How do geologists differentiate between geologic events that took place over thousands of years and those that happened very suddenly or very quickly?

Question

I’m interested in reading about sudden or rapid geologic events, like earthquakes, volcanic activity, and so forth, but haven’t heard a satisfying explanation of this, i.e. whatever I read didn’t stick with me. If someone can give me a basic enough explanation, that would help as I look for stuff like this.

Thanks!

In: Earth Science

Answer 1

For volcanic events we have several ways to date the volcanic deposits. There are for example certain radioactive elements that are embedded in minerals. Over time those elements decay radioactively into other isotopes or whole new elements. By comparing the ratios between the original element and the new ones we can determine an approximate age of the deposits. There are a lot of different dating methods, which all work best for a specific timeframe.
Often we can also use fossils to determine age. If for example the eruption killed specific animals and we know when they were around we can get an approximate age. Even spores and pollen can be embedded in these layers and we can use those for dating.

As for earthquakes We can’t actually know when past ones happend if they weren’t recorded by humans.