3 September 2008
Hawaii Field Work – Ground Truth
Posted by Ryan Anderson
I posted a while back, right before I left to participate in the NASA Planetary Volcanology Field Workshop, describing what exactly I was going to be doing there. Our goal was to study a set of images and come up with a geologic map of the area, and then actually go to that location (something we can’t usually do for other planets) and see how good our geologic maps were.
Since I’ve been back, I’ve been making a bunch of posts about volcanoes. This was partly because volcanoes are awesome and I wanted to share the awesomeness, but it was also to provide you with some background and vocab for my discussion of the mapping exercise. So, go read my other volcano posts and then come back. [Shield Volcanoes, Hot Lava: Where does it come from? , Hot Lava: Types of Lava, What Makes Volcanoes Erupt?]
Now without further ado, here’s my map.
Now, I know, you look at that and first think “that looks like it was drawn by a third grader!” and then think “and he didn’t label it! those colors could mean anything!” To that I say, yes it’s not the prettiest map, and it’s true that I didn’t label it. You’ll just have to trust me.
I called the black areas pahoehoe flows, which they were for the most part. There were places where older a’a flows remained, and there was also a lot of complexity because the pahoehoe didn’t all arrive in one flow. The picture below shows pretty clearly that there were several stages. The light-colored pahoehoe is oldest, followed by the rough a’a, which was then almost engulfed by fresher, darker pahoehoe.
The ridge down the center of the mapping area, which I marked in pink on the map, turned out to be a lava channel. During the eruption it was essentially a river of molten rock, which would occasionally overflow its banks. When it overflowed, the lava would cool and harden, so it eventually built up raised banks. Here’s a photo from the small shield volcano north of the mapping area, looking south along the channel. The landscape was spectacularly barren, and when misty rain blew in later that day, I was reminded of Modor from the Lord of the Rings (yes, I’m a dork.)
The yellow stuff on my map was indeed a’a, though there were some places where it interacted weirdly with pahoehoe-like flows. The picture below is a good example. The confusing thing about this location is that it looks like liquid lava welled up onto the smooth areas from the a’a, but a’a lava is very viscous and chunky, not liquidy. My mapping partner and I came up with the hypothesis that perhaps where the edge of the a’a flow contacted the “wall”, it tore open the a’a and allowed more liquid lave to flow from the interior. You can see there is a transition from a’a on the left, to stuff that looks all chewed up because a’a has been sticking to it or tearing at it, and then smooth lobes. I don’t know if our idea was right. But we explained it to one of the instructors and he couldn’t say that it was wrong…
The orange and red areas on my map were what I guessed to be old a’a flows, because they were bright in radar. It turned out they were vegetated areas including both a’a and pahoehoe. The picture below shows one of the instructors picking his way across pahoehoe and some a’a, after crossing the vegetated area in the background (the roughly triangular orange area on my map). When we met up with him he said: “If you’re thinking of going across that grassy area I have some advice for you: don’t” It turns out that it is a mess of high grass and scrubby bushes that totally conceal the uneven lava surface underneath and are actually harder to cross than just plain lava.
We also talked with him about the effect of slope on the radar data. The southern part of the mapping area is a steep hill and in the radar data it shows up as a strange bright area that is very hard to interpret. It turns out this is because the slope was reflecting the radar signal back to the detector much more than it would have if the same surface had been flat. Here’s a nice view over the edge of the “cliff” from where I ate lunch. I expected that the pahoehoe flows might turn to a’a when they were forced to flow faster down the hill, but you can see that they stay smooth and silvery-looking all the way down.
Two more areas of the map left! I thought the light blue was going to be sparse vegetation. This was partly correct. There was vegetation, but again, it was surprisingly thick. As for the green stuff, the eastern green blob on my map was basically the same as the blue: vegetated old flows. The western green area on my map was a surprise though. It was a mixture of trees and shrubs growing out of a layer of ash that had draped over older a’a and pahoehoe! In the picture, you can see grey ash and small golden looking clumps filling in the nooks and crannies of the old a’a.
Here’s a close up of one of those golden blobs. They are a volcanic rock called “reticulite” and are extremely porous and fragile. They form when an eruption has tons of gas in the lava, so it becomes a sort of foam. You may have heard that pumice is so light that it floats on water. Well, reticulite is even more porous, and has so much interconnected empty space that it acts like a sponge, fills with water and sinks!
In the end, my map was pretty good, but there were still some surprises and a lot of complexity to figure out that was not at all apparent in the orbital and aerial data. I also found that there were places where it was more difficult to see what was going on from the ground, but if I had been twenty feet up in the air, it would have been much clearer! It is humbling to attempt to map an area using only remote sensing and then go out and look at it in person and see how much more is going on there. We try to do the same thing on Mars, but in the vast majority of cases, we can’t go and look closely at the surface and figure things out. Still, the fact that my map was mostly right gives me some hope that what we do on Mars is not just guesswork. With all of the new orbital instruments like HiRISE, CRISM, and CTX, we have a much better chance of understanding what is happening on the surface of Mars than ever before. But at the same time, we should always expect surprises. After all, surprises are what make exploration so exciting, and I’m sure Mars has plenty in store for us!
The next step is to go to Mars and do what we did in Hawaii: hike around, take pictures, kick the rocks, pick up interesting ones, and try to piece together what they are telling us.
Great article on Hawaii cool read for the day.