Basalt Survey Near Clay Hill At The Santa Rosa Plateau

This page gives the details of our basalt survey that is reported on Ten million year old drainages seen at the Santa Rosa Plateau: Overview.

In November 2007 we made a detailed fine-scale map of the basalt at Clay Hill and the west side of the Mesa de Burro. This was an opportune time to do so, since it was at the end of a severe drought year, which followed a previous drought year. The vegetation cover was therefore minimal, allowing us to easily see the rocks exposed at the surface.

Our mapping was done primarily from hand-sized to boulder-sized rocks on the surface, although we did find a number of outcrops where the lava was directly exposed. Most of the rocks used in the survey are what geologists call float, loose rocks. Mapping using float is of course subject to contamination by movement of rocks downslope, which must be taken into account in any analysis of such mapping. Fortunately, the effect of such movement is negligible in this analysis, since the contours at Clay Hill include the top of Clay Hill, and the important contours below the Mesa de Burro are far from the basalt outcrops at the top edge of that Mesa.

We decided to include in our basalt contours any location in which any basalt rock was found. It turned out this decision did not make much difference in the derived contours. Mixed rocks were only found in a fairly narrow zone at the perimeter of the contours. At the scale of the map presented in this work, contours based on only including locations with 100% basalt rocks would appear the same.

There most difficult parts of this mapping were:

In our mapping, we demanded high reliability in determining a rock as basalt. We had to see clear large bubble holes that are never present in the mudstone. Also, we demanded that the rocks showed a distribution of bubble sizes, and depths of the bubbles, that were never seen in rocks of the Santiago Peak Volcanics. If we had any doubt about a given rock, we did not use it in the mapping of the basalt, or we split it open to be sure.

The following images show mudstone on the left and basalt on the right:

In the middle picture above, the middle rock (smallest one) is basalt, but it has very few bubbles. In the bottom picture above, the mudstone seen in the middle picture was split, which revealed that a fresh exposure is clearly different from basalt. (The rock halves were placed next to each other in the middle picture, which therefore shows the fracture line.) This rock splits very easily, and can even be broken by knocking it by hand against a basalt boulder.

The following picture shows the variation in the amount of bubbles in the basalt:

Note how similar the left hand rock is to the mudstone two pictures before.

However, basalt is a much harder rock to split than mudstone. Attempts to split open that left hand basalt rock failed, only resulting in the five chips that expose fresh surface. Furthermore, the fresh basalt surface is very different from the fresh mudstone surface.

The following pictures show basalt of the Santiago Peak Volcanics on the left, with Santa Rosa Basalt on the right:

The top picture immediately above has three rocks of the Santiago Peak Volcanics on the left; the bottom picture has two such rocks.

The following picture shows how similar the large pits in the weathered Santiago Peak Volcanics can be to the pits in the Santa Rosa Basalt:

Note the numerous large spherical pits on the weathered surface of the rock that are virtually indistinguishable from basalt bubbles at a glance, but the completely-different appearance on the fresh surface! (The whitish marks on the ridges on the bottom weathered surface were made by the hammer crushing those ridges. Note how hard the rock is; those ridges only got minorly abraded!)

Compare that rock with any of the Santa Rosa Basalt rocks above; at a glance, they look very similar.

The size of the pits in the rock above are 3-8 mm in diameter, and 2-4 mm in depth. In contrast, the bubbles in the Santa Rosa Basalt are 2-15 mm in diameter, and up to 15 mm in depth. Also, the pits in the Santiago Peak Volcanics tend to be very similar in size and shape, whereas the bubbles in one bubble-iferous sample of the Santa Rosa Basalt have a wide variation in both size and shape.

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Copyright © 2007 by Tom Chester, Wayne Armstrong and Kay Madore.
Permission is freely granted to reproduce any or all of this page as long as credit is given to us at this source:
Comments and feedback: Tom Chester
Updated 30 November 2007.