. He set out looking for a place that would prove that there just had to be more time in, well, time itself. He found various places where there were unconformities or places where the rocks didn’t meld into each other but existed in two different sharp and very distinct layers. It wasn’t until he got to Siccar point that he was able to get the full picture.

Hutton's Unconformity at Siccar Point, Scotland- photo by M.B. Griggs

Here, at a rocky outcrop into the sea along the coast of Scotland, he found evidence for his suspicion that there had to have been more time since the world started. What he saw was a rock formation, known as the Old Red Sandstone, lying horizontally across another sedimentary rock that was gray and had vertical layers. Hutton was ridiculously excited by this. He even found ripple marks on the vertical layers indicating that they had been originally deposited horizontally in water. This fit with Nicolas Steno‘s long established Principles of Stratigraphy, which had been around for about as long as Ussher’s chronology.

Hutton was thrilled, because (get ready to follow the bouncing ball, kids) the bottom layer of rocks would have had to be deposited, compacted and lithified (turned into rock), tilted like a see-saw with Andre the Giant on one end and a two year old on the other, then uplifted, eroded and sunk again into the sea where eventually the horizontal layers of the Old Red Sandstone would be deposited, compacted, lithified, and uplifted to the surface, where eventually weathering and erosion would create the lovely spot called Siccar Point where Hutton was having his really excited moment.

And why was he so excited? Because like that last ridiculous run-on sentence, a lot had to happen, which meant that a lot of time had to take place. And it had to be a lot more time than the accepted age of the earth, which in 1788 was 5,792 years old. Because Hutton established that the earth had to be a lot older than Ussher’s estimate, he has become known as the Father of Modern Geology. Because without accurate estimates of how old the rocks that geologists are looking at are it becomes far more difficult to actually get any decent information out of them. Others of his colleagues took his work on the subject, and actually made it readable (he was a great scientist and a horrible writer) and he became all but worshipped by other people that really liked rocks.

It wasn’t until the advent of modern technology that we were able to figure out exactly how old the earth was. Scientists in Hutton’s time used relative dating to figure out how old things were, simply putting the puzzle pieces together and figuring out that this rock was older than that rock which is the same age as that rock over there. They had no numbers to work with on their time scale, just fossils and places where the rocks met each other.

Eventually though we figured out the numbers that went with their relative dating scale, using absolute dating which measured how long ago rocks had been formed using radio-active isotopes found in tiny zircon crystals. The oldest rocks that we know about now date to 4.28 Billion years old! They can be found in Canada along with the previous record holder the Acasta Gneiss, at a sprightly 4.03 billion years old.

So, how old is the earth?

Modern estimates using old rocks and new technology put the Earth’s birthday somewhere around 4.5 billion years ago, a lot longer than the 6,014 years that the world would be turning this October 22 (MARK YOUR CALENDERS) if we kept with Ussher’s original calculation. But, while Ussher was wrong, and while Hutton only was able to prove that Ussher was wrong, their contributions to the field led modern geologists to look at the rocks and figure out that the rocks did in fact know exactly how old they were.

Next time: Wealth in a warzone; why the rocks in Afghanistan are such a big deal