Episode 391 Valles Marineris

In today's episode we’re going to space. Specifically, Mars. You didn’t really think that earth science is really limited to the earth, did you? Our topic today will be the Valles Marineris.

The Valles Marineris is a long series of canyons east of Olympus Mons, the largest mountain in the solar system. These canyons are about 4,000 km long, 200 km wide and up to 7 km (23,000 ft) deep. On terrestrial scales, the Valles Marineris is as long as the distance from New York to Los Angeles. That’s about the same as Beijing to Hong Kong or Madrid to Copenhagen for our international listeners. They are as wide as central Florida, central Italy, or the middle of the Korean peninsula. Two and a half times deeper than Death Valley, though only about 60 percent of the depth of the Marianas Trench, the lowest point on earth.

Valles Marineris Image Courtesy NASA/JPL-Caltech

Not to be outdone, our planet, Earth, has even bigger valleys. These occur at the oceanic ridges, where plate spreading takes place. The longest rift valley on earth lies in the middle of the Mid-Atlantic Ridge, and it is more than double the length of the Valles Marineris. But let’s not belittle Mars. After all, while we have a pretty good idea for how oceanic rifts form on earth, there is quite a bit of debate about how Mars’ great valley formed.

The most popular theory suggests that the Valles Marineris are an analog to our oceanic rifts, and formed by the same process. As the volcanoes of the nearby Tharsis region developed, the Martian crust bowed down toward the center of the planet due to the weight of the new volcanic rocks. In time, the crust began to crack. This crack is what we see in the Valles Marineris. Unlike on Earth, this rift valley did not continue expanding, but shut down as the Tharsis Region, and Mars as a whole, cooled. Remember that unlike Earth, Mars does not have plate tectonics. It doesn’t have a continual process of hot material (like lava) rising to the surface, while relatively cold material (like the oceanic crust) is brought down towards the planet’s center.

More recent work has used satellite images, and high resolution elevation data to develop new insight into how the Valles Marineris formed. While images from the 1970’s Mariner 9 orbiter were quite blurry by today’s standards, new missions in the late 90’s to early 2000’s have given us a better view of the Martian surface than we have available for the earth. The Mars Reconnaissance Orbiter can take images where each pixel is about 0.5 m or 20 inches. That is, the color on each image is an average of an area of 0.25 square meters, or 2.5 square feet. It can then use image pairs to estimate the elevation of any point on the Martian surface with a pixel size of 0.25 m, or about 10 inches.

These new satellite images include multispectral data, or images that look at different wavelengths of light. The camera on your phone works in the same way: There are sensors that pick up, red light, green light, and blue light. Your phone records the intensity of each color in each part of the image, and then plays it back on your phone’s screen to create a picture.

Some of the satellites orbiting Mars take this to the next level. They don’t just take different slices of colored light, but also longer wavelength, infrared light. If you’ve ever seen an image from a thermal imaging camera, you know what this is. Parts of you show up as hotter or colder on the screen. It’s the same with the surface of the earth, or Mars. Scientists can compare the intensity of different wavelengths of light from each point on the surface. They can then compare these values, with what would be expected for different rock types. In other words, we’re able to roughly determine the types of rocks on the Martian surface without ever setting a boot, or rover tread, on the red planet.

Data from these images has shown that the Valles Marineris have layered rock formations both on the sides of the canyons, and within them. The great valley has seen many landslides over the last 3.5 Billion years of its existence, as well as new and smaller canyons carved into it. Scientists now speculate that rather than just forming as a big crack in the Martian surface, the Valles Marineris have been sculpted by flowing water, either in its liquid form as rivers, or in its solid form as glaciers.

An alternative hypothesis proposes that the Valles Marineris formed as a crack during a massive, planetary scale landslide. This landslide was about half the size of the US or China. How do you form a landslide that big? Well, you need a large pile of relatively weak rock, and high elevations for the landslide to flow from.

A key player here is salt. Salt is relatively weak as compared to rock, and can deform easier when squeezed. It can also hold water, which can be driven off by heating. On Earth, weak salt layers are partly responsible for undersea landslides in the Gulf of Mexico. The Opportunity rover had found some salt layers during its mission on Mars, so we know salt is present on the red planet.

Some scientists interpret the layers on the sides of the Valles Merinaris to be made of salt, and possibly include pockets of ice. This would imply that those layers are weak, and could potentially move downhill under the right circumstances.

Heating in the Tharsis region helped de-water salts under the future landslide, melted ice pockets, and created high elevations on one side of it. Think of it like putting a can on a wet metal sheet. If you raise one side of the sheet, the can will slide to the lower side. Just like that, the salty Martian crust broke, and slid downhill.

A crack in the side of this landslide allowed massive amounts of underground water to escape. As the water flowed downhill, it eroded the crack to form a massive canyon. This canyon is the Valles Marineris. The flood that helped form the Valles Marineris was probably bigger than any seen on earth. Bigger than the massive glacial outburst floods that formed the channeled scablands of the northwestern United States. Dick Gibson discussed outburst flooding in the December 27, 2014 episode. Unlike the Earth, the Martian surface has been relatively quiet since the Valles Marineris formed 3.5 billion years ago.

—Petr Yakovlev

This episode was recorded at the studios of KBMF-LP 102.5 in beautiful and historic Butte, Montana. KBMF is a local low-power radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.

Episode 386 Dynamic Topography

What is dynamic topography? Well, it depends on who you ask. Dynamic topography is similar to other terms, like uplift, that have been used in so many different ways that you really have to look at the document you’re reading to understand what the author is talking about. This term has been applied to places around the world, like the Colorado Plateau in the United States, South Africa, the Aegean, and East Asia, which makes it even more complicated to tease out its meaning.

Most broadly, dynamic topography refers to a change in the elevation of the surface of the earth in response to something going on in the mantle. This “something” can include both the flow of the mantle, as well as differences in mantle temperature or density. For the purposes of this podcast, I will use a more strict definition: Dynamic topography is the change in the elevation of the surface of the earth in response to the upward or downward flow of the mantle.

How much higher or lower can dynamic topography make the earth’s surface? Well, that’s a matter of debate. Earlier studies have suggested that several kilometers, or over 6000 feet of modern elevations can be explained by things going on in the mantle. More recent work instead suggests that dynamic topography creates changes of at most a three hundred meters, or a thousand feet.

A good example of a place where this process is thought to be active is Yellowstone. As Dick Gibson discussed in the December 19th, 2014 episode, Yellowstone is thought to be a hot spot. That is, an area of the earth where hot material moves from deep within the mantle to the base of the crust, causing significant volcanism at the surface of the earth. Other well-known hot spots are located in Hawaii, and Iceland.

So how can a hot spot like Yellowstone cause dynamic topography? Well, you’ve probably seen a similar process at play the last time you played in a pool or a lake. Think of the surface of the pool like the surface of the earth. If you start moving your hands up and down under water, the surface of the pool starts to move up and down. If you ever tried to shoot a water gun upwards underwater when you were a kid, you probably remember it pushing up the surface of the water, and being disappointed that it didn’t shoot out at your friend or sibling. As an adult, you could try holding a hose upwards in a pool. Again, it probably won’t shoot out, but will gently push upwards on the surface of the pool.

Dynamic topography concept. © Commonwealth of Australia (Geoscience Australia)
 2017, used under Creative Commons Attribution 4.0 International Licence 

The principle for a hot spot creating dynamic topography is the same. The flow of the mantle pushes upwards, warping the crust and increasing the elevation of the earth’s surface above the hot spot. Near Yellowstone, this results in an area of high elevation which lies next to the Snake River plain.

But Dynamic Topography doesn’t just cause increases in elevation, it can also pull the earth’s surface downward. In North America, dynamic topography is thought to have been in part responsible for the creation of the Cretaceous interior seaway. 

As a reminder, the Cretaceous interior seaway was a shallow sea that covered parts of western North America, in middle to late Cretaceous time, about 100 to 79 million years ago. Its size varied, but at its greatest extent the seaway stretched through Texas and Wyoming in the US, and Alberta and to the Northwest Territories in Canada. It was widest near the US-Canadian border, where it stretched from Montana to western Minnesota.

Low elevations in western North America that allowed the ocean to flood in and form this shallow sea may have been caused by downwards flow in the mantle. This downwards flow was likely caused by oceanic crust that was subducted at the western margin of North America. That is, oceanic crust that went underneath the North American plate and into the mantle. Because this crust was part of the Farallon oceanic plate, it is often referred to as the Farallon slab.

As oceanic crust associated with the Farallon plate continued to sink into the mantle, it continued to cause changes in the elevation of North America. This drop in elevation likely decreased in size as the Farallon slab moved towards the eastern edge of North America, and deeper into the mantle.

Since Eocene time, or about 55 million years ago, dynamic topography associated with the Farallon slab is thought to have been in part responsible for lower elevations in the eastern United States. A wave cut escarpment called the Orangeburg Scarp is now located 50 to 100 miles inland from the coasts of Virginia, Georgia and the Carolinas. It formed at sea level and now lies up to 50 meters, or about 165 feet above the modern coast line. In fact, a good part of the southeastern US to the east of this escarpment contains marine sediments, and smooth topography as a record of its time underwater.

Differences in the elevation of the Orangeburg Scarp along its length suggest that rather than just going up and down, the Atlantic coast experienced a broad warping caused by mantle flow. The most recent phase of warping brought this area to modern elevations, as warm material moved into the upper mantle beneath the Atlantic coast. This warm material helped push the crust up to higher elevations, creating the southeastern US as we see it today.

This example also highlights an important part of dynamic topography: If you are already at really high or really low elevations, you might not notice it much. If you are near the coast, it can have a big impact as the sea starts to flood in and out due to changes in the mantle. Provided of course, you’re there for the millions to tens of millions of years it takes for the mantle to flow this way and that. That’s why geologists typically rely on the rock record to provide evidence for processes like dynamic topography.

—Petr Yakovlev

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This episode was recorded at the studios of KBMF-LP 102.5 in beautiful Butte, Montana. KBMF is a local low-power community radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.

Links:

What is Dynamic Topography?

Surface expressions of mantle dynamics

Dynamic topography, eastern US

Episode 383 Himalayas, Catskills, and more

For today’s episode of the podcast I’m introducing you to Dr. Petr Yakovlev, a friend and geologist here in Butte at the Montana Bureau of Mines and Geology. Petr will be doing occasional guest episodes to give you all a break from my voice, as well as information about some of the diverse things he's worked on.

Petr Yakovlev with a Cenozoic conglomerate near Cardwell, Montana.
Photo by Dick Gibson

Petr got his undergrad geologic education at Boston College and his PhD at the University of Michigan.

In this episode Petr and I talk about his work in Tibet, which has implications for the fundamental nature of the India-Eurasia collision; another structural geology project he worked on in the Catskills of New York; and the projects he’s working on here in Montana. And he gives us some teasers about the kinds of topics he plans to cover for History of the Earth.

Running time 13 minutes.

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