Question about geologic column

[BH]

I have a question about the geologic column. And a radio metric dating. Why do we get older dates for rocks on the bottom of the column and younger ones at the top if the soil composing these layers is simply material blown or washed from other places and these layers are composed of atoms that go back to the beginning whenever it was anyway. Thanks

 

[Jokodo]

I have a question about the geologic column. And a radio metric dating. Why do we get older dates for rocks on the bottom of the column and younger ones at the top if the soil composing these layers is simply material blown or washed from other places and these layers are composed of atoms that go back to the beginning whenever it was anyway. Thanks

Not all atoms "go back to the beginning". That's exactly the basis of radio dating. If we find a lot of thorium in places where due to their chemical properties we only expect uranium, we conclude that it used to be uranium when the rock formed.

 

[BH]

Please be my friend and explain in more detail. Are you saying we should find older elements in only older strata and younger elements only in the top strata,assuming there has been no shifting due to faulting, one section of rock being pushed under another,ect.

 

[skepticalbip]

Please be my friend and explain in more detail. Are you saying we should find older elements in only older strata and younger elements only in the top strata,assuming there has been no shifting due to faulting, one section of rock being pushed under another,ect.

Dating of a rock sample is determined by the ratio of radioactive elements to their decay products. For example, basalt is cooled lava. Any U235 in the rock will be fixed in the matrix. As time passes the Uranium will continually decay building up an increasing amount of lead associated with the remaining U235. Since we know the half life of Uranium, the ratio of U235 to associated lead can tell us how long ago the lava flow cooled to form solid basalt.


ETA:
Here's a Wiki article that likely explains it a bit better:
https://en.wikipedia.org/wiki/Radiometric_dating

 

[BH]

Uranium turns into Thorium. What element did the Uranium used to be?

 

[skepticalbip]

Elements heavier than Hydrogen and some Helium up to Iron were created by fusion in a star's core. Uranium and all elements heavier than Iron were created in a supernova explosion. This explosion scattered the elements into space to become interstellar gas clouds (nebula) which were mostly hydrogen but included the higher elements created by the star and subsequent supernova. A section of this gas cloud eventually collapsed under mutual gravity to become our solar system, including the Earth.

So the Uranium we see today was created through several fusion steps by a massive star that went supernova starting from hydrogen.

ETA:
Here's a short description of nucleogenesis, the process of the creation of the elements:
http://www.dynamicscience.com.au/tester/solutions1/space%20science/nucleogenesis.html

 

[Loren Pechtel]

I have a question about the geologic column. And a radio metric dating. Why do we get older dates for rocks on the bottom of the column and younger ones at the top if the soil composing these layers is simply material blown or washed from other places and these layers are composed of atoms that go back to the beginning whenever it was anyway. Thanks

Dating sedimentary rock doesn't produce information that is of much use most of the time.

Uranium turns into Thorium. What element did the Uranium used to be?

It was created in a supernova or a neutron star. On Earth it's primordial. It's just long enough lived that some of it is still around.

 

[BH]

If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.

 

[skepticalbip]

If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.

It isn't uranium that is used to date rock. It is the ratio of the amount of uranium with respect to the amount of its associated decay products locked in the rock matrix around it.

There is no way to tell how old a sample of uranium is unless all its decay products are kept with it so a ratio of the two can be measured..

 

[Loren Pechtel]

I have a question about the geologic column. And a radio metric dating. Why do we get older dates for rocks on the bottom of the column and younger ones at the top if the soil composing these layers is simply material blown or washed from other places and these layers are composed of atoms that go back to the beginning whenever it was anyway. Thanks

Uranium turns into Thorium. What element did the Uranium used to be?

If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.

Radioactive material does not reveal a date. Rather, it's the ratio of the material to other material in the sample that gives the date. In most cases this is the radioactive material and it's decay product, but you also see things like carbon dating that are based on a known original ratio (the ratio of C-14::C-12 when the material was laid down) and seeing what the ratio is now. We typically identify radioisotope dating methods by naming only the radioisotope, but that doesn't mean it uses only the radioisotope.

 

[BH]

But how can it be different ages in the different strata if it was formed billions of years before the different strata was even formed?

 

[Minnemooseus]

Oversimplified version.

You don't radiometric date sedimentary rocks. You do date igneous rocks. Sedimentary rocks are made of fragments of igneous rocks of various ages.

When an igneous rock crystallizes from a magma, the radiometric clock is set to zero. The radiometric daughter elements (those formed from radioactive decay) are excluded from the initial crystal structure because of chemistry reasons. But the new daughter elements are trapped in the crystals once the igneous rock is solid.

The age of a sedimentary rock (let's call it a sandstone) is the time that it was deposited, the earliest at the bottom, getting younger as you go up. The sandstone is made of of fragments of various aged older rocks, and thus a radiometric dating of the sandstone will be some pretty meaningless date prior to the deposition. About all you can say is that the sandstone is younger than that date.

An old sandstone will be made up of old fragments and will whole rock radiometric date some average of those fragments ages. A younger sandstone will be made up of possibly also younger fragment, and would tend to whole rock radiometric date younger.

I gotta go and get something else done.

Moose

 

[BH]

Okay. That makes sense now that I reflect on it.

 

[barbos]

If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.

They measure how much time has passed since rock was molten. When rock solidifies tiny crystals of pure uranium are grown inside rock. After a while some of the uranium inside crystals decays into other elements but they stay inside the crystals. By measuring amount of these elements you can measure the age of the crystals and rock containing it. Asteroids are dated like that too.

 

[Wiploc]

If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.

They measure how much time has passed since rock was molten. When rock solidifies tiny crystals of pure uranium are grown inside rock. After a while some of the uranium inside crystals decays into other elements but they stay inside the crystals. By measuring amount of these elements you can measure the age of the crystals and rock containing it. Asteroids are dated like that too.

Thanks. That helps.

 

[Bronzeage]

The oldest principle of modern geology is "The rocks at the bottom of the pile are older than the rocks at the top of the pile. The second oldest principle is "Sometimes the pile is on its side and sometimes it's upside down." What we can observe of the Earth's geology is such a very thin layer and what we see actually gives very little clue as to how it got to be what it is.

 

[Politesse]

The oldest principle of modern geology is "The rocks at the bottom of the pile are older than the rocks at the top of the pile. The second oldest principle is "Sometimes the pile is on its side and sometimes it's upside down." What we can observe of the Earth's geology is such a very thin layer and what we see actually gives very little clue as to how it got to be what it is.

Steno's Laws.

 

[Cheerful Charlie]

If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.

Because when Uranium is trapped in a mineral, as it decays, it's daughter elements remain there. Generally. Some will actually decay themselves yielding other daughter elements. Knowing how these elements decay gives us a good measuring stick. Sometimes some minerals trap daughter elements in very stable ways. And are very tough and don't easily change their composition. Zircon for example. So grains of zircon may become part of sedimentary minerals. Such rocks then must be younger than the zircon minerals. Thus volcanic material can be used to date sedimentary materials trapped between them. When zircons form, they stop the clock and start radioactive decay in a consistent manner, trapping their daughter elements that do not leach out.

Historical geology is intensely logical and has built up large catalogs of techniques for dating geological assemblages. Including being aware of how one can be mislead if one is not careful.

 

[lpetrich]

The relative ages of the rocks were determined long before radiometric dating became practical. Those ages were determined with the help of 17th-cy geologist Nicholas Steno's principles of stratigraphy (Steno's Principles of Stratigraphy)

• Superposition: In a sequence of strata, any stratum is younger than the sequence of strata on which it rests, and is older than the strata that rest upon it.
  "...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." Steno, 1669.
• Initial Horizontality: Strata are deposited horizontally and then deformed to various attitudes later.
  "Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon." Steno, 1669.
• Stratum Continuity: Strata can be assumed to have continued laterally far from where they presently end.
  "Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno, 1669
• Cross Cutting: Things that cross-cut layers probably postdate them.
  "If a body or discontinuity cuts across a stratum, it must have formed after that stratum." Steno, 1669\

He also stated:

1. "If a solid body is enclosed on all sides by another solid body, of the two bodies that one first became hard which, in the mutual contact, expresses on its own surface the properties of the other surface."
2. "If a solid substance is in every other way like another solid substance, not only as regards the conditions of surface, but also as regards the inner arrangement of parts and particles, it will also be like it as regards the manner and place of production . . ."
3. "If a solid body has been produced according to the laws of nature, it has been produced from a fluid."

Steno considered some odd structures called glossopetrae or tongue stones that were present in some rocks of Tuscany in Italy (roughly west central Italy). He noticed that they had a strong resemblance to sharks' teeth, and he concluded that they were the teeth of long-ago sharks that got buried in sediment - sediment that got turned into rock and then pushed upward.

 

[lpetrich]

With these principles of stratigraphy, one could work out which order the Earth's crust's rocks had been deposited in, and that was done long before radiometric dating became feasible. Before that, geologists had used estimates from sedimentation rates and the like - much less precise.

The first division of strata was by 18th cy. Italian geologist Giovanni Arduino, who divided the rocks of the southern Alps into Primary, Secondary, Tertiary, and Quaternary ones -- 1, 2, 3, 4. But British geologists of the early 19th cy. went into much more detail, naming many of the stratigraphic divisions that we use today. Primary and Secondary were eventually dropped, though Tertiary and Quaternary survived. I'll summarize our present-day division. The divisions have this hierarchy:

Supereon, eon, era, period, epoch, age, stage

Here they are:

Supereon, eon:

• Precambrian
  • Hadean
  • Archean
  • Proterozoic
• (Phanerozoic)
  • Phanerozoic

Phanerozoic Eon: era, period:

• Paleozoic
  • Cambrian
  • Ordovician
  • Silurian
  • Devonian
  • Carboniferous
  • Permian
• Mesozoic
  • Triassic
  • Jurassic
  • Cretaceous
• Cenozoic
  • Paleogene
  • Neogene
  • Quaternary

Paleogene + Neogene = Tertiary

Cenozoic Era: period, epoch

• Paleogene
  • Paleocene
  • Eocene
  • Oligocene
• Neogene
  • Miocene
  • Pliocene
• Quaternary
  • Pleistocene
  • Holocene