Ask a geologist thread

[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.

[Faithfree]

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.

Maybe we should start an "ask a chemist" thread.

The accretion disc is supposed to have formed from the relics of earlier supernovas and thus contained a smattering of heavier elements. Most of the disc was still H and He but those heavier elements that were present got sorted in such a way as to form the rocky planets (I remember there is a good explanation for this). Earth is actually around 45% oxygen; apart from the atmosphere and core most of the planet is dominated of oxygen compounds (predominant silicates). The little bit of Hydrogen that exists on our planet has efficiently bonded itself to any free O or C atom it can find. The free O in our current atmosphere is there curtsey of life on Earth; if all life went extinct tomorrow, free oxygen would eventually cease to exist on the planet.

Not sure if that answered the question.

[Geoff]

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.

Maybe we should start an "ask a chemist" thread.

The accretion disc is supposed to have formed from the relics of earlier supernovas and thus contained a smattering of heavier elements. Most of the disc was still H and He but those heavier elements that were present got sorted in such a way as to form the rocky planets (I remember there is a good explanation for this). Earth is actually around 45% oxygen; apart from the atmosphere and core most of the planet is dominated of oxygen compounds (predominant silicates). The little bit of Hydrogen that exists on our planet has efficiently bonded itself to any free O or C atom it can find. The free O in our current atmosphere is there curtsey of life on Earth; if all life went extinct tomorrow, free oxygen would eventually cease to exist on the planet.

Not sure if that answered the question.

Kinda supports my point, really. Any free oxygen at the start would combine with other elements more readily than with H (just mixing the two gases won't work as there's a thermodynamic energy barrier to overcome).

[Faithfree]

Kinda supports my point, really. Any free oxygen at the start would combine with other elements more readily than with H (just mixing the two gases won't work as there's a thermodynamic energy barrier to overcome).

There was no "long lasting" free oxygen at the start, but I'll leave it up to any chemists to explain how and why small proportions got partitioned into H-O vs Si-O relationships, other than the H was there.

[Thinking Aloud]

Going back to the accretion stage, there is also a pretty well supported hypothesis* that very early in the planet's history, it was struck by something about the size of Mars, tearing a whopping big chunk out of the planet and sending it into orbit. The planet re-coalesced, as did the chunk, which formed the Moon.

*From memory this is supported by the chemical composition of the Moon, which is very similar to that of Earth, suggesting it came from the same source, rather than a separate initial coalescing.

[Geoff]

Going back to the accretion stage, there is also a pretty well supported hypothesis* that very early in the planet's history, it was struck by something about the size of Mars, tearing a whopping big chunk out of the planet and sending it into orbit. The planet re-coalesced, as did the chunk, which formed the Moon.

*From memory this is supported by the chemical composition of the Moon, which is very similar to that of Earth, suggesting it came from the same source, rather than a separate initial coalescing.

Yep, I remember reading about that somewhere. I think they also worked out that it fitted with the angular momentum of the Earth-Moon system, as another supporting datum.

[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?

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.

Krauss mentioned something in his book ATOM about the amount of Oxygen expected in the accretion period of the Earth to be much less than we see today and uses the idea of massive snowballs as a means to compensate for the extra water we have observed.

On another note about planet formation, I understand that our sun is a second or third generation star, during the previous supernovae could magnetism in the initial stages of the explosion and molecular density be responsible for the larger quantity of heavier elements closer to the sun since the forces of magnetism and gravity would work to slow them down more than the lighter / non metallic elements. Also expanding on this is it then possible for the distances of certain planets to be more predictable than we currently believe? like the same predictability we have of different bands of clouds in the atmosphere. Not sure if I've worded that last bit as well as I should have.

[ScholasticSpastic]

I'm working on tackling the question of water w/out snowballs in the Chemistry Thread.

[Faithfree]

Going back to the accretion stage, there is also a pretty well supported hypothesis* that very early in the planet's history, it was struck by something about the size of Mars, tearing a whopping big chunk out of the planet and sending it into orbit. The planet re-coalesced, as did the chunk, which formed the Moon.

*From memory this is supported by the chemical composition of the Moon, which is very similar to that of Earth, suggesting it came from the same source, rather than a separate initial coalescing.

Yep, I remember reading about that somewhere. I think they also worked out that it fitted with the angular momentum of the Earth-Moon system, as another supporting datum.

I think this is still the prevailing hypotheses for the origin of the Moon. There are several other lines of evidence, such as the unusually small iron core of the Moon compared to the other rocky inner planets. Depending on the angle of impact, the debris that formed the Moon would be composed mainly of the silicate mantle and crust of the proto-Earth and impactor, hence be depleted in iron that had already sunk to form the cores of those bodies. I recall there are also some isotopic arguments, but would have to go look up the details.

[Taryn]

How do the flint layers form within chalk?

This is an example of what I mean, these are the Dover cliffs in Kent, UK, where I have been fossil hunting.

[Faithfree]

How do the flint layers form within chalk?

This is an example of what I mean, these are the Dover cliffs in Kent, UK, where I have been fossil hunting.

Good question Taryn.

Chalk, a type of limestone, is made predominantly from the calcium carbonate skeletons of microscopic marine creatures that accumulate on the sea floor. Also present in less abundance are microscopic silica skeletons of such organisms as radiolarians, diatoms and the fine needle-like spicules from silica sponges.

After burial by successive layers of sediment the process of diagenesis starts, whereby some of the chemical components of the original sediment dissolves in trapped fluids, which then move to allow re-precipitation of the material elsewhere. Re-precipitation of calcium carbonate acts to cement all the grains together to form a rock – chalk. Dissolved silica tends to re-precipitate in isolated ‘nodules’ or layers to form flint. Sometimes it forms within or around fossils, or fills old burrows, replacing the original calcium carbonate sediment that was there. It picks out layers or locations that were originally a little more porous, or had some slightly different chemical properties that facilitate silica precipitation.

Flint is simply composed of the mineral quartz (SiO2), but in a form characterised by an interlocking aggregate of microscopic crystals. A more general term for this type of quartz is chert, flint being the ‘common name’ for the variety of chert found in chalk and similar limestone. Chert can form in a number of different ways, what I describe above is specific to chalk situation you describe.