Why were there many active volcanoes in southern California about 10,000 years ago, but not now?

I will give you two hypothesis.

The first one is that perhaps the volcanoes in that region (which part of California you're talking about, I don't know) aren't really extinct. There are a number of volcanoes that have average repose periods between eruptions that are measured in thousands of years. To a human it might seem as if the volcano is dead, but in fact it is just between eruptions.

The second idea is that magma is required to produce volcanoes and the tectonic setting of California has changed from one that produces magma to one that doesn't. Tectonic changes such as these take millions of years to occur though, so I don't think this second hypothesis is very likely.

Scott Rowland

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