[skydiamonds (OP)]

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.

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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.

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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!

[RD]

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?

https://en.wikipedia.org/wiki/Geochronology#Radiometric_dating

[Bass]

The rocks in your photo certainly could be volcanic- but I couldn't make our enough detail to tell for sure. Did you see any crystals in the rocks? Andesites commonly have both black and white crystals.
As to the age, probably Potassium-Argon would give you the best radiometric readings to determine age if it's volcanic. Why do you think this is "last" eruption? Volcanic lava flows are notoriously local and tend to fill low spots in the topography, so this may have been covered by even younger flows.

[skydiamonds (OP)]

Thanks so much for your reply and apologies for the delay in mine.

About the crystals, I did not see any here. The local I was with thought it was andesite--I have no idea whether it is or not at this stage. For me, the strange formation of the rocks made it seem foreign to the surroundings. The jagged edges are nothing like the smooth rock one normally sees protruding along the river banks or in the nearby rice-fields. I'm attaching another photo to this message which perhaps shows the formation better.

In terms of dating, should I contact a specialist or is taking a sample of the rock something I could attempt on my own? I would send the sample to a professional lab for testing.

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About its age, the river in which I found this rock flows directly from a volcano now considered extinct. There is no record of its eruptive history. So I assumed that if it was the case that this rock had been left there by an ancient eruption that sent lahar down the river (or perhaps just lava flows, although this is many kilometers from the summit) and coated the walls with volcanic debris, then this might be evidence of the most recent and final eruption of that particular volcano. The volcano is considered extinct but I assume there is still magma flowing underneath because of the vast quantity of hot springs in the area. Walking just down from this photo one sees sulphur (?) stained ponds with bubbles emerging in heated water.

[skydiamonds (OP)]

I'm attaching another photo with a little higher resolution of the same place but of a different wall, this time on the other side of where the waterfall drops into the river downstream.

Hopefully you can make out the difference between what I assume is the original underlying rock that extends upwards from the water line, and the other kind of material above it, which seems to literally have things growing out of it. Is that significant? Is that rock or something else?

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[skydiamonds (OP)]

This last photo is of a place nearby that has equally strange rocks by its river. This river again originates in the same mountain which is considered an extinct volcano with no detailed eruptive history.

These rocks are very different than those found by the waterfall pictured above. They are giant boulders of a completely different material to the rocks lining the river below them. Some are approximately 3 x 2 metres with a height of over 2 metres. Some even larger, some much smaller. There are many in this particular location and I've not yet found any others close-by.

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I hope the photo shows enough to give an estimate of the kind of rock, and whether it is of volcanic origin. If they are of volcanic origin, could they be sampled for age?

[Bass]

The last photo you posted, if that is a boulder, certainly looks like lahar. The USGS describes " Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments that flows down the slopes of a volcano and typically enters a river valley."- basically a mudflow from a volcano that is usually full of fragments.
Interesting that the volcano is considered "extinct" if there is still magma below and hot springs- maybe "dormant"?
Yes, it can be age dated. The most accurate way to date the rocks is radiometric age-dating, which requires a lab (or university) that specializes in that sort of work, and probably best to have the sample taken by a professional. Depending on the age of the rock, K-Ar might be best.

[GrizzlyBoom]

Theoretically possible. But if you ask who can do it, what firm, I don't know

[skydiamonds (OP)]

Good, thanks. I will contact someone at the largest university here to see if they can help with sample collection and dating.

Yes, the volcano could well be dormant only, given the profusion of hot springs. I presume there is magma below because of the existence of these springs, not because of any other knowledge. As I say, there is no detailed history here so I had hoped some kind of dating on what looks like evidence of previous eruptions might help, given that both kinds of rocks are located besides rivers that originate on the slopes of the ancient volcano.

I'm curious about this personally because the local people tell me that some have been here since the volcano was much larger, and they survived its massive eruption in ancient times. I first assumed this was mythology, but if the dating shows evidence of a major eruption within the last 50,000 years, I suppose this oral tradition could relate an actual geological event.

[Bass]

Let me know what you find...

[bennieAligh]

Just saw a prettig nasty 5 quake of the oregon coast.
Is this volcano in the same area?

[priyateke]

That's great.Thanks for sharing..

[topmaduras]

Interesante foro sobre ciencia

[Colin2B]

Interesante foro sobre ciencia

débil intento de enviarnos un correo electrónico con su sitio web de sexo.  Lo hemos cambiado por el nuestro y te deseamos un buen viaje.

[Monox D. I-Fly]

Hey, I am also an Indonesian. In which mountain was that?