Happy Old Rock Day!
Oldest Earth Rocks
Most Earth rocks are youngsters compared to other rocks in the solar system. That’s because Earth is the only planet with active plate tectonics: our crust is constantly being recycled. Earth also has active erosion, which grinds down mountains and deposits sediments that turn into new rocks. And of course, if you happen to live near a volcano, you may have brand new “baby” rocks in your backyard!
There are some parts of the Earth’s crust that are truly ancient though: the continental “shields.” These rocks form the “cores” of the continents and have avoided destruction by plate tectonics. The oldest in-place Earth rock is thought to be from the Acasta Gneiss in the Canadian Shield. Scientists use dating techniques on the zircon crystals in the rock, determining the age of this rock to be about 4.0 billion years. This places the origin of the rock somewhere near the Archean/Hadean boundary, before the emergence of life on Earth. This rock has seen a lot, and if it could talk would probably have a lot of interesting stories to tell!
The Acasta Gneiss may contain the oldest Earth rocks that are still “in place” in the ground, but interestingly, these are not the oldest rocks on Earth, nor are they even the oldest Earth rocks! The oldest known Earth rock was not found on Earth – it was discovered in a sample brought back from the Moon by the Apollo astronauts! Lunar sample 14321 (a.k.a. “Big Bertha”), is a breccia that contains a piece of rock that was blasted off the surface of the Earth by an impact and landed as a meteorite on the Moon. This well-traveled piece of Earth has been dated to be about 4.46 billion years old (only slightly younger than the Earth itself).
Oldest Non-Earth Rocks
Ok, but what’s the oldest non-Earth rock on Earth? That’s not exactly clear: many meteorites and lunar samples have ages of more than 4 billion years, and as we saw with Big Bertha, they are often made of bits and pieces with different ages. Possibly the oldest material on Earth is found in the Murchison meteorite. Tiny silicon carbide grains in the meteorite are thought to be particles of interstellar dust dated to 7 billion years old – 2.5 billion years older than the Sun itself!
If we look beyond what we have on Earth, most planetary bodies aren’t as active as the Earth, and rocks that are billions of years old are common. The oldest rocks in the solar system date back to first stages of solar system formation – before we had the planets as we know them, and even before the Sun had fully formed. These remnants of the formation of our solar system are mostly preserved as asteroids, comets, and dwarf planets in the asteroid belt, Kuiper Belt, and Oort Cloud.
Dating Rocks on other Planetary Bodies
All this discussion of the ages of rocks may have you wondering: how do we know how old rocks are? The best way to measure a rock’s age is to use radiometric dating like Argon-Argon dating or Uranium-Lead dating. Radioactive elements decay into a daughter element or isotope at a predictable rate, and scientists can use that rate and the amount of decay products in the rock to determine how much time has passed since that rock formed. The problem with this is that it can only be done in a laboratory, so we need the sample in-hand. For rocks from out in the solar system, that means either they need to fall to Earth as a meteorite, or they need to be collected and returned by a mission. Sample return missions are especially valuable because they allow the age of the rock to be linked to the location where the sample was collected. In addition to the search for evidence of life, getting radiometric dates for a location on Mars is one of the major motivations for the Perseverance Mars rover mission’s sample collection efforts. Future missions hope to retrieve the samples cached by Perseverance and bring them back to the lab on Earth.
Dating Planetary Surfaces
Understanding the age of planetary surfaces is an important part of planetary science – it gives us a vast amount of information about the formation of the solar system, each planetary body, and how each body has evolved over time. Impact craters are almost everywhere in the solar system, and generally the more craters a surface has, the older it is. We use the lunar surface as a model for other planetary bodies in the solar system because it doesn’t get re-surfaced the way that Earth and other active planetary bodies do, and we have samples from various areas that have been brought back and can be directly age-dated using the radiometric techniques. We can then compare the measured radiometric ages to the number of craters to come up with a relationship between the two. Adding in our best understanding of how the cratering rate varies throughout the solar system, we can build on the lunar model to estimate the age of any planetary surface big enough for us to count the craters!
Do you know how old the rocks are where you live? Celebrate Old Rock Day with us and share your favorite old rocks!