Saturday, July 11, 2015

The Crew and the Cosmapa Lahar

Here’s the problem.

The Cosmapa site is buried. In Shovel Test 8, the northeasternmost shovel test, the buried surface is particularly clear (Figure 1).

Figure 1. Sharp Contact between the sterile superposed volcanic sand and the underlying buried soil rich in artifacts in Shovel Test 8, Cosmapa
The buried soil appears at about 100 cm. The first sherd appeared at 102 cm. The contact between the overlying ash or sand and the clayier buried soil is particularly sharp. Last year, while writing the informe, I noticed on Google Earth that there was what looked like a large lava flow that came down the southern flank of San Cristóbal straight toward Cosmapa and stopped just south of the Highway, about 1000 m north of the site (Figure 2).

Figure 2. Lava Flow or lahar? In Yellow. Site is in blue.

A couple of days ago, I was looking at it again in Google Earth, and I came down to ground level and "flew" toward San Cristóbal, and I saw that the “lava flow” looked like a gigantic lahar that came straight from a fissure in the cone of San Cristóbal, where the collapse looked quite sharp and fresh (Figures 3-6).

Figure 3. At the summit, it looks like a lahar. Notice the sharp fissure where part of the cone has given way and a giant avalanche has cascaded downward.

Figure 4. The collapse feature is clearer from an angle in this Google Earth image. See how fresh it looks.

Figure 5. Here you can see the relation between the flow and the lahar above. They are almost, but not quite, aligned.
Figure 6. Same as the previous map, but zoomed in a little.

So, was it a lahar or a lava flow? We drove out to look. It’s only 7.2 km from the rotunda at the entrance to Chinandega. We parked at the north entrance to the Cosmapa road. From there, you can see that the road is humped up where it crosses over the flow, whatever it is. We walked along the highway and looked at the section visible in the drainage ditch. We got to a bridge and went down into the dry streambed. There we could see what looked like a lahar deposit in the lowest stratum with ash deposits sitting unconformably atop it. The lahar deposit had lots of sharp stones (not rounded river stones) all mixed in it and lightly cemented, but I could pry bits off with my trowel. We went back to the car and drove east to a dirt road that runs north just shy of the stream. Then we drove north along the road, which follows the eastern edge of the flow. The flow is pitted with big quarries, which makes sense: the volcanoes must be the only places to get stone in the coastal plain. And this recent flow, whatever it is, is clearly recent—there’s only stunted vegetation growing on it, as you can see on Google Earth—so the stone must be easily accessible. In the quarries and road cuts along this track, we could see that the flow is composed of a jumbled mix of totally unsorted, rough, angular, volcanic stone. The particle size varies from silt to house-sized rock, all tumbled together in piles and ridges (Figure 7).  Although there is some scoria mixed in, it looks to me like a lahar. I drove up the Posoltega lahar in 2000, only two years after it happened, and I have seen ancient, consolidated lahar deposits at Cusirisna in Boaco. They pretty much look like this one on San Cristobal (Figure 8), allowing for differences in parent material and age.

Figure 7. Lahar deposits in a quarry face
Figure 8. Ancient lahar deposit from the Las Lajas Crater near Cusirisna, Boaco, Nicaragua.

So, I think it is a lahar. It doesn’t look a lava flow. It’s largely unconsolidated.

If it's a lahar, it's huge. Look at it at the same scale as the Posoltega lahar, which killed a couple of thousand people in few minutes during Hurricane Mitch in 1998 (Figures 9 and 10). The two images are at the same scale.

Figure 9. Posoltega Lahar, from 1998
Figure 10. San Cristobal Lahar at the Same Scale as the Posoltega Lahar

Here's the crew. They're great (Figure 11).
Figure 11. Kelsey, Grazia, and Ashley, with Agateyte smoking pacifically in the background

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