Hi folks,

I'm currently in Indonesia and today I was taken to a special place by one of the local people. It consists of a river and waterfall around which the villager thought andesite (his suggestion, but perhaps some other kind of volcanic rock) lines the walls of the clearing where the river falls down from above and continues downstream. This river has its source high up on a volcano many miles away and it ends in the sea.


I know next to nothing about geology, but I assume that for 10-metre high walls of volcanic rock to exist along the banks of a falling river, a massive lahar or flow of lava and rock debris traveled down here during the time of the volcano's last eruption. Is that a reasonable explanation? The volcanic rock (if that is what it is) extends for about 10 meters from the waterfall down the river.


I've attached a few pictures of the wall in question. In some of these you can see the normal river stones and the different rock type above it. I'm clueless about rocks so it could be the case that the villager is completely wrong and that rock wall is not at all related to a volcanic eruption !!

My main question, of course, is can anyone tell me if it's possible to take a sample of this rock wall and test it in a laboratory to determine the age of the rock and therefore the time of the last eruption?

Thanks!