Stone Is Memory: History in the Rocks

There's a thin layer of sedimentary rock found in locales scattered all around the world with high levels of iridium, a metal more often found in meteors than on Earth. It's known as the K-T boundary, and it marks the end of the Cretaceous and the extinction of the dinosaurs. The iridium is debris from the massive Chixculub impactor, a meteor that smashed into the Yucatan peninsula of Mexico 65 million years ago. As terrifying and amazing as the event was, however, it's not what I'm here to talk about today- instead, I'd like to discuss how geologists learn about the past.

The rock layers of the Grand Canyon are clearly visible here. [Image source]

The name geology literally means "study of the Earth". Geologists read the story of the world in rock. It's by no means a simple task- it takes years to learn the languages of stone. The basic principles, however, are fairly simple. Much of it can be accounted for in a few simple rules. These rules are known as Steno's Laws, named after the early geologist Nicholas Steno.

The Law of Superposition

Younger layers of rock sit atop older layers of rock.

The Law of Original Horizontality
Layers of sedimentary rock are always originally deposited in a flat orientation.

Law of Cross-Cutting Relationships
When one geologic feature cuts across another, it is younger than the feature it cuts across.

Law of Lateral Continuity
Rock layers are laterally continuous- that is, they continue along along a flat plane unless interrupted by another feature. Rocks that are apparently similar but divided by a valley or other erosional feature can usually be safely assumed to be from the same original rock layer.

These laws are simple enough in theory, but can often get hellishly complicated in practice. Tectonic activity, for instance, can warp and distort rock layers. The Rock of Gibraltar, notably, was flipped almost entirely upside down by it. Erosion can create bizarre formations known as unconformity, where rock layers are eroded away and then new layers are laid atop them. Often the eroded layers have been uplifted at an angle by tectonic forces, leaving a truly odd pattern in the rocks.

Hutton's Unconformity, the rock outcrop that is often credited for helping kick off the science of geology. [Image source]

Beyond the organization of the rock layers, the composition and structures of the layers themselves have much to tell us. A limestone layer, for instance, is formed by the carbonaceous remains of corals, molluscs, and other shelled sea life. Chalk is entirely made of fossilized aquatic shelled microorganisms. Sandstone is deposited in relatively shallow marine environments, while siltstones and other mudstones are often deposited in deeper marine environments, or alternatively in lacustrian (lakebed) environments. Basalt occurs in free-flowing layers, and often intrudes into other rock layers. There are dozens of common types of rocks, and hundreds of less common rock types, and they all tell the story of their environment at formation, as well as forces exerted on the rocks through their lifespan.

The stratigraphic column of the Grand Canyon.[Image source]

Even dating rocks is an immensely complex task. It's done in a huge variety of ways (a dozen or so at least), but it can generally be divided into two types- exact and relative dating. Exact dating uses various chemical methods to perform its task, generally measuring radioactive decay. Extremely young sedimentary rocks can occasionally be dated via carbon dating organic deposits trapped within, but other methods of dating are often used. Radiometric dating is extremely reliable over great ages.

Relative dating compares the ages of rock layers to one another. Fossil dating is one of the most heavily used methods, and tracks the age of rocks in comparison to others by knowing when a species was present on the earth in relation to other species- if two different species show up in the same rock, geologists can pin the age of the rock down to a time when both species were on the planet together. Index fossils are best for this task- they're species that had relatively short reigns, preferably of a million years or less, but nonetheless had high levels of success, with high populations around the planet or continent.

Chemical analysis of rock composition, thickness of rock layers, the range of different grain sizes in a rock, the roundedness of rock grains, the alignment of larger grains in the rock- these all tell geologists important information about the time and place in which the rock was deposited. The warping of metamorphic rocks tell us about the internal movements of the Earth. And learning even just a little bit about it will completely change the way you look at the world around you.

In a very literal sense, all of the above is just the tip of the iceberg in geology. I've only just touched on the duties and tasks geology has been appointed. We study the history of the entire physical world across four and a half billion years. We'll never run out of things to learn, either- the world is still living and changing.

---

Bibliography:

https://en.wikipedia.org/wiki/Nicolas_Steno#Geology_and_stratigraphy

https://en.wikipedia.org/wiki/Geology

https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_boundary

Me. I don't know how to cite myself in a bibliography properly, but I'm doing it anyways.