Ask a geologist thread

[Gawdzilla Sama]

Can you lend me $100?

No

Oh, go on, it's not real money anyway.

[cowiz]

Can you lend me $100?

No

Meanie :pissed:

[Psi Wavefunction]

I'll start with a simple one, before Gawd gets to it:

How did the Earth come to be as it is now? How was it formed?

The creationists have it so easy - just turn the Genesis 1 and no further brain exercise is required.

Ok executive summary of Earth history:
Earth accreted from the condensation of a rotating solar nebular, simultaneously with the other planets, about 4.5 billion years ago. Initially very hot with frequent volcanism and impacts from space and in the final stages of accretion. After a few hundred million years the temperature dropped to the point where liquid oceans formed - a Goldilocks planet rife for the appearance of life. Life probably started before 3.5 billion years ago (controversial) and by 2.5 - 2 billion years ago (plus or minus quit a bit) had changed the Earth to an oxygen bearing planet rather like today. Animals emerged around 600 million years ago and have infested the Earth ever since.

Bacteria disagree. And they have 99% of the votes...

[ScholasticSpastic]

Bacteria disagree. And they have 99% of the votes...

<image snipped for easier reading>

Probably referring to multicellular eukaryotic life with the 600Myr thing.

[Psi Wavefunction]

Bacteria disagree. And they have 99% of the votes...

<image snipped for easier reading>

Probably referring to multicellular eukaryotic life with the 600Myr thing.

Wouldn't call it an 'infestation' though! Not that significant, we are

[Trolldor]

I is cannot see image...

[Psi Wavefunction]

I is cannot see image...

Right click and select 'view image'

[Faithfree]

Bacteria disagree. And they have 99% of the votes...

<image snipped for easier reading>

Probably referring to multicellular eukaryotic life with the 600Myr thing.

Wouldn't call it an 'infestation' though! Not that significant, we are

I'm not forgetting bacteria and their relatives. They are of course the organisms I refer to appearing by at least 3.5 billion years ago and effectively 'terraforming' the planet long before animals and plants ever appeared, and remain by far the most invasive and voluminous organisms on the planet.

[Psi Wavefunction]

I'm not forgetting bacteria and their relatives. They are of course the organisms I refer to appearing by at least 3.5 billion years ago and effectively 'terraforming' the planet long before animals and plants ever appeared, and remain by far the most invasive and voluminous organisms on the planet.

Right, I shouldn't be so critical -- you're a geologist, and therefore still view biology in some sort of perspective. Those zoologists, on the other hand... :twisted:




Our botany dep't logo is a stylised phylogenetic/botanical tree with "never mind the animals" underneath Love wearing the t-shirt when wandering around the zoology hallways

Anyway, I will unhijack your thread now...


Question: In this place in northern California, about 45min drive south of San Francisco, is a beach with a very flimsy/flaky cliff. You can literally dig into the cliff with your bare hands, pulling out chunks of highly compressed sediment. There's a certain 'line' of what appears to be fossilised bivalve shells -- they're brownish in colour, and some appear to consist of something other than calcium carbonate. How likely are they to actually be fossils, as opposed to random shells stuck in very old sediment -- where is the line between a shell in a rock, and a fossil?

Also, approx. how long would it take for the shell to become a fossil? Or do they tend to retain their CaCO3 nature throughout the ages?

[Faithfree]

Question: In this place in northern California, about 45min drive south of San Francisco, is a beach with a very flimsy/flaky cliff. You can literally dig into the cliff with your bare hands, pulling out chunks of highly compressed sediment. There's a certain 'line' of what appears to be fossilised bivalve shells -- they're brownish in colour, and some appear to consist of something other than calcium carbonate. How likely are they to actually be fossils, as opposed to random shells stuck in very old sediment -- where is the line between a shell in a rock, and a fossil?

Also, approx. how long would it take for the shell to become a fossil? Or do they tend to retain their CaCO3 nature throughout the ages?

If they are actually in the rock, ie the same age as it, no matter if their composition has changed or not, I'd call them a fossil. Depending on the chemical conditions in the rock, calcium carbonate can survive for a very long time, eg. there are plenty of examples of calcium carbonate shells surviving from the Cambrian Period when such shells first became abundant. Calcium carbonate shells can either be secreted as aragonite (more common) or calcite, just different crystal structures of CaCO3. Aragonite is less stable and more readily dissolves, or in time converts to more stable calcite.

I have seen plenty of cases where shells turn brownish, while remaining calcium carbonate, especially if the calcium carbonate was originally aragonite - most bivalves are aragonite, except oysters and scallops. The easiest way to tell if its still calcium carbonate, rather than replaced by another mineral, is a drop of weak HCL (~10% ). If calcium carbonate it will fizz vigorously. Shells replaced by phosphate can also be brownish, but won't fizz.

The answer to how long it takes to form a fossil is - it depends on which definition you take. In sediments dominated by calcium carbonate particles it is possible for solid rock to form on a human time scale (this excites the creationists), while sediments lacking cementing agents and not buried deeply can remain unconsolidated for tens or even potentially hundreds of millions of years. For this reason some have sort to set an arbitrary boundary of 10, 000 years before something can be called a fossil.

[leo-rcc]

Okay, this question is going to be very stupid probably but for some reason it came up over lunch. As there are layers and layers of sediment deposited over the Earth, does that mean the Earth is getting larger, or is everything shifting below the tectonic plates?

[Faithfree]

Okay, this question is going to be very stupid probably but for some reason it came up over lunch. As there are layers and layers of sediment deposited over the Earth, does that mean the Earth is getting larger, or is everything shifting below the tectonic plates?

The Earth is slowly accumulating dust and larger meteorite fragments from space (a slow drizzle of the process that formed it in the first place), but this increase is utterly insignificant. Earth effectively stays the same size. There is a constant exchange of material between the mantle and crust. Mantle material comes to the surface at mid-ocean ridges and other volcanically active sites, and such material, including some sediments, gets returned to the mantle at subduction zones. The interchange does not seem to be equal, such that it is widely considered that the proportion of continental crust is expanding overtime. However, the size of the Earth remains effectively the same, it is just the segregation of the Earth's light elements into the continental crust, versus heavies in the mantle that is progressing with time.

There are a very small number of nuts geologists that still believe in the "expanding Earth" hypothesis, but they are progressively dying of old age.

[The Curious Squid]

I'll start with a simple one, before Gawd gets to it:

How did the Earth come to be as it is now? How was it formed?

The creationists have it so easy - just turn the Genesis 1 and no further brain exercise is required.

Ok executive summary of Earth history:
Earth accreted from the condensation of a rotating solar nebular, simultaneously with the other planets, about 4.5 billion years ago. Initially very hot with frequent volcanism and impacts from space and in the final stages of accretion. After a few hundred million years the temperature dropped to the point where liquid oceans formed - a Goldilocks planet rife for the appearance of life. Life probably started before 3.5 billion years ago (controversial) and by 2.5 - 2 billion years ago (plus or minus quit a bit) had changed the Earth to an oxygen bearing planet rather like today. Animals emerged around 600 million years ago and have infested the Earth ever since.

In this post you mention that the Earth eventually cooled to a point where liquid oceans could form, was the water always there or did we aquire it from the snowball effect believed by some astronomers like Lawrance Krauss?

[Faithfree]

I'll start with a simple one, before Gawd gets to it:

How did the Earth come to be as it is now? How was it formed?

The creationists have it so easy - just turn the Genesis 1 and no further brain exercise is required.

Ok executive summary of Earth history:
Earth accreted from the condensation of a rotating solar nebular, simultaneously with the other planets, about 4.5 billion years ago. Initially very hot with frequent volcanism and impacts from space and in the final stages of accretion. After a few hundred million years the temperature dropped to the point where liquid oceans formed - a Goldilocks planet rife for the appearance of life. Life probably started before 3.5 billion years ago (controversial) and by 2.5 - 2 billion years ago (plus or minus quit a bit) had changed the Earth to an oxygen bearing planet rather like today. Animals emerged around 600 million years ago and have infested the Earth ever since.

In this post you mention that the Earth eventually cooled to a point where liquid oceans could form, was the water always there or did we aquire it from the snowball effect believed by some astronomers like Lawrance Krauss?

Water, or more correctly the elements to form water, should have been present in the original mix of particles and planetesimals that accreted to form the early Earth, and such elements have continued to be added at a much slower rate, eg by the rare impact of comets. I'm not familiar with the ideas of Lawrance Krauss, but I think most people in this field think that the majority of water derives from the components of the initial accretion. Exactly when the first oceans formed is a matter of some debate.

[Geoff]

I'll start with a simple one, before Gawd gets to it:

How did the Earth come to be as it is now? How was it formed?

The creationists have it so easy - just turn the Genesis 1 and no further brain exercise is required.

Ok executive summary of Earth history:
Earth accreted from the condensation of a rotating solar nebular, simultaneously with the other planets, about 4.5 billion years ago. Initially very hot with frequent volcanism and impacts from space and in the final stages of accretion. After a few hundred million years the temperature dropped to the point where liquid oceans formed - a Goldilocks planet rife for the appearance of life. Life probably started before 3.5 billion years ago (controversial) and by 2.5 - 2 billion years ago (plus or minus quit a bit) had changed the Earth to an oxygen bearing planet rather like today. Animals emerged around 600 million years ago and have infested the Earth ever since.

In this post you mention that the Earth eventually cooled to a point where liquid oceans could form, was the water always there or did we aquire it from the snowball effect believed by some astronomers like Lawrance Krauss?

Water, or more correctly the elements to form water, should have been present in the original mix of particles and planetesimals that accreted to form the early Earth, and such elements have continued to be added at a much slower rate, eg by the rare impact of comets. I'm not familiar with the ideas of Lawrance Krauss, but I think most people in this field think that the majority of water derives from the components of the initial accretion. Exactly when the first oceans formed is a matter of some debate.

I've forgotten most of my chemistry, but I don't see how that would work - there would have been little or no free oxygen in the original accretion disc (mostly H and He, plus solids), and what there was would combine more readily with other elements (silicon, carbon, iron...) than with hydrogen.