[QUOTE=japelprime;574394]Scientists took the lava under the scope and found out that this is the same type of lava found in older period of time when volcano activity was 6000 - 7000 years ago. It is different type than was coming up 800 years ago. News say this lava is from estimated deep of 17 - 20 km . The lava is Olivine Tholeiitic Basalts type from that time and have not erupted here last 7000 year.[/quote]Thank you for your quick response.

But how did they determine that it [i]was[/i] that long ago? That's what I'm asking. I read that the last eruption on the peninsula was after human settlement, and was a different volcano. So the last eruption of Fagradals Mountain was before that.

[i]Something[/i] is saying that the basalt from the last eruption of Fagradals Mountain is 6000 to 7000 years old, and not 3000 years old or 10000 years old. What?

[QUOTE=Dr Sardonicus;574393]Anybody here know? Thanks![/QUOTE]
Some random guy passed that way when the lava blobs were still hot and inscribed his name and the date on the cool red hot wall.

[QUOTE=LaurV;574403]Some random guy passed that way when the lava blobs were still hot and inscribed his name and the date on the cool red hot wall.[/QUOTE]

:mooc:

[QUOTE=LaurV;574403]Some random guy passed that way when the lava blobs were still hot and inscribed his name and the date on the cool red hot wall.[/QUOTE]Sounds like what I heard some YECs saying...

[b]EDIT: (update)[/b] I did see Páll Einarsson, an emeritus Professor of Geophysics, University of Iceland, named as a source in some of the news reports. I'm guessing he told the reporters how long the volcano had been dormant.

The last eruption on the Reykjanes Peninsula was 781 years ago. That's in recorded history. Some of the news reports muffed that, saying that the Fagradals Mountain volcano (the one currently erupting) had erupted 781 years ago.

[b]Further update:[/b] Professor Einarsson has kindly informed me that the last lava flows around Fagradals before the current eruption were [highlight]dated by C14 and "tephrochronology," the dating of ash flows deposited on top of the lava flows.[/highlight] He also sent me a 20-plus page paper from JÖKULL No. 58, 2008 which looks to be interesting reading.

[QUOTE=Dr Sardonicus;574396]Thank you for your quick response.

But how did they determine that it [i]was[/i] that long ago? That's what I'm asking. I read that the last eruption on the peninsula was after human settlement, and was a different volcano. So the last eruption of Fagradals Mountain was before that.

[i]Something[/i] is saying that the basalt from the last eruption of Fagradals Mountain is 6000 to 7000 years old, and not 3000 years old or 10000 years old. What?[/QUOTE]

It is mostly two parameters. The richness of Magnesium Oxid and Titan in the material. In this case it have the same amount as the old lava. Specially the low quantity of Titan. I suppose it is also matching more to materials in older-history of earth live.

But this tells you only The dept of where the material is coming from. How they know the time ? When geologists digs through the earth layers they are counting the years in the different layers finding this ashes in the layers from that periode of time.

[QUOTE=japelprime;574501]How they know the time ?[/QUOTE]I don't know but can think of several possibilities. Each could be used on either the material in question or material with similar chemistry, in particular with similar isotopic concentrations.

Endogenous methods include measuring the amount of decay products of U and Th since the material was last liquid. Until then the concentrations of each will be independent of the chemical composition of the bulk. Afterwards, the different minerals will (I believe, I don't know) be different in each mineral. That solidification starts the clock ticking. Similarly, magnetism induced by the Earth's magnetic field will be frozen in both direction and intensity at the time of solidification. We have pretty good estimates for where the terrestrial field has been for rather a long time.

Exogenous methods applicable to surface deposits include counting cosmic ray tracks or measuring erosion evidence, whether by rainfall or by abrasion.

There are doubtless other possibilities.