Cleopatra’s needle, an early granite build.

We wandered along the Thames embankment in late summer.

Cleoatra’s misnamed edifice stands out like a large needle on the bank of the River Thames. Ancient history and a geological story wrapped in a monument.

The stone quarries of ancient Egypt provided granite and limestone by the pyramid load. Aswan quarries beside the Nile supplied granite for the needle.

The obelisk was erected in Heliopolis around 1400 BCE, however, the granite crystallised long before then.

How Old is the Aswan Granite?

You can either flick to the answer in highlight at the end of this section, or plough through the science first.

Zircon crystals form when granitic magma, initially above 1200 degrees Celsius, cools deep in the earth’s crust.

Uranium atoms locked in the Zircon crystal behave like tiny atomic stopwatches. Each Uranium atom in a zircon crystal will decay in stages, shedding parts of its nucleus till it emerges as a lead atom.

There are four stages in the the life cycle and metamorphosis of a butterfly. (Egg, caterpillar, pupae, butterfly)

Lead atoms are the final product of a 9 stage metamorphosis beginning with Uranium. We don’t call it a metamorphosis in Physics, it’s a decay series, but you get the idea.

The decay series from Uranium to lead is completely random, it is not possible to predict which atom will flip next or when. Overall the process is remarkably time constant as determined by experiment.

The key number defining a radioisotope is the 1/2 life, the time taken for half the atoms in a sample to decay. This is why Zircon crystals with trapped uranium atom make good stopwatches.

The stopwatch starts timing when magma solidifies and Uranium atoms are sealed in their personal Zircon crystals. Their happy place through geological time and space, as tectonic plates slide across the Planet.

Timing stops when crystals are removed from their rock and contents are passed through a mass spectrometer. Mass spectrometers count atoms with a high degree of accuracy.

By counting Uranium atoms and the lead atoms they eventually decay to, we can calculate how much time has passed since the magma cooled and crystallised.

Zircon stopwatches are accurate over huge time intervals. They reliably measure ages between 1 million and 4.5 billion years.

After all that, samples of granite used in monument construction have been reliably dated at 606.2 MYA

(Finger, Dorr et al Ur Pb Determination of Monumental Granite Ages)

Hieroglyphic translation.

Hieroglyphs on the obelisk acknowledge Egyptian Deities. They celebrate successes in wars fought by Ramesses the Great against the Nubians and Syrians. Status was important in Egytptian society and good solid media exposure was important.

Graffitti artists love Granite.

Graffitti artists were attracted to light coloured granite surfaces as an early as 1280 BEC. Their work still survives at Bigeh Island near Aswan.

Ancient Egyptian graffiti tells a story we can read today. Modern graffiti scrawled over the statue has no history. My search assistant could only turn up “no relevant information”.

Ancient Romans used to visit even more ancient Egypt on expensive package tours. If they were underwhelmed by the view a graffiti artist in the party left a comment for everyone who followed to marvel at. An early “trip advisor”rating.

No self respecting graffiti artist in ancient Egypt would use a spray can. Stonemasonry gear is heavy and monument tourists probably brought a slave along to do the job.

Classy work in classical times.