Oldest Volcano In The World

Old rocks, a billion years or more, tend to be beat up a bit (or more). They might be folded and faulted. The might have their original minerals replaced by a new set of minerals. The original volcanic edifice (stratovolcano, shield, etc.) probably won't be recognizable. None of these would stop a volcanologist from getting all the information she could out of the rock. An age can be determined and the chemistry can tell a lot. Sometimes pillow lava or ropy pahoehoe lava can be found.

Volcanologists have found old volcanic rocks in the cratons of most continents. Here's a brief list of places and ages:

Canada: 3.5 to 2.8 billion years
Rhodesia (Zimbabwe): 3.4 billion years
South Africa: 3.3 billion years
West Australia: 2.8 billion years
West Australia: 3.1 billion years

3.5 billion years seems old to me. Since the oldest known rocks are 3.96 billion years old, you might ask why there are not any volcanic rocks older than 3.5 billion years. There might be and they have not been found (funding mapping projects is not very popular these days). More likely they did exist and were changed by high pressure and temperature into metamorphic rock. If I spent a day in the library I could probably find the age of the oldest metamorphic rock that used to be volcanic