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Seminars on Science
Earth: Inside and Out
Week 3
Violation of the Law of Uniformitarianism
There’s a very important principle in geology, and that’s the
principle of uniformitarianism. It states that the processes that are
operating today also operated in the past. But here we see an apparent
violation of that principle, because these rocks are not forming today, and
they only formed in the past. Something different was happening in the
Pre-Cambrian than today.
The early atmosphere was very different from the present-day
atmosphere. In particular, it probably contained no oxygen. And the
evidence for that is in rocks such as this. This is known as a “banded iron
formation.” As you can see, it’s beautifully laminated, with alternating red
layers which aremade of jasper. Jasper is a fine-grained form of quartz
with tiny grains of iron oxide mixed in, and these dark layers, these black
layers, are made of the iron oxide mineral magnetite. And the very finegrained, finely laminated nature of these rocks is very typical. I should
say, as an aside, that banded iron formations are of more than just
academic interest because they provide about 90 percent of our iron.