Geology and time tables

[YoursTrue]

An eruption on La Palma caught my interest. So a volcano erupted and lava spilled out. Sorry for the residents. La Palma volcano: Visual guide to what happened - BBC
So the question is: since it's obvious eruptions have been happening over the centuries, what about dating methods with accumulated sediment going over from the depth of the volcano? Wouldn't those later sediments be older than the lower strata? Possibly much older? This question is hopefully for those who are trained in these sciences. (Thanks.)

 

[Secret Chief]

An eruption on La Palma caught my interest. So a volcano erupted and lava spilled out. Sorry for the residents. La Palma volcano: Visual guide to what happened - BBC
So the question is: since it's obvious eruptions have been happening over the centuries, what about dating methods with accumulated sediment going over from the depth of the volcano? Wouldn't those later sediments be older than the lower strata? Possibly much older? This question is hopefully for those who are trained in these sciences. (Thanks.)

The younger sediments (igneous) sit on top (unless folding has occured) of the older (presumably) sedimentary strata. Relative dating is done by such stratigraphic principles. Fossils feed into this. Absolute dating is by radiometric means.

(I took geology at college...during the Devonian).

 

[Polymath257]

An eruption on La Palma caught my interest. So a volcano erupted and lava spilled out. Sorry for the residents. La Palma volcano: Visual guide to what happened - BBC
So the question is: since it's obvious eruptions have been happening over the centuries, what about dating methods with accumulated sediment going over from the depth of the volcano? Wouldn't those later sediments be older than the lower strata? Possibly much older? This question is hopefully for those who are trained in these sciences. (Thanks.)

Yes.

And this is verified by radiometric dating techniques. One caution is that high temperatures can 'reset' the radioactive clocks in some cases (by releasing accumulated gases, for example). But it is usually clear when that happens.

Another caution is that rocks included in the lava that are not melted (and whose clocks are not 'reset') may well gives dates for their original formation, not he date of the eruption. Again, this is something that is easily tested for.

 

[exchemist]

An eruption on La Palma caught my interest. So a volcano erupted and lava spilled out. Sorry for the residents. La Palma volcano: Visual guide to what happened - BBC
So the question is: since it's obvious eruptions have been happening over the centuries, what about dating methods with accumulated sediment going over from the depth of the volcano? Wouldn't those later sediments be older than the lower strata? Possibly much older? This question is hopefully for those who are trained in these sciences. (Thanks.)

If you are talking about sediments, i.e. sedimentary rocks, found in lava, these obviously would not themselves be volcanic. However if the volcano lies above sedimentary rocks, then there could be some present in the lava, torn from the sides of the conduit through which the magma forces its way to the surface. One might then expect a mixture of ages, depending on the depth from which they have come.

But I have a feeling this may not be exactly what you are driving at. Do you mean, perhaps that magma erupted first would be younger than what is erupted last, on the basis that it erupts from the top of the magma chamber first? I am not sure this is necessarily true, since magma chambers can be filled fairly rapidly in geological terms. Also as it is liquid, it is not generally in layers. However what you can get is partial crystallisation inside the magma, so that lower melting point material erupts first and then later you get a sort of sludge of partly solidified stuff that has settled out in the chamber. But this kind of stratification would not be related to age but to melting point.

Above is what I think is the case.... I stand to be corrected by any geologists here.....

 

[ecco]

So the question is: since it's obvious eruptions have been happening over the centuries, what about dating methods with accumulated sediment going over from the depth of the volcano?

Now what?
Now that there have been several answers that appear to be grounded in science, where are you going to go? We I know you didn't ask the question because you actually wanted to gain any knowledge.

 

[YoursTrue]

The younger sediments (igneous) sit on top (unless folding has occured) of the older (presumably) sedimentary strata. Relative dating is done by such stratigraphic principles. Fossils feed into this. Absolute dating is by radiometric means.

(I took geology at college...during the Devonian).

Ha. (about when you took geology.) OK, I'm still wondering, but thanks for your answer.

 

[YoursTrue]

If you are talking about sediments, i.e. sedimentary rocks, found in lava, these obviously would not themselves be volcanic. However if the volcano lies above sedimentary rocks, then there could be some present in the lava, torn from the sides of the conduit through which the magma forces its way to the surface. One might then expect a mixture of ages, depending on the depth from which they have come.

But I have a feeling this may not be exactly what you are driving at. Do you mean, perhaps that magma erupted first would be younger than what is erupted last, on the basis that it erupts from the top of the magma chamber first? I am not sure this is necessarily true, since magma chambers can be filled fairly rapidly in geological terms. Also as it is liquid, it is not generally in layers. However what you can get is partial crystallisation inside the magma, so that lower melting point material erupts first and then later you get a sort of sludge of partly solidified stuff that has settled out in the chamber. But this kind of stratification would not be related to age but to melting point.

Above is what I think is the case.... I stand to be corrected by any geologists here.....

Thanks for asking. I did not take geology and am only beginning to get interested in these things. Because mainly of the time-dating procedures. So let's say you're correct--that the eruption of magma from the top of the chamber, stuff (?) comes after the top erupts, and likely it is mixed from top to whatever comes under that. So let's say that's true. Unless it's not true, but it sounds logical. Who knows anyway when that top layer in the volcano was filled. (Not sure if filled is the right word...now I'll have to start reading about volcanoes.) But! the idea is that the soil outside the volcano within who knows how many miles changes. And then floods do happen. So -- this in relation to the dating process.

 

[YoursTrue]

If you are talking about sediments, i.e. sedimentary rocks, found in lava, these obviously would not themselves be volcanic. However if the volcano lies above sedimentary rocks, then there could be some present in the lava, torn from the sides of the conduit through which the magma forces its way to the surface. One might then expect a mixture of ages, depending on the depth from which they have come.

But I have a feeling this may not be exactly what you are driving at. Do you mean, perhaps that magma erupted first would be younger than what is erupted last, on the basis that it erupts from the top of the magma chamber first? I am not sure this is necessarily true, since magma chambers can be filled fairly rapidly in geological terms. Also as it is liquid, it is not generally in layers. However what you can get is partial crystallisation inside the magma, so that lower melting point material erupts first and then later you get a sort of sludge of partly solidified stuff that has settled out in the chamber. But this kind of stratification would not be related to age but to melting point.

Above is what I think is the case.... I stand to be corrected by any geologists here.....

(Yes, I'm talking about sediments around and near the volcano, and also that it may have slushed over the years to other areas. In fact, as it stands now as I'm writing this, there is no doubt in my mind that the top layer after the volcano erupted on the ground around there would have sediment from a different timing underneath it. Maybe older?)

 

[YoursTrue]

Yes.

And this is verified by radiometric dating techniques. One caution is that high temperatures can 'reset' the radioactive clocks in some cases (by releasing accumulated gases, for example). But it is usually clear when that happens.

Another caution is that rocks included in the lava that are not melted (and whose clocks are not 'reset') may well gives dates for their original formation, not he date of the eruption. Again, this is something that is easily tested for.

Could be easily tested you say. I'd be interested in looking into that point as I keep reading about footprints taken from soil and other artifacts found in soil, topsoil or beneath that. I'm beginning to the think that dating of soil can be very off in terms of the effect it has on bones and other objects around it. So the question is: what happens to the magma when it erupts after a long while when it covers the ground? Does it solidify and become soil?

 

[YoursTrue]

The younger sediments (igneous) sit on top (unless folding has occured) of the older (presumably) sedimentary strata. Relative dating is done by such stratigraphic principles. Fossils feed into this. Absolute dating is by radiometric means.

(I took geology at college...during the Devonian).

OK, re-read your nice answer. So let me start. The younger sediments (igneous) as you relate above sit on top of the ground that the magma spills onto?

 

[Polymath257]

Could be easily tested you say. I'd be interested in looking into that point as I keep reading about footprints taken from soil and other artifacts found in soil, topsoil or beneath that. I'm beginning to the think that dating of soil can be very off in terms of the effect it has on bones and other objects around it. So the question is: what happens to the magma when it erupts after a long while when it covers the ground? Does it solidify and become soil?

Well, the lava will cool and become hard, yes. This is igneous rock.

Over time, that rock will erode because of the action of water, wind, and repeated cycles of heating and cooling. This produced smaller pieces, some the size of sand grains. These tend to get moved by water and air to other locations, producing sedimentary deposits. because many different igneous rocks will be eroding, sedimentary deposits tend to be a mixture from different sources. Over time, those sedimentary deposits will harden also and become sedimentary rock.

Soil is the stage where you have a mixture of sedimentary deposits and the remains of living things, as well as actual living things that help to 'churn' the soil. Worms, for example, do a lot to keep the soil loose.

The effect on bones and objects depends on the specific mixture in the soil: is it acidic? basic? what minerals are in it? etc.

But this won't have a major effect on radioactive dating methods. Chemistry, pressure, temperature, etc are all irrelevant to radioactive decay (unless you get to really high temperatures or pressures like in the center of stars).

Do you have any specific questions about how dating can be affected by the environment?

 

[Polymath257]

OK, re-read your nice answer. So let me start. The younger sediments (igneous) as you relate above sit on top of the ground that the magma spills onto?

Well, yes. The lava is spilling out over the ground. That means the lava (and the igneous rock it forms) will be on top.

Now, there are always things that can cause folding, but that is the basic setup.

But, for example, suppose there is a mountain and the lava comes out of the top of the mountain. The lava will cover the ground on the mountain, but because the mountain itself is not all at the same height, neither will the lava (or the igneous rock) be at the same height. So it is quite possible for the 'bottom' layer at the top of the mountain to be 'above' the 'top' layer from the lava at the bottom of the mountain.

 

[Subduction Zone]

An eruption on La Palma caught my interest. So a volcano erupted and lava spilled out. Sorry for the residents. La Palma volcano: Visual guide to what happened - BBC
So the question is: since it's obvious eruptions have been happening over the centuries, what about dating methods with accumulated sediment going over from the depth of the volcano? Wouldn't those later sediments be older than the lower strata? Possibly much older? This question is hopefully for those who are trained in these sciences. (Thanks.)

How would you get younger sediments under older sediments?

There is a way, but it also leaves evidence behind.

 

[Subduction Zone]

There are several different forms of magma that come out of a volcano. Some volcanoes emit mostly "ash". These tend to be volcanoes like Mt. St. Helens. And we know what happened to her. In Hawaii the source of magma is quite different and it is in the form of molten magma that solidifies into basalt. Those volcanoes are much more user friendly, but you still do not want to get in the way of a river of molten magma.

 

[Subduction Zone]

Also one has to be careful dating very recent volcanic depositions. The process of eruption is not exactly smooth. As the magma flows within the Earth it can pick up bits of the rock that it flows through or past. Those pieces often do not melt and their "age" will be much older than the magma. If one does a whole rock dating one will get a date that is much older than it actually is. It does not take a lot of contamination to skew a date so that something new can date as being over a million years old. That is why one needs to know what one is doing when one tries to date a rock.

 

[YoursTrue]

Well, yes. The lava is spilling out over the ground. That means the lava (and the igneous rock it forms) will be on top.

Now, there are always things that can cause folding, but that is the basic setup.

But, for example, suppose there is a mountain and the lava comes out of the top of the mountain. The lava will cover the ground on the mountain, but because the mountain itself is not all at the same height, neither will the lava (or the igneous rock) be at the same height. So it is quite possible for the 'bottom' layer at the top of the mountain to be 'above' the 'top' layer from the lava at the bottom of the mountain.

Can is the lava pushed up from deep in the cone of the volcano?

 

[YoursTrue]

Well, the lava will cool and become hard, yes. This is igneous rock.

Over time, that rock will erode because of the action of water, wind, and repeated cycles of heating and cooling. This produced smaller pieces, some the size of sand grains. These tend to get moved by water and air to other locations, producing sedimentary deposits. because many different igneous rocks will be eroding, sedimentary deposits tend to be a mixture from different sources. Over time, those sedimentary deposits will harden also and become sedimentary rock.

Soil is the stage where you have a mixture of sedimentary deposits and the remains of living things, as well as actual living things that help to 'churn' the soil. Worms, for example, do a lot to keep the soil loose.

The effect on bones and objects depends on the specific mixture in the soil: is it acidic? basic? what minerals are in it? etc.

But this won't have a major effect on radioactive dating methods. Chemistry, pressure, temperature, etc are all irrelevant to radioactive decay (unless you get to really high temperatures or pressures like in the center of stars).

Do you have any specific questions about how dating can be affected by the environment?

Yes. Because when fossils are found embedded in rock, isn't it possible that the fossil itself can be younger than the ground it is embedded in? that is, if there was a volcano some time before the fossil got there*. And the residue of the rock, especially if the fossil is ground into the surface of the dirt, becomes ground into the fossil itself?
*OK, that question might answer itself but please -- you answer.

 

[Secret Chief]

OK, re-read your nice answer. So let me start. The younger sediments (igneous) as you relate above sit on top of the ground that the magma spills onto?

The magma cools and becomes the younger-formed igneous rock layer, sitting above the older layer (which may be sedimentary, igneous or metamorphic).

 

[Secret Chief]

@Subduction Zone
Boy, have you got the right username

 

[YoursTrue]

• The magma cools and becomes the younger-formed igneous rock layer, sitting above the older layer (which may be sedimentary, igneous or metamorphic).

  That's what I thought, thank you, unless of course, some of it leeches into deeper ground. That's another subject I suppose.
• Well, oh well...thanks for answering, I have more questions, but that answer makes sense to me. I'm beginning to realize why I don't understand certain things. It's because (and I've been reading) when a science textbook (such as that on chemicals, chemical reactions, etc.) makes statements that I want to scratch my head about, when I was a student in school, I figured they were somehow telling me something that they really "knew" about, which was the truth and nothing but -- not realizing people (and I'm talking about 'experts') make scientific statements that are really not fact in the absolute sense of the word. Or do not explain. So I could memorize and did well because of that trying in many cases to follow their logic, thinking it's true. Thanks. Now I'm beginning to understand more.