nickle toxicity, and to a lesser extent, chromium toxicity. While of vanishing importance in typical continental crust, these two elements are much more abundant in ultramafic rocks
The second reason is a little more complicated, but boils down to a single word: clay. Nearly a year ago, I wrote about the importance of feldspar and clays in dune deposits, and the issue raises its head here, again. In this case, the problem is that there is proportionally very little feldspar in these rocks, so when they weather, you get various metal oxides, hydroxides, and hydrates, but little clay. That in turn means that the soils don't do well at retaining water and plant nutrients.
The two factors of heavy metal toxicity and poor soils mean that plant communities on ultramafic rocks and serpentinite are limited, and often pretty marginal-looking, that is, they look as if they're barely hanging on. On the other hand, some plants have exquisitely adapted to these conditions, a topic that came up twice in the most recent Accretionary Wedge. Update: Dan McShane (of Reading the Washington Landscape) points out a couple of other stress factors for plants in this geological setting in the comments.
What all this means to geo-enthusiasts is that even doing 60-MPH reconnaissance geology, it's pretty easy to tell when you're going by these sorts of rocks.
Photo unmodified. May 8, 2013. FlashEarth Location. Indexed
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