This question comes from a local fifth grader, courtesy of the Science Outreach Program (SOP)’s question box. Teams from the Science Outreach Program make weekly visits to elementary school classrooms in the Portland area to encourage scientific learning in younger students—most importantly, by making it fun. One of the ways they do this is by having a question box, where any student can submit a question they have about the world, and the Science Outreach team will explore the answer with them. Now, this spirit of curiosity has taken on a new form in the Quest. If you have any serious SOP questions, please contact scienceoutreach@reed.edu, but if you just have a silly science question you’ve always wanted answered, we have a Reedie question box in the shape of a turtle outside of Bio 246. Drop us a question!
As one might expect, this answer depends entirely on what country you come from!
Before we discuss the answer you might find on National Geographic or in a standard geology textbook, I’d like to bring up a few other perspectives on the question.
Without making any generalizations, one might note reoccurring themes throughout the creation mythology of many indigenous groups across the globe. From the Sami of Northern Scandinavia to the Mexica of Mexico, the Earth is/was understood to have been formed from the body of a great deity. For the Sami, the world originated from a majestic White Reindeer, as “its fur became forest, its stomach became the ocean, and its horns became mountains.” In the slightly more gruesome Mexica story, the giant crocodile-like monster Cipactli was slain by Quetzalcoatl and Tezcatlipoca, who then formed the Earth from her body.
Other instances of ‘first rocks’ appear in Shinto belief in Japan, where Izanagi and Izanami drew up the first islands from a primordial ocean using a jeweled fishing spear. In the traditions shared across much of Polynesia, Māui fished up the first islands of Polynesia by tricking his brothers into thinking he was just “catching a really big fish.”
One might even question what ‘the first’ means if we accept that not everybody shares the same perspective on time. In many aboriginal myths from what is today called Australia, creation exists in a continuum in a certain sense. Howard Morphy, of the Australian National University, summarized that the hazy (but as real as our reality) time of mythological creation called Dreamtime “has never ceased to exist and, from the viewpoint of the present, it is as much a feature of the future as it is of the past.” So, when the Rainbow Snake began to move and carved the mountains and valleys of the Earth, the mythologically significant event of creation is understood to still be real and meaningful in the present.
A final story to consider (although it doesn’t explicitly answer the given question) comes from the Chinook people of the Pacific Northwest, which tells how the first men were born from the broken eggs of Thunderbird. They came down to the valley floor, and pulled women from a rock they found. Although it probably wasn’t ‘the first’ rock, it was the first rock humans had meaningful interaction with, which feels like it counts for something! This rock has a hole through where the arms of the first woman were pulled through still exists to this day, and although its precise location is not disclosed, it is said to be located in the area around Saddle Mountain (where Thunderbird originally laid the eggs).
Basically, humans have been asking ‘what was the first rock?’ for as long as humans have been humans, and the answers we came up with impacted—and continue to impact—the cultures of all humans. I provide this context before presenting the scientific response not to pit these answers against each other, but to show that humans will use whatever tools and skills they have to answer questions which arise universally. Moreover, regardless of what country you’re in, ‘the first rock in your country’ was there long before your country existed. Really, national claims to ‘the oldest rock’ are a bit silly. Regardless, this column is based on providing serious answers to silly questions, so on with the show!
With all of that in mind, what does science have to say in regard to the question: “What is the oldest rock in your country?” In order to answer this, we’ll need to do a bit of geology. First and foremost, the Earth has a thin rocky crust and a liquid magma core in about the same ratio as the skin of an apple to its fruit. The core is (very slowly) mixing and moving around, powered by convection currents, gravitational pulls from other stellar objects, and other things we don’t yet understand. This fluidity causes the plates of rocky crust that float atop to be slowly jumbled around, not only causing earthquakes and volcanoes, but also occasionally ‘subducting’ the plates back down into the core. When one plate rams up against another, the heavier one is forced below and melts back into liquid, and all of the lovely rocks which constitute it are effectively destroyed.
The hunt for the oldest rocks, then, is the hunt for the regions of the planet which have gone the longest without getting sucked down into the molten core of the planet. Because of how the plates knock up against each other, in the history of Earth it is exceedingly rare for an entire plate to be subducted and hence destroyed. Typically, only the edges of plates are nibbled on by the mantle, while the centers stay high and dry. Hence, “geological shields”, as they are called, are often referred to as continental nuclei. The central shield of a plate is frequently formed of Precambrian (that is, 500 million to 4 billion years old) rock, while more often the edges of a plate are Postcambrian. So, in order to go looking for old rocks, we go looking for geological shields.
The diagram highlights some of the main shields of the Earth, and also makes the useful distinction that shields can frequently be buried below miles of other rocks or water, and so become inaccessible in our quest for the oldest rock. The Archean core areas are the oldest of the old, the Archean being defined as the period from 2.5 billion to 4 billion years ago. Recalling that the Earth is only 4.5 billion years old, rocks from this era are about as old as we can get!
Unfortunately, that’s all we have room for in this edition of SOPxQuest. Be sure to tune in next week as we consider some more ‘out of this world’ geology facts, and finally learn what was the first rock in your country.