Posts Tagged ‘geology’

Once Molehill, Now Mountain

For most people, mountains are part of the landscape. We don’t think about them except when we’re directly interacting with them, and even then, we don’t always ask ourselves where they come from. That’s a shame, because there’s some really interesting geology in mountains, and I’m not talking about the types of rocks, either. The processes that create mountains are fascinating. Molten rock rising through the crust? Continental plates hitting each other and forcing the crust into folds and waves? What’s not to love?

White Horse Bluff, which erupted under a lake

Volcanoes occur any time a rupture in the Earth’s crust allows magma to rise to the surface. This magma, or rather lava because it’s exposed to air (or water), can form all kinds of protrusions from the crust, depending on the composition and fluidity of the lava and the surrounding environment. A volcano that erupts to open air is different from one erupting underwater, which is different again from volcanoes that come up under a glacier. Submarine volcanoes often occur around ocean ridges, where there is tectonic activity. Land volcanoes often occur near tectonic plates as well, but they’re also formed by hotspots or weak points in the crust. (Hotspots are particularly neat because multiple volcanoes can form from a single one as the plates move across it.)

As for the shapes of land volcanoes … boy. Volcanoes are frequently complex and boast any number of features, but the main types are stratovolcanoes, cinder cones, and shields. Stratovolcanoes are what most people think of when they hear ‘volcano’. They’re conical, with lava, rock, and ash spewing out of the top in all directions, and build up over multiple eruptions. Mount Etna is a stratovolcano. So are Mount Fuji, Mount St. Helens, Mount Vesuvius, and Eyjafjallajökull. They occur at subduction zones, where one plate of the crust is moving underneath another, and tend to be rather explosive.

Parícutin, a cinder cone

Cinder cones are similar to stratovolcanoes, except that instead of lava they spew pyroclasts (also called tephra, also called rocks and ash). They can appear in isolation, as with Parícutin, or as part of another type of volcano when there are multiple vents.

Shield volcanoes, on the other hand, are made of flowing lava, and tend to be wider and flatter than other volcanoes as a result. They’ll also erupt for longer periods. The Hawaiian volcanoes are excellent examples of this type, though shield volcanoes pop up worldwide. They’re frequently the result of hotspots, but can occur at rifts and subduction zones as well.

Any and all off these can also be worn down by erosion, so that only the hardened lava from the vent or intrusions remains, or a shield volcano gets split into multiple mountains. It also bears mentioning that volcanoes are complex and that assuming a world only has the three classic types with no variation is erroneous. On Earth alone, you’ll find craters, calderas, and volcanoes inside calderas. Some volcanoes have fissures, linear vents that ooze more than they explode, and which yield a lot of the spectacular photos from Hawaii. And of course, a stratovolcano may sprout cinder cones or a small shield volcano, or a caldera may yield a lava dome, or, or, or.

The other major method of creating mountains, without any messy flying or liquid rocks, is by smashing tectonic plates into each other. This is how the bulk of Earth’s mountain ranges are formed, and why we find seashells and fossilized ocean creatures on top of mountains. When two plates meet and form a subduction zone, the top plate can be wrinkled by the force of the subduction, producing fold mountains such as the Rockies. Faults can also create fault-block mountains by raising a chuck of bedrock above another.

Of course, thanks to NASA and its probes, we now know a fair bit about the geology of the other planets and moons in our solar system. Starting close to the sun…

Pancake domes on Venus

Venus doesn’t have plate tectonics, but that doesn’t mean it has no mountains. Eighty percent of its surface is covered by lava plains and volcanoes. Venus boasts a large number of shield volcanoes, as well as coronae, scalloped margin domes, arachnoids, and pancake domes, features not found on Earth. (The only other spot we’ve found coronae is Uranus’ moon Miranda.) Venus is still volcanically active and its shield volcanoes tend to have far greater diameters than those on Earth.

The Moon’s maria are basalt planes formed by lava flowing into impact craters. Some of these maria also yield shield volcanoes and domes. The Moon also lacks tectonics, but this hasn’t stopped it from having mountain ranges. It’s also believed that the far side of the Moon was volcanically active longer than the near side.

Pretty much every volcano nut has heard about Mars’ Olympus Mons, the highest mountain in the solar system at 27 km. Pretty much every conspiracy buff has heard of the Face on Mars, which turned out to be a butte rather than an alien sculpture. It should surprise no one to learn that these aren’t the only mountainous features on the planet, or that Mars also seems to lack tectonic plates. Mars’s volcanoes likely come from hotspots, as the Moon’s do, and while they don’t erupt as often as the volcanoes on Earth, they’re of a much larger scale when they do finally happen, since lower gravity means magma bodies must be bigger if they’re going to reach the surface before cooling. Mars has several major volcanic regions, such as Tharsis where Olympus Mons is located, which boast giant volcanoes along with smaller ones called tholi. Tharsis also contains Alba Mons, another volcano with no Earth counterpart (it has an immense area but isn’t very high). Mars also has a number of volcanic plains, and may still be volcanically active—which may increase our chances of finding life there.

Due to tidal friction from Jupiter, the moon Io is the most volcanically active spot in the solar system. Its volcanoes eject a lot of sulphur and sulphurous compounds, along with the igneous rocks one expects. However, volcanic mountains are rare on Io, and most of Io’s mountains are formed by thrust faulting resulting from crust compression. What volcanic mountains there are, are shields.

Saturn’s Titan, the moon of methane lakes and NASA buzz, has both what may be cryovolcanoes spewing water, methane, and ammonia, and an icy mountain range in the southern hemisphere. This range may be caused by tectonic plates shifted after a meteor impact. Since we’re still studying Titan, though, and having a hard time seeing through its atmosphere at times, it’s uncertain whether the volcanoes are actually volcanoes. Further study is obviously needed.

The other major gas giant moons have many interesting and unexplained surface features as well as tectonic activity, but since none of them seem to actually have mountains, I’m skipping them here.

Any planet your characters are on, science fictional or fantastic, is likely to have mountains, volcanoes, plate tectonics, and the like. Earth provides a good model for just about any geographic feature you might want, but as we’ve seen with Venus and Mars, there are types of mountains and volcanoes that are only possible under different conditions. Higher or lower gravity, ratios of elements within the lava, the pull of a star or a planet, and plate tectonics will all come into play—and if there aren’t plates, there won’t be analogues to the Rockies, Alps, or Himalayas, which arise at plate boundaries.

  • We’ve seen the devastation and chaos a volcano like Vesuvius, Mount St. Helens, or Eyjafjallajökull causes. What happens when a supervolcano such as the one in Yellowstone blows? It’s going to take out a fair chunk of the continent.
  • Would it eventually be possible to sculpt mountains out of lava? To create islands on a water world from the planet’s own mantel?
  • If a planet’s mountains are made of different substances than Earth’s are, how will that effect what grows on them, or erosion?
  • Earth cultures near volcanoes tend to have volcano gods, and some cultures posit gods living on mountains. What might change if the mountains are different?

The Science of Red Mars

Mars. There’s something about the Red Planet that gets people excited. Sure, part of it may be that it is the most Earth-like planet in the solar system (except for Earth…) but even more powerful are the stories that are told about it. Mars has been the subject of myths since before recorded history, and more recently has been the setting or inspiration for reams of science fiction.

For those of us who like a little science in our fiction, the good news is that in the last decade our understanding of the Red Planet has grown by leaps and bounds thanks to a whole armada of space probes: Mars Global Surveyor, Mars Odyssey, Mars Pathfinder, Mars Express, the two Mars Exploration Rovers, Phoenix, and Mars Reconnaissance Orbiter.

All of these missions came after the publication of the classic hard sci-fi masterpiece “Red Mars” by Kim Stanley Robinson, so I thought it would be interesting to take a look at how Red Mars holds up.

The short answer is: pretty well! Kim Stanley Robinson did a phenomenal amount of research for the book, and the Mars that he describes is still remarkably accurate. The book is a tome, so I can’t go through and critique every bit of Mars science in it, so I will focus on a few key sections.

First, let’s look at the beginning of Part Three: The Crucible. (If you’d like to follow along, a PDF version of the book is available here, legally, for free) This section begins with a description of how Mars formed, and how it acquired its geography (areography?). The language takes some poetic license, but is generally accurate: Mars did indeed form along with the rest of the solar system about 4.5 billion years ago (I’ll forgive Robinson for rounding up to 5) from the gradual accretion of planetesimals, and it did have a short-lived magnetic field.

What Robinson didn’t know was that the evidence of that magnetic field is still preserved in the ancient rocks of the southern hemisphere! The magnetization was obliterated by the giant impacts Hellas and Argyre, but elsewhere in the southern highlands there are broad bands of opposing magnetic fields. These are similar to the alternating bands of magnetization preserved near tectonic spreading centers on the earth caused by the switching of the Earth’s magnetic field, but on a much larger scale. Some scientists have used this similarity to argue in favor of plate tectonics on early Mars, but there’s not a lot of other evidence for plate tectonics so this hypothesis isn’t very popular these days.

Robinson is also correct that the huge Tharsis bulge and its towering volcanoes probably are caused by convection in the mantle, and that a leading theory for the formation of the northern lowlands is that they are a single gigantic impact basin. This giant impact theory has been making a comeback lately. By using careful measurements of the martian gravity, scientists have been able to figure out how thick the crust is, and effectively “subtract” the Tharsis bulge. What’s left in the northern hemisphere is a gargantuan elliptical crater, 10,000 km by 8,500 km.

Oh how Robinson must wish he had this topographic map of Mars when writing! Blue is low, red and white are high. Tharsis is the huge high elevation area dotted with volcanoes west of the giant canyons of Valles Marineris.

The idea of Mars’ moons Phobos and Deimos as ejecta  from a similar giant impact is still alive and kicking. Mars Express has found evidence of clay minerals on Phobos, which only form when water is available. This, combined with the moon’s low density suggest that it is probably a piece of the ancient martian crust rather than a captured asteroid.

The intro to Part Three is also pretty accurate in its description of water on Mars. We now know for sure that there is water on Mars, and that there is likely a lot of ice beneath the surface. It’s not necessarily all in the form of pure-ice lenses like Robinson describes, but it’s there, and there seem to be lenses at least in some places, like the Phoenix landing site.

Finally, he’s spot on in his description of life on Mars (or the lack thereof) at the end of the intro to Part Three. Even the references to sulfur and clays and hot springs are accurate! Mars Express and Mars Reconnaissance Orbiter have both found evidence of clays and sulfates in the ancient rocks all over mars, and the Spirit rover has dug up sulfate- and silica- rich soil interpreted as the result of hydrothermal activity. But so far, no evidence for life. All the claims of life in Martian meteorites have been met with plausible inorganic explanations, and Phoenix has discovered perchlorates in the martian soil – powerful oxidizers that help explain the confusing results of the Viking landers’ life detection experiments.

Let’s fast forward a bit to page 131 of the pdf. Some of the colonists are on a road trip to the north polar cap to set up some automated ice-mining. Setting aside the implausibility of being able to drive halfway across the planet without any trouble, there is a statement made in passing here that really does reflect a fundamental change in our understanding of Mars.

“The rocks you see here come from late meteor action. The total accumulation of loose rock from meteor strikes is much greater than what we can see, that’s what gardened regolith is. And the regolith is a kilometer deep.”

This is an idea that was carried over to Mars by scientists who cut their teeth studying the moon. It’s certainly true for much of the moon, where there are few geologic processes other than volcanism and impacts. But one of the most significant things that has been learned about Mars since Red Mars was written is that it is not just a big red version of the moon. With the first really high resolution images of Mars returned by the Mars Orbital Camera on Mars Global Surveyor, and the spectacular even-higher-resolution images that continue to stream down from HiRISE on MRO, we can start to see Mars as it really is. Yes, there are lots of craters, and the regolith is made of ejecta from eons of impacts, but it’s not ejecta all the way down like it is on the moon. Mars has sedimentary rocks, and lots of them. Volcanic ash lofted thousands of kilometers by the martian atmosphere and deposited in blanketing layers, sand seas cemented in place by hydrated minerals, sediment settling through the icy waters of short-lived lakes. All of these combine to form thick sequences of layered rocks. Sure, there are also craters interbedded with the other layers, but cratering is just one of many processes at work on Mars.

An illustration of the idea of the martian crust as a "cratered volume" rather than just a bunch of megabreccia. Here, craters are interbedded with various layered rocks, going down many kilometers. (image from Malin and Edgett, 2001)

Even more amazing is how much the landscape has changed. There is evidence that huge swaths of the planet were once buried and have been exhumed by billions of years of erosion. In Gale Crater, one of the potential landing sites for the next Mars rover, there is a 6km high mountain of sedimentary rock that is taller than the rim of the crater it sits in. How far did those rocks once extend? In some places, erosion has partially uncovered craters that look for all the world like they should be freshly formed. Except that they are half buried under a billion years of rock.

It is difficult, even for those of us who study Mars, to look at the surface as it is now and remember that we are just seeing the upper layer of an intricate intermingling of processes, and that kilometers of rock may have been stripped away from above the present surface. It’s amazing what four billion years of erosion can do to a planet. Even though Robinson isn’t quite right when he writes that the regolith goes down a kilometer, he was right about the most fundamental difference between Earth and Mars:

“It’s billions of years. That’s the difference between here and Earth, the age of the land goes from millions of years to billions. It’s such a big difference it’s hard to imagine.”

Later in the same section of the book, Robinson reveals another case where the information available when he was writing pales compared to what we have now:

“Ann was crouching, a scoop of sand in her palm.

“What’s it made of?” Nadia asked.

“Dark solid mineral particles.”

Nadia snorted. “I could have told you that.”

“Not before we got here you couldn’t. It might have been fines aggregated with salts. But it’s bits of rock instead.”

“Why so dark?”

“Volcanic. On Earth sand is mostly quartz, you see, because there’s a lot of granite there. But Mars doesn’t have much granite. These grains are probably volcanic silicates. Obsidian, flint, some garnet. Beautiful, isn’t it?”

I thought this was quite telling because these days we have a very good idea of what the sand on Mars is made of, thanks to the spectrometers on Mars Express and MRO, and the results of the MER rovers. The nice thing about sand dunes is that they are self-cleaning: the saltation of the sand grains knocks the dust off of them, so the dunes on Mars often have nice clean spectra revealing a basaltic composition, with lots of minerals containing iron and magnesium. Robinson shows his earthly biases when he lists things like obsidian, flint and garnet – these all have much higher silica and aluminum than basalts, a good sign that the rock containing them has been recycled by plate tectonics. On Mars, just about everything is basaltic, or the result of weathering basalt. The same spectrometers that found the composition of sand on Mars have also found hundreds of examples of minerals formed by the interaction of water with basaltic rocks, but there are not many places where more processed, high-Si and high-Al rocks are found.

Jumping forward again to page 241 of the pdf, there are some interesting comments about glaciers and oceans on Mars:

“The glacial theory, however, and the oceanic model of which it was part, had always been more persistent than most. First, because almost every model of the planet’s formation indicated that there should have been a lot of water outgassing, and it had to have gone somewhere. And second, John thought, because there were a lot of people who would be comforted if the oceanic model were true; they would feel less uneasy about the morality of terraforming.”

We aren’t too concerned about the morality of terraforming yet, but the rest of this excerpt is pretty spot-on. The ocean hypothesis for Mars still lives on, partially because of new discoveries, but partially because people want it to be true. One interesting recent result took a look at all the features on Mars that might be deltas formed by rivers dumping their sediment into a standing body of water. Many of these are in craters, so you only need to invoke enough water to fill the crater, but there are some that empty into the northern lowlands, and they are all at about the same elevation. Likewise, the density of drainage channels on Mars has been mapped and it shows a similar cutoff with elevation. Are these results evidence of a northern ocean? Maybe, maybe not.

A map of the density of drainage channels on Mars, in relation to a hypothetical northern ocean. Image credit: Wei Luo/Northern Illinois University/PA Wire

As for glaciers, this is another area that has made a lot of progress since Red mars was published. Mars Express and MRO both carry ground-penetrating radar instruments designed to look for ice on the surface of Mars. These have given some spectacular cross-section views of the polar caps, but maybe even more important is the discovery that there are huge lobate masses of ice – some might call them glaciers – buried under thin layers of rocky debris on Mars. I’m a little surprised that Robinson did not mention these because they had been seen even before MOC and HiRISE sent back high-resolution photos. The radar results just confirmed that these lobate features were mostly ice rather than mostly rock.

A map of glacial ice (blue) discovered in the mid-latitudes of mars by ground-penetrating radar. The lobes of ice are buried under a thin layer of soil which protects it from sublimating away.

Overall, the science in Red Mars is remarkably good even today. If you want to get a good basic understanding of Mars science (or if you want to read an epic story of colonization, human drama, etc.) Red Mars is a great place to start. Still, we know a lot more about some things than we did when the book was written. We know that Mars has honest-to-goodness sedimentary rocks and a stratigraphic record that is more complex than just craters and volcanoes. We know a phenomenal amount about the composition of the surface thanks to orbiting spectrometers and the Pathfinder, MER and Phoenix landers. And we know that there is ice, not just at the poles, not just beneath the surface in the arctic, but in vast buried glaciers even in the low latitudes.

With all that we do know, it’s striking to me how many of the big questions about Mars that Robinson mentions in the book are still open to debate today. Did Mars have an ocean? Maybe. Was it ever warm and wet? Maybe. Is the northern hemisphere a giant crater? Maybe.

Is there life on Mars? Maybe. Maybe not. Maybe not yet.