Archive for the ‘Sand Dunes’ category

Big Picture HiRISE Gallery!

November 6, 2009

Speaking of Mars art, the Big Picture blog (which all of you should be following by now) has a feature on images of Mars taken by HiRISE. Head on over and take a look. Mars is a really pretty and often bizarre-looking place.

[PS – Have you voted today?]

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Mars Art: HiRISE Dust Devils and Dusty Dunes

October 15, 2009

It’s been a while since I posted any “Mars art” but I just came across this Bad Astronomy post and had to share. The short explanation of the photo is that dust devils spiraling across these sand dunes have removed the red dust but left behind dark sand in artistic swirls. For a more detailed description, check out Bad Astronomy, and to take a closer look at the image itself, head on over to the HiRISE site.

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Pretty Dunes in Gale Crater

April 27, 2009

These dunes and ripples share a valley with an anceint, infilled channel on the mound of sediment in Gale Crater, near the end of the proposed MSL traverse. They are especially striking because they are made of dark volcanic sand, mixed with light-toned sand from the sediments filling the channel.

These dunes and ripples share a valley with an ancient, infilled channel on the mound of sediment in Gale Crater, near the end of the proposed MSL traverse. They are especially striking because they are made of dark volcanic sand, mixed with light-toned sand from the sediments filling the channel.


This is a tiny subframe from the HiRISE image PSP_009294_1750.

Dune Mars, Visiting Mars and Carnival #98!

April 13, 2009

It seems that the astro-blogs have Mars on the brain today! Bad Astronomy has a post about some mind-bendingly cool HiRISE pictures of dunes on Mars, and The Spacewriter has a post about Mars as a whole and Ganges Chasma in particular.

And, if you’d like a little more diversity in your space-blogging, go check out the 98th Carnival of Space at Universe Today!

Grand Falls and Sand Dunes

March 20, 2009
An aerial view of Grand Falls and the dune field that we visited. Grand Falls is indicated by the marker. You can clearly see where lava blocked the previous course of the river.

An aerial view of Grand Falls and the dune field that we visited. Grand Falls is indicated by the marker. You can clearly see where lava blocked the previous course of the river.

(This is day 6 of a week-long field trip in Arizona. Get caught up with days 1,2,3,4,5)

Today we visited Grand Falls and the nearby dune field. Grand Falls is especially interesting because it combines many of the processes that are active in shaping planetary surfaces. The falls are the result of a huge lava flow pouring into the ancient canyon of the Little Colorado river, filling the canyon and flowing both up and downstream for many miles. Obviously this had quite an impact on the river! It formed a dam and a lake upstream until finally the lake spilled over the top of the lava dam and began carving a new course for the river. Basalts and other lava rocks are very hard compared to the Kaibab limestone and Moenkopi siltstones of the original canyon, so the huge tongue of lava is preserved, byt the river is currently working on carving a path around it in the softer rocks. The result is Grand Falls:

On the left you can see the dark, erosion resistant basalt that dammed the original canyon. On the right, Grand Falls are busy carving a new course for the river into the softer rock around the obstruction.

On the left you can see the dark, erosion resistant basalt that dammed the original canyon. On the right, Grand Falls are busy carving a new course for the river into the softer rock around the obstruction.

A mosaic of Grand Falls. Huge blocks of limestone sit at the bottom of the falls, showing that they are a powerful erosive force. (Click for full-resolution)

A mosaic of Grand Falls. Huge blocks of limestone sit at the bottom of the falls, showing that the falls are a powerful erosive force. (Click for full-resolution)

The other main point of interest at Grand Falls were the interesting patterns of cracks in the massive lava rock. These cracks, of “joints” tend to form perpendicular to surfaces in the flow that have the same temperature. In very simple flows, the joins often are vertical and the rock looks like it is made out of hexagonal columns. At Grand falls, the joints are mostly very jumbled, which probably means that steam was percolating through the rock as it cooled. This might mean that there was water in the river when the canyon was filled with lava (the Little Colorado doesn’t always have running water in it).

Columnar joints in the basalt flow at Grand Falls. This is one of the more ordered sets of joints; in many places there is no clear texture to the rock, suggesting complicated interactions with water as the lava cooled. Note that the normally-black basalt is stained tan-colored by the silt-bearing mist from the falls, yet more evidence that they are constantly eroding the rock that they flow over.

Columnar joints in the basalt flow at Grand Falls. This is one of the more ordered sets of joints; in many places there is no clear texture to the rock, suggesting complicated interactions with water as the lava cooled. Note that the normally-black basalt is stained tan-colored by the silt-bearing mist from the falls, yet more evidence that they are constantly eroding the rock that they flow over.

After Grand Falls, we drove our rental minivans through a shallow part of the river upstream, over some very rough roads, and arrived at a nearby dune field. This field is quite young: in the 1930s there were no dunes, but by the 1950s there were, and now no new dunes seem to be forming. The source of the sand is the bed of the Little Colorado, but there are also lots of dark volcanic cinders in the dunes. Larger gravel-sized particles get pushed around by small sand sized particles and form “granule ripples”. These are extremely common on Mars; the Opportunity rover is currently in the middle of an expansive plain of similar ripples.

Granule ripples. The larger, more dense basalt granules end up at the crest of the ripple, where finer-grained sand is blown away or settles below the granules. (in the background are the San Francisco Peaks, an extict stratovolcano)

Granule ripples. The larger, more dense basalt granules end up at the crest of the ripple, where finer-grained sand is blown away or settles below the granules. (in the background are the San Francisco Peaks and a few much smaller cinder cones)

Larger piles of sand form dunes. Dunes move as the wind blows sand up their slope and deposits it at the top until it becomes too steep and avalanches down the “slip face”. Here is an example of a boomerang-shaped “barchan dune” with a nice slip-face. This type of dune is very common on Mars.

A boomerang-shaped "barchan dune". The points, or "horns" of the dune point in the direction that the wind is blowing. In this image, the wind that formed the dune was blowing roughly from right to left.

A boomerang-shaped "barchan dune". The points, or "horns" of the dune point in the direction that the wind is blowing. In this image, the wind that formed the dune was blowing roughly from right to left.

That’s all for today. Tomorrow we will be visiting the painted desert, a location that may be similar to Mawrth Vallis on Mars, one of the potential MSL landing sites.

Meteor Crater, Walnut Canyon, and Red Mountain

March 19, 2009
Meteor Crater is the best preserved (and the first recognized) impact crater on Earth.

Meteor Crater is the best preserved (and the first recognized) impact crater on Earth.

(This is day 5 of a week-long planetary geology field trip to Arizona. Get caught up with days 1,2,3,4)

Today was a long and awesome day. We started out at meteor crater, the youngest and best preserved impact crater on Earth! Our guide today was Shaun Wright, a colleague from the Hawaii field workshop, among other places. He showed us infrared images of the crater taken from an airplane and we walked around the rim trying to identify the main compositions detected. Meteor crater is especially nice for this because it excavated into three distinct layers: the red Moenkopi siltstone (the surface of the surrounding plains), the yellowish Kaibab limestone (normally beneath the Moenkopi), and the white Coconino sandstone (below the Kaibab).

Back in the early 1900s, people were trying to dig and find the iron meteorite that they thought was buried under the crater. (it turns out the meteorite was blasted into thousands of pieces upon impact) Luckily, the mining work carved a notch in the rim that lets you see the three units of the crater where they have been overturned by the impact. When a large impact occurs, it lifts up the ground and forms an “overturned flap” at the rim. You can see in the picture that the Moenkopi goes from relatively solid-looking to very fractured-looking, and is then overlain by blocks of Kaibab and Coconino.

At the rim of the crater, the impact has reversed the sequence of layers. The red Moenkopi would normally be on top but here it is overlain by blocks of Kaibab limestone and Coconino sandstone that have been excavated by the impact.

At the rim of the crater, the impact has reversed the sequence of layers. The red Moenkopi would normally be on top but here it is overlain by blocks of Kaibab limestone and Coconino sandstone that have been excavated by the impact.

Another very interesting part of the crater is that the impact pulverized the coconino sandstone, crushing the sand grans into powder. This powder was actually mined for a while because it is a very high grade silica “rock flour” used in things like makeup. Amazingly enough, even though it has been subjected to one of the most violent forces imagineable, the crushed sandstone still maintains its original structure, and you can even see crossbeds preserved!

The shocked sandstone still preserved very fine cross-bedded layers, but can be crumbled into a power with your hand.

The shocked sandstone still preserved very fine cross-bedded layers, but can be crumbled into a power with your hand.

After Meteor Crater, we made a short stop at Walnut Canyon, where the Coconino sandstone is not shocked and the crossbeds are displayed prominently. Remember, cross-bedded layers typically form when sand dunes are lithified in place and turned into sand stone, preserving the layers within the dune. For  more info about crossbeds, check the USGS site about them.

Crossbeds at Walnut canyon are essentially fossilized sand dunes from when Arizona was a coastal desert. The direction that the layers are tilted tells us that the prevailing winds blew from north to south.

Crossbeds at Walnut canyon are essentially fossilized sand dunes from when Arizona was a coastal desert. The direction that the layers are tilted tells us that the prevailing winds blew from north to south, although the various sets of layers in this image actually reflect several wind directions.

Finally, after Walnut canyon we drove up to Red mountain, which is a cinder cone volcanoe that has been carved open by erosion. Not only does it give a great view of the interior structure of the cone, it also erodes into a very bizarre landscape that looks like it belongs in a Dr. Seuss book.

The interior of Red mountain cinder cone. The layers are from different stages of the eruption that deposited cinders with slightly different composition or weathering properites. The bizarre shapes are due entirely to erosion, mostly by water.

The interior of Red mountain cinder cone. The layers are from different stages of the eruption that deposited cinders with slightly different composition or weathering properites. The bizarre shapes are due entirely to erosion, mostly by water.

That’s all for today. Tomorrow we are off to Grand Falls and the nearby dune field!

Sedona and Oak Creek Canyon

March 16, 2009

Today we made our way from Phoenix north to Flagstaff, and on the way stopped to check out some interesting geology in Sedona and Oak Creek Canyon.

Bell Rock in Sedona, Arizona is an outcrop of red sandstone deposited 275 million years ago in a broad tidal zone.

Bell Rock in Sedona, Arizona is an outcrop of red sandstone deposited 275 million years ago in a broad tidal zone.

Sedona is famous for its spectacular red rocks, such as Bell Rock, which we clambered around on today. Bell Rock is made mostly of very fine-grained sandstone formed by windblown sand reworked by the advance and retreat of oceans in the early Permian period (~275 million years ago).

A view of the layered butes and mesas near Sedona. All of the open space between the outcrops was once solid rock, and thousands of feet more was once on top of that.

A view of the layered buttes and mesas near Sedona. All of the open space between the outcrops was once solid rock, and thousands of feet more was once on top of that.

One of the things that is always impressive about the geology of Arizona (and most other locations on Earth, and other planets, for that matter) is that the surface that you’re looking at was once buried under miles of rock. This view out over Sedona from Bell Rock, shows the distant mesas and buttes with continuous colorful red and white layers of sandstone and limestone. The entire valley was once filled with rock, and there were thousands of feet above the very tops of the mesas that are present today!

In many sandstones, you see diagonal layers that form graceful curves and swooping shapes. These are called cross-beds and are the preserved cross sections of ancient dunes (or ripples). There was a light-colored layer that showed very clear cross-bedding across from Bell Rock.

The light-toned layer shows clear cross-bedding, indicating that it is probably a sandstone formed by lithified sand dunes.

The light-toned layer shows clear cross-bedding, indicating that it is probably a sandstone formed by lithified sand dunes.

We saw some nice examples of cross-bedding on all different scales in the rocks of Oak Creek canyon. This picture shows a small oblong chunk of sandstone near the creek bed with cross-bedding on the scale of inches. This may have been due to small ripples rather than large dunes.

The rectangular block shows cross-bedding on a very small scale. It is about 5 inches across and a couple feet long.

The rectangular block shows cross-bedding on a very small scale. It is about 5 inches across and a couple feet long.

Oak Creek canyon is also interesting because it follows a fault in the plateau, so the layers on one side of the canyon have dropped down several hundred feet compared to the other side!

Coming up tomorrow, we will be exploring the San Francisco volcanic field north of Flagstaff, focusing on the very young SP Mountain volcano and flow.