Any geologist worth their weight in kimberlite cannot resist the temptation to take landscape photos as much for their beauty as for their utility providing insight into our Earth's past. Most people don't understand how geologists can infer from a picture historical events that extend perhaps eons into the past. Drawing from my own travelogue of "nerdy geology pictures," I will demonstrate the thought process of a trained geologist. If you're not into using the geologist's prism to unweave the stratigraphic rainbow, just scroll to each photo—I think most of them are pretty, no matter who you are.
Let's start with volcanoes, everyone's favorite natural agent of destruction. This first shot comes from the infamous Crater Lake in southern Oregon. We infer that the "crater" was caused by a volcano as opposed to meteor impact because of the lake's position above the main land surface rather than as a depression in an otherwise flat landscape. This isn't a particularly hard one for a geologist to figure out: the rim of the crater is almost completely circular, and if you imagined the volcano's shape before its top blew, you might conjure up with something akin to Japan's Mt. Fuji. This shape is indicative of stratovolcanoes, which erupt in alternating explosive and non-explosive episodes. The fact that so much material (i.e. rock) is missing from the crater either means that the ancient volcano (called Mount Mazama) blew it's top (like Mt. St. Helens in 1980) or collapsed in on itself after an eruption as a result of the vacated magma chamber. All this we can tell just from looking at the landscape. Further geological examination of the rocks indicates that the latter is true- Mount Mazama did have episodes of explosive eruptions, but the formation of Crater Lake was caused by the collapse of a magma chamber. Wizard Island in the center of the lake was formed from eruptions after the initial collapse.
Let's continue on the theme of volcanoes, but take a broader view with this next photo. Crater Lake and Mount St. Helens are part of the Cascades Range, a mountain chain that parallels the Pacific Northwest coast. This photo was taken from my airplane descending into the Portland, OR airport. The mountain in the foreground is the Portland's iconic Mt. Hood. This photo looks south, and we can also see Mt. Jefferson and two of the three sisters at increasing distances in the background. When a geologist notices these regular, linear mountain chains on a coastline, they can immediately conclude that the mountains are a result of a certain type of plate tectonics. Most people have an idea that plate tectonics is a bit like crackers sliding around on Jell-O (or some other food analogy you learned in 5th grade), and in this case, one plate is Melba Toast and the other is a Saltine: the Melba Toast is much more dense and when it collides with the Saltine, it ends up underneath. Here, the Melba Toast is the Juan de Fuca plate, moving eastward into the North American plate just offshore the Washington/Oregon/California coastline. Once the Juan de Fuca plate has descended into the hot mantle, it begins to melt and the resulting magma rises through the crust, supplying the source of the Cascades range volcanoes. Because the descending plate melts at the same distance inland over the length of the plate boundary, the resulting mountains form a linear chain. A very similar process is occurring to form the Andes in South America. There are other distinctive styles of mountain building that geologists recognize (Saltine vs. Saltine- the Himalayas, "hotspots"- Hawaii, etc) but we'll stop there for now.
On a recent trip to Sedona, Arizona, I was mesmerized by the dramatic red rock formations. The most interesting feature to me though, was where the red transitions almost instantaneously into tan. This photo shows the ubiquitous pattern around the Sedona area: red rocks on the bottom and tan rocks on top. This problem was more reluctant to relinquish its secrets. Red coloring like this usually indicates the presence of oxidized iron, called "rust" by non-geologists. The most common source of the rust is the mineral hematite (from the Greek haima meaning "blood," the same root as hemoglobin) which can occur only when there is enough oxygen in the atmosphere at the time the mineral forms. It takes a lot of hematite to make this deep red color. Therefore, a geologist might look at a cliff like this and hypothesize about a dramatic environmental or climatic change taking place around the time the rocks changed from red to tan. It turns out that the border between red and tan in this picture is right around the boundary between the Permian and Triassic periods in geologic history, around 255 million years ago. Geologists and paleontologists know of something very dramatic that happened around this time: the Permian mass extinction. This is the most destructive extinction known in earth's history. It wiped out over 95% of marine species and 70% of terrestrial animals. The exact cause of the extinction is not known, but the pattern of the extinction (more marine than terrestrial animals died, among other things) is indicative of hypoxic (lack of oxygen) conditions in the ocean, where most sediments are deposited. This is consistent with the disappearance of hematite, which requires oxygen to form! Other exciting things were happening at this time period, including the final formation of Pangaea, as well as large amounts of volcanic eruptions, both of which can cause extreme changes in the earth's atmosphere.
Another example of the "red color = oxidized iron" rule comes from the Dry Valleys in Antarctica. I spent three months there measuring glacier melt, and while on continent, I was lucky enough to see this curious feature known to researchers as Blood Falls. The picture here shows the terminus of the Taylor Glacier and Lake Bonney in the foreground. The glacier cliff face is about 10 meters high. Here again, the red color indicates the presence of oxidized iron flowing out from the glacier, but this time dissolved in water rather than in the mineral hematite. The conclusions of a geologist unfamiliar with the area would have to end here, and mine did initially. After some investigation I found out that the reservoir of iron-enriched water inside the glacier is actually left over from the last glacial maximum. At that time, the glacier reached the ocean and incorporated pockets of seawater into its structure. The ocean is relatively enriched in dissolved iron, and now we observe it melting out when the glacier retreats.
On a hike in New Zealand, I came across a great overlook of the picturesque Dart River valley. In geomorphic terms this is a called braided river, for obvious reasons. Braided rivers are geologically on the other end of the spectrum from meandering rivers such as the Mississippi. When a geologist sees a braided river, they immediately know two things: 1) there is a high supply of sediment (and thus high erosion) in the watershed of this river and 2) the slope of the river is relatively high, falling many more feet per mile than a meandering river. Braided rivers usually have many active channels, and the river frequently changes course due to sediment supply clogging one channel or another. Meandering rivers are confined to one channel, and have well developed sinuous curves that change relatively infrequently. In this particular situation, a trained geologist will also recognize that the channels on the left side of the photo are "older" than the rest- the river has not flowed there in a while allowing vegetation to grow on the islands.
So, the next time you gaze over a picturesque landscape, don't forget about the geologists' toolbox. Unlike John Keats, whose rainbow was no longer beautiful after it was explained, I believe that trying to understand the natural world only adds to its exquisiteness.
