A Large Impact on Earth’s Geography

Title: On a Possible Giant Impact Origin for the Colorado Plateau

Author: Xiaolei Zhang

Lead Author’s Institution: Department of Physics and Astronomy, George Mason University

Status: Submitted to Earth and Planetary Science Letters [open access]

Author’s note: The discussion in this astrobite was written from the point of view that the hypothesis in the paper is correct – which of course remains to be seen by future studies.

Science requires collaboration, and often that collaboration leads to projects that span across different fields of science. This paper caught my eye mainly because the Colorado Plateau is one of my favorite places to travel, and though I’m not a geologist, its unique and beautiful rock formations have always piqued my interest. As I read this paper, I was surprised to find that this author also was not a geologist by trade. Xiaolei Zhang is an astrophysicist, but decided to carry on her late father’s unfinished project to uncover the formation of the Colorado Plateau and the possibility of giant-impact-plate tectonic motion.

By looking at planetary geology through an astronomer’s lens, it is clear that though discoveries of exo-solar planets are exciting, sometimes discoveries about humanity’s first known planet can be just as intriguing. There are still many things to learn about our home planet Earth. For example, geographical features on Earth can tell us about its astronomical history— its track record with meteors and other space debris.

Today’s paper discusses evidence Earth collided with a Mars-sized object roughly 750 Million years ago to form today’s Colorado Plateau. To put this in perspective, the impact that contributed to the mass extinction of dinosaurs was likely caused by an asteroid only about 10 km across.

This shows a bird’s eye view of the Colorado Plateau region, which could have been formed from a glancing impact with a Mars-sized planet.

The Colorado Plateau is the region around the Four Corners region of the southwestern United States. It is about 337,000 square kilometers and consists mostly of a high-desert plateau of a shallow bowl shape. The Plateau is a surprisingly stable region; the entire area does not have fault lines or features that indicate strong plate movement, which is a bit unusual given that its surroundings have these features. Sedimentary record shows that the entire North America continent, including the Colorado Plateau region, was very flat in early times, but now it has many mountains and sharp geographic features surrounding the area. These geographical peculiarities point towards an impact with an astronomical object significantly different from Earth forming this plateau.

The features discussed above indicate that this region was very likely formed by an impact, and since we are able to know the age of some of the features originating from the impact, we know at what time this collision happened. Zhang discusses that this impact likely happened 750 million years ago.

At this point in time, out solar system would have been very stable in its structure, so if an impact occurred, where could this object have come from? Zhang suggests the impacting object came from a spiral density wave as the solar system orbited the Milky Way; as the solar system orbited the Milky Way, it could have passed through a spiral arm of the galaxy. If this happened, a free-floating Mars exoplanet could have been drawn into the solar system and then crashed into Earth.

Zhang discusses multiple geographical features that provide evidence for the giant-impact theory about the Colorado Plateau. She also provides quantitative arguments to support the claim of the Mars impact. To review, these arguments include

  • Despite much igneous and orogenic activity at its boundaries, the plateau has remained structurally stable.
  • The type of rock throughout this area indicate evidence of shock metamorphism.
  • The basalt throughout the lowest level of rock also shows shock geographic formation. These features can also be dated around the time of the theorized impact.

This rare paper combines both geology and astrophysics spectacularly. By using the geological evidence and providing an astronomical theory, Zhang shows in a unique way how our planet got some of its features. Using interdisciplinary studies like this, we could learn much more about other planets within our solar system and newly-found exoplanets.

Zhang presents a quantifiable theory of a giant impact formation of the Colorado Plateau; though others have qualitatively mentioned this theory, this paper is the first to give a quantifiable estimate of size based on collisional dynamics. Zhang successfully completed this geological project by approaching it from her astrophysicist point of view, which is an incredibly important part of interdisciplinary science.  To me, this project exemplifies the collaborative nature of scientific work; not only does it span across different studies, it spans beyond the lifetime of just one person.

About Mara Zimmerman

Mara is working on her PhD in Astronomy at the University of Wyoming. She has done research with Heartbeat binary stars, and currently works on modeling the composition of debris disks. When not at her computer writing code, she likes hiking, baking scones, and making puns.


  1. I’m a little disappointed by how un-critically this paper was read. Zhang is not a geologist, and in this case that’s a major liability, as she ignores decades of geological knowledge about how things like mountain ranges form. The confidence with which she characterizes rock types, again without consulting geologists, is extremely concerning. That would be like me, as an astrophysicist, looking at the internal anatomy of a bird and trying to guess its evolutionary history *without talking to a biologist*. And on top of the likely incorrect geological suppositions, Zhang even gets her *astrophysics* horribly wrong. It would take as many pages as the paper itself to fully document all the errors, but here a few major ones: a) it is unlikely in the extreme for a rogue planet to collide with Earth simply given the mean free path of small rocks in the Milky Way mid-plane, b) Zhang’s impact velocities are unphysical for her proposed scenario, c) Zhang’s speculation on the fate of the impactor is unphysical and inconsistent with her proposed origin (again the velocities are wrong), d) egregious misuse of crater scaling relations (if it gives you an unphysical answer that’s telling you something about its suitability), and e) an impact like this would indeed resurface the planet with a lava ocean and *unquestionably* extinguish all life, at a time when we know biodiversity was increasing exponentially.

    If anything, this paper is a great example of interdisciplinary science done incorrectly and arrogantly with disregard for the contributions of experts in relevant fields. And definitely an argument for not posting papers on arxiv as a single author without having other scientists critique the paper first. Zhang really should have included others in this work; they would have made a big difference.

  2. Such an impact should have far reaching effects – atmospheric, oceanic and identifiable debris outside of impact area. Not easy for me to accept premise without examination of these variables.

  3. Mars has a diameter of ~6800 km. The Colorado Plateau has an area of ~386000 km^2, which corresponds to a circular patch of ~1400 km diameter. This is far less than Mars-sized. Then throw in a ratio of ~20:1 for crater:impactor diameters.
    Mars-sized becomes even more unlikely.

  4. 20 to 1 ratio of impactors to crater diameters is correct for meteors traveling 30 to 40 thousand mile an hour – essentially asteoids. Must assume this impactor is in earth similar orbit with similar speed.

  5. C-mon! Mars-sized object would shatter half of our planet and likely create another moon. If you say 1000 km object, i would think “hmmm, very unlikely”, 100 km object, yeah, maybe, possibly… But even that would be already very obvious in global geological record, would it?

  6. A Mars-sized exoplanet impactor? Well, if you are pushing dubious ideas, you might as well go for broke.

  7. The paper clarifies (as this summary should have!) that the proposed impact was a glancing blow.

    However…the velocities specified for the impactor, both before and after, are vastly less than the Solar System’s escape velocity (by a factor of two or three). This seems entirely inconsistent with the idea of a “rogue exoplanet,” i.e., one that fell into Sol’s potential from afar. I certainly see no possible way it could have gotten back out again, at least not without a ludicrous coincidence (i.e., passing close enough to one or more other planets to get a gravitational assist, or falling into the Sun without passing through and without triggering other solar changes).

    The geometry of the glancing blow in Figure 6 (16 km depth) also seems unphysical to me, implying a seriously large “bounce” effect; based on simulations I’ve seen, planets are more like jelly blobs when they interact. Even without direct touching, there appears to be no mention of tidal effects at all. (Caveat: I haven’t read the whole paper, but a search for the word “tidal” turns up nothing.) I don’t have a lot of intuition for this, but if you have two planetary bodies in extreme proximity, even for just a few seconds, I’d naively expect a huge amount of crustal stress, compounded with massive oceanic inundation, and maybe even sufficient torque to tweak Earth’s axial tilt.

    Finally, the abstract speaks of the correlation between mass extinctions and Earth’s passage through the spiral arms (density waves), which I understood to have been debunked on statistical grounds by now.


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