Before my last post, I had a week away from blogging. My laptop died and I was doing fieldwork so there was no chance to fix it (and I was too tired after dinner and drinks to write anyway). We were in New Mexico, along the transition between the Rio Grande Rift and the Colorado Plateau, testing operational strategies for planetary rovers by comparing the data and maps generated by field geologists to the geologic interpretations of a simulated rover. I was on the human team, and to gain value from the exercise we documented all of the small steps, observations, and shifting hypotheses that make up what passes for intuition in experienced fieldworkers. New Mexico is beautiful and I learned a great deal from my field partners, each of whom is an expert volcanologist or planetary scientist or both. I wish all my work was like that.
Cabezon Peak seen from the truck.
On day one, we mapped part of a maar volcano. Maars are low, flat volcanoes that form when magma explosively interacts with water (usually groundwater or permafrost), and the craters commonly host shallow lakes. The aerial photos showed this one to be about 800 meters across, though we only mapped one side. The rim and lower units were inward-dipping, loosely consolidated volcanic material overlain by debris flows and relatively flat-lying lake sediments deposited in the crater. I’ve read about maars and even taught about them, but I’d never before been close enough to see the structures and the outcrop-scale features. As a petrologist, I’m usually focused on the magmatic rocks, but in phreatomagmatic systems like this one the basalt composes only 5-10% of the material, and most of it’s hydrothermally altered. The bulk of the material is old country rock, sediment, and mud brought up and churned by the eruption.
From inside a maar crater. The dark layers in the foreground are continuous with the ridge in the right of the photo (though this interpretation was somewhat contentious). The cliffs to the back left are a massive layer of volcanic debris topped by thinly-bedded crater lake deposits.
Slump block of layered volcaniclastic material in the massive debris layer.
Prismatically jointed bomb (PJB), formed when molten basalt blobs land in water or wet mud.
Day two was a little more familiar. It was a volcanic neck that came up through Mesozoic sedimentary rocks. We worked from the base with an aerial photo so the only volcanic material available to examine was brought down in long debris flows (a very realistic scenario for a rover). There were reddish boulders of vesicular cinders and scoria in a matrix of altered basaltic glass (palagonite) and there were blocky flows of extraordinarily fresh aphyric basalt. We interpreted the former as deriving from early cinder cones or rim deposits and the latter as a crater-filling lava lake or flows outside the crater. Both rocks contained beautiful and abundant mantle xenoliths. Because such xenoliths occur almost exclusively in alkali basalts and related rocks (these magmas typically form from very deep melting and have a low enough viscosity to ascend quickly) we could infer the basalt composition in the field even though it lacked phenocrysts.
A volcanic neck. The tan layers in the mid-slope are Cretaceous sedimentary rocks. The top is capped by dark red-brown scoriaceous basalt and darker blocky basalt. The boulder in the foreground is the former and the latter litters the ground and fills the far drainages.
Looking down from the base of the sedimentary cliffs. Blocky basalt in the foreground.
An aside: xenoliths are our primary source of knowledge about the Earth’s mantle. Although sections of uppermost mantle are occasionally tectonically emplaced in ophiolite complexes, alkali basalts can sample much deeper material – from greater than 60 km. (Xenoliths in kimberlites provide the deepest mantle samples we have, from depths down to 200 km, and the great pressures are why kimberlites are the primary diamond-bearing rocks). Here’s a pretty good paper on some xenoliths in the area we were mapping.
Two mantle xenoliths in basalt. The front sample is a pyroxenite (mostly clinopyroxene) visible on a slightly weathered surface. The rear sample is a lherzolite (mostly olivine + pyroxene) seen in a freshly broken surface.
I don’t have any operational experience with rovers and I haven’t (yet) been on a rover science team, so the experience was edifying. A rover is limited by time, tools, and energy expenditure. A two-legged geologist can cover hundreds of square feet in minutes, breaking rocks to expose fresh surfaces and holding them up for examination with a hand lens. If a rover even has an arm (and many designs don’t), it would be a major decision to take the time and energy to turn over a rock, and even the best rover cameras are limited in field-of-view and height as to what they can image. It’s reassuring that both of our teams came to similar general interpretations at both sites; however, it took a rather long time for the rover team to spot the xenoliths, which would be a major find on Mars or the Moon. On the human side, we seemed to have a bias that the layered rocks would be volcaniclastics (not surprising, given our backgrounds) or even pyroclastic surge deposits. The lake sediments in the maar undoubtedly had some reworked volcanic input but the sandstones and shales from the neck contained little if any obvious volcanic material. This was much clearer when we could examine them up close, but a rover team doesn’t always have that option. I think this shows how important these analog studies can be in identifying the pitfalls of rover geology, which is the first step in avoiding them or compensating. I’m pleased to have been a part of it.
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