Earth’s continents as they appeared 750 million years ago. Courtesy The Planetary Habitability Laboratory @ UPR Arecibo, NASA, Ron Blakey /Colorado Plateau Geosystems, Inc./The PaleoMap Project. Click to embiggen.
We talk a lot about climate change these days. It’s clear that human activity has played a role, but our planet also changes naturally on very long time scales. With that in mind, it’s always worth looking at how our planet looked in the past.
What Did Earth Look like Hundreds of Millions of Years Ago?
I found a great site that has videos and image collections that help you understand the large-scale surface changes Earth has experienced in its past. The site is called the Visible Paleo Earth and you can explore it at the Planetary Habitability Lab. It’s a creation of the University of Puerto Rico at Arecibo. They have created a bunch of different visualizations that also link our planet and its features to other exoplanets.
For example, here’s a set of images showing what Earth looked like nearly three-quarters of a billion years ago, about the time complex life forms were starting to migrate across the planet. The continents began as large landmasses called supercontinents that eventually broke apart and reformed. You may have heard of Pangaea, which was a supercontinent in the recent geologic past. There were others before that, such as Ur, Valbaara, Rodinia, and Laurentia. In the future, the continents, which ride along on the tectonic plates, will form a new supercontinent that has been nicknamed Ultima Pangaea. The charts on the PLH’s site show the migration of continents in the past.
Also check out their YouTube channel, where you can see this video showing the major life extinctions our world experienced.
I really like this site — it looks at Earth as a planet, from the viewpoint that we look at other planets and exoplanets. Take some time to browse through — lots of cool educational info there!
In the center left of Pluto’s vast heart-shaped feature – informally named “Tombaugh Regio” – lies a vast, craterless plain that appears to be no more than 100 million years old, and is possibly still being shaped by geologic processes. This frozen region is north of Pluto’s icy mountains and has been informally named Sputnik Planum (Sputnik Plain), after Earth’s first artificial satellite. The surface appears to be divided into irregularly-shaped segments that are ringed by narrow troughs. Features that appear to be groups of mounds and fields of small pits are also visible. The blocky appearance of some features is due to compression of the image. NASA/JHUAPL/SWRI
It’s great news that Pluto is geologically active, based on the fantastic images from New Horizons. I was hoping it would be, but the level of activity just simply implied in these images is amazing. The New Horizons team has its collective hands full with the flood of data, and the ultimate story of Pluto’s activity will probably be even more complex and cool than we think right now.
What Does it Mean to be Geologically Active?
When we talk about “geologically active” as it relates to Earth, we know what that means: mountain-building processes, volcanic flows, earthquakes, canyon-creating processes, plate motions, erosion (by wind and water), and so on. These are processes that geologists study. They also use an understanding of chemistry and physics to explain the complex details of how rocks form, and interact with each other and the atmosphere.
I studied geology for several semesters when I was in school, and one learns quickly that it is the basis for understanding how our planet’s surface has changed over the billions of years since it formed. Here are just a few examples of what I mean:
the Rocky Mountains (where I live), formed hundreds of millions of years ago, pushed up by the action of tectonic plates sliding under the North American plate — which carried much of the North American continent. Before these mountains formed, the area was covered with an ocean which deposited many layers of sandstone, limestone, and shale over what is called Precambrian bedrock. When the tectonic plates began their action, they forced the bedrock up through the layers, creating the jagged mountains we see today.
Tectonic plate motions also spur volcanic activity in the Pacific Northwest of North America, where plates subduct (dive under) others or spread apart from each other. Tectonic plate motions also cause the earthquakes that countries around the Pacific Rim experience each day.
The Hawaiian Island chain was built by volcanoes that formed as a result of plate motions over a hotspot (or a plume) in Earth’s mantle (the layer below the surface). As the plate moves, the spot creates new volcanoes in a sort of “arc” across the mid-Pacific.
In addition to those activities, there are other spreading zones — the most prominent being in the middle of the Atlantic. There the spreading zone splits the crust apart, which allows the upwelling of new mantle material to the surface. In this case, they’re under the Atlantic Ocean. The action is pushing Europe and Africa apart from North and South America.
Among other things, geology examines the rocks that are “built” through these processes, and the surface formations that are created. By looking at rocks and landscapes, geologists can get a good idea of what happened to create the various surface units we see on Earth — from continents and mountains to deep-sea canyons and impact craters.
How Does Geology Help us Understand Other Planets?
There’s not room here to go into all the many details of how geology helps us understand other worlds. It’s enough to know at this point that the same principles of geological processes that help us understand Earth’s physical history also explain features we see on other worlds: volcanoes on Venus, Mars, and Jupiter’s moon Io, for example. There’s very clear evidence of volcanism on our own Moon, as well as the planet Mercury. Tectonic motions of rock most certainly helped form the giant Valles Marineris canyon on Mars.
But, geological principles don’t just apply to rocky worlds. They can be applied to icy worlds, as well. For example, the concept of “cryovolcanism” is relatively new, but perfectly explains the plumes of material we see on the Neptunian moon Triton, as well as plumes emanating from Enceladus at Saturn and what look like flow features on other icy moons. In the outer solar system, ice acts as the “lava” that flows from volcanoes driven by internal action on the icy moons.
So, when planetary scientists talk about “geologically active” at Pluto, they are referring to some kind of activity being driven from within that is affecting and changing the surface of Pluto (and probably Charon, too). As on Earth and the other worlds with “geological activity”, you need some kind of heat to drive the processes of volcanism and tectonism. Pluto clearly has had its surface “repaved” in places. Some physical process inside the planet is driving that action. We’re not sure what it is, but the evidence is laid out there in ice before us. I expect that we’ll hear about cryovolcanism on Pluto once more images and data come down from the spacecraft. I hope we’ll learn that there’s a heat source in the planet. It could be driven by the decay of radioactive materials that provides heat. Or it could be something else.
Whatever it is, Pluto has experienced mountain-building processes (just look at the mountains in the video below!) and what looks like volcanism (albeit with ice as the “repaving material”). This little world promises a fascinating time of discovery for all of us, and I’ve no doubt the New Horizons scientists will be delivering surprises for us for years!
