Comments on: Dating an active volcano. And I don't mean metaphorically.

By: Nigel Depledge

Nigel Depledge — Wed, 06 Jun 2012 12:49:34 +0000

The BA said:

If you look at other flows around that volcano, you can almost guess which ones are older by how much greener they are. In fact, relative ages of flows sometimes can be found that way.

Moreover, there are some parts of the world where lava flows overlay one another so their stratigraphy (and hence relative age) is there to see. Just such a set of formations (on Sicily, IIUC) formed the first evidence that the Earth was really and truly old.

In the 17th century (IIUC), a fellow whose name escapes my memory was walking over an old lava flow on Sicily and noticed that the lava was still in the process of being colonised by plant life, whereas other nearby lava flows possessed rich and varied plant life. He deduced that this was an indication that the lava flow upon which he walked was younger. The lava flow upon which he walked was from a documented eruption, so he could determine its age precisely. It had formed from an eruption that occurred when the Romans were invading Sicily, placing the youngest lava flow at approximately 1600 years old.

He went on to document all the lava flows from the volcano and concluded that the Earth was probably at least 100,000 years old – far older than any previous estimate. This story is related in the book Measuring Eternity by Martin Gorst.

By: Georgijs

Georgijs — Sun, 03 Jun 2012 22:40:22 +0000

For some reason I want to call volcanoes as the planets zits. When one pops, watch out!

By: bassmanpete

bassmanpete — Sat, 02 Jun 2012 07:59:01 +0000

Bipedal Tetrapod, I thought the same thing for a second then realised that Phil probably meant hike across the area in the photograph in an hour.

By: NWimby

NWimby — Sat, 02 Jun 2012 06:04:06 +0000

Erik, the northerner is right about the various influences on A’a and Pahoehoe, more to do with deposition environment than chemistry. Not too familiar with Garibaldi, but it looks pretty complicated, chemistry wise: http://www.eos.ubc.ca/~krussell/epapers/KRH_GSL_2002.pdf

By: Randy A.

Randy A. — Sat, 02 Jun 2012 03:41:04 +0000

Lava with an andesitic composition usually create flows with a “blocky” texture.

By: Ron Schott

Ron Schott — Sat, 02 Jun 2012 03:27:16 +0000

I have to agree with Erik. I haven’t been to Garibaldi, but I’ve never seen intermediate composition lavas (andesite or dacite) with what are conventionally described as a’a or pahoehoe morphologies. In my experience those morphologies are restricted to basaltic (+/- basaltic andesite) compositions. If UP North has documentation of this, I’d be very eager to see it.

By: Erik

Erik — Sat, 02 Jun 2012 02:43:22 +0000

I have to disagree with Up North on this one. I’ve never heard of anyone refer to andesite as a’a or pahoehoe – maybe basaltic andesite, which has less silica than andesite – but not andesite. If there are andesitic a’a or pahoehoe at Garibaldi, I’d love to see evidence, but I thought Garibaldi was mostly dacite anyway.

By: UP North

UP North — Sat, 02 Jun 2012 00:02:20 +0000

Sorry, Bob, both a’a and pahoehoe can form from the same lava chemistry. In some places, a pahoehoe flows can turn onto an a’a flow along the flow route due to change in gradient. (I have observed this in Hawai’i). The terms do not indicate a lava type, but the depositional form resulting from complex interplay of source volume, lava temperature and insulation of the overlying cooled layer, gradient, and atmospheric conditions during flow. Andesitic a’a and pahoehoe lavas are prevalent at Mount Garibaldi (a Cascade volcano in Cananda)

At the risk of being yet another nit-picker on this post, you are not likely to get “pumice” resulting from any flow, pahoehoe or a’a. Pumice is formed by violent eruptions throwing liquid lava in the air which cools as the volatiles are being released, but before they can bubble out.

By: Matt B.

Matt B. — Fri, 01 Jun 2012 22:50:41 +0000

@Randy, I know what you mean. I had a classmate in high school that thought all rivers flowed south.

By: Randy A.

Randy A. — Fri, 01 Jun 2012 21:32:15 +0000

Phil, when you said “The big tongue of lava flowing up and to the right is new…”, you really meant to say the down and to the northeast, right?

After all, lava, like all fluids except liquid helium, flows downhill!

It’s a common thing to say “up” when we mean “north”, but it causes endless confusion for young people, and adults too. I recently has a student draw rain clouds on a diagram of the Earth — in the southern hemisphere the student drew rain falling toward the bottom of the page — in other words, it was raining up!

So I encourage you to reserve “up” and “down” for directions away or toward the local center of gravity.