Question about geologic column

[lpetrich]

These divisions were worked out before biological evolution became generally accepted, and they do not require any notion of evolutionary progression.

For instance, bivalves and brachiopods are shelled invertebrates whose shells superficially look much alike. They coexisted from when they emerged in the Cambrian, and they continue to coexist in the present. But brachiopods were very common in the Paleozoic, and not nearly as common afterward. But after the Paleozoic, bivalves became much more common. So one can use brachiopods to mark out Paleozoic time and bivalves to mark out Mesozoic and Cenozoic time. However, bivalves are not descended from brachiopods -- their ancestors invented shells separately.

 

[Loren Pechtel]

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.

Nitpick on his #2: A volcano or impact event can deposit a non-horizontal layer.

 

[Politesse]

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.

I visited his tomb when I was last in Florence. It is covered in notes from geology students and the like wishing for luck in their field studies, and I was amused to note that the sticky notes and so forth were starting to form proper strata themselves, in accordance with his principles. Even original horizontality, though caused by social convention rather than gravity.

 

[lpetrich]

Radiometric dating was worked out in the early 20th century, and it remains the most successful absolute dating method.

Radiometric dating has been done with several radioactive nuclides (proton + neutron combinations), with four main kinds of decay:

• Quantum-mechanical tunneling
  • Alpha decay: emission of He-4 nucleus
  • Spontaneous fission: splitting into approximately equal halves
• Weak elementary-particle interaction
  • Beta decay: emission of electron or positron with a (anti)neutrino
  • Electron capture: with emission of a neutrino

Radiometric dating can be done with different radionuclides, and there is no evidence of relative changes in decay rates. If it happens, then it is below the error bars of the estimated ages, something like 1% or less.

 

[lpetrich]

Another form of dating is with Milankovitch astronomical cycles. These are the result of a combination of our planet's spin precession and orbit precessions. These produce three kinds of cycle:

• Precession - of our planet's perihelion direction through the seasons - about 20,000 years
• Obliquity - interaction of spin and orbit precession to make the Earth's axial tilt vary - about 40,000 years
• Eccentricity - variations in our planet's orbit eccentricity due to overlapping orbit-precession cycles - about 100,000 and 400,000 years

The eccentricity cycle controls the strength of the precession cycle - more eccentricity means more perihelion/aphelion difference, likewise for less.

These cycles are responsible for the comings and goings of continental glaciers across the Pleistocene. When the high-northern summer is hot, the glaciers melt, while when the high-northern summer is mild, the glaciers stay and accumulate.

These cycles extend beyond the Pleistocene, and they have been used to get a date for the base of the Miocene: 23.03 million years. Going back earlier, one can find partial coverage of the geological timescale with cycle-containing sediments, coverage that extends from the Cenozoic into the Mesozoic and Paleozoic and even into the Proterozoic.