h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Given that the 'Earth' article is now to be found by following the chain:
'Top / Everything / Science, Technology & Medicine / Earth Sciences'
from the main page, I thought I'd try and start a proper Geology / Earth Science discussion, in case anyone else is interested in the planet itself.

For anyone out there who watched and liked the TV series 'Earth Story', and maybe bought the book, but still wants to learn more, I heartily recommend reading the book 'Stepping Stones' by Stephen Drury.
It covers the subject of how the Earth has developed over the last 4.5 billion years in greater detail, but doesn't rely on a massive amount of scientific knowledge. I doubt you could find a more comprehensive book that's anything like as accessible. Much of what I've read elsewhere fits together so much better now.

The number of immense upheavals the planet has undergone is truly awe-inspiring, as is the profoundly intimate two-way connection between life and geology. It really puts humanity in its proper perspective.

The only thing about this book I didn't like was the realisation that there isn't much chance of a sequel. If you like geology, you really should buy this book, or get your local library to get a copy.

Anyone with any suggestions of other reading, Earth Science questions, or other comments on geology, please reply.

Well, I am of course quite familiar with the geoloy of my home planet (Zlob) but not at all familiar with that of the Earth.

I did see a picture of a cross-section in a child's geography book showing a volcano as having its source at the centre of the earth. This is not the case on Zlob, where the centre of the planet, although liquid, is made up mostly of iron, and not the MgFeSi one finds in volcanoes.

Is it different for you on earth?

On earth, the origin of the material erupted through volcanoes is the mantle, the semi-solid (it flows, but very slowly) material between the solid crust, and the iron core.

The liquid magma arises from partial melting of the mantle, either due to heat travelling outwards from the core, a drop in pressure, or possibly (I think) a change in the concentration of dissolved water, or any combination of all three.

The composition, viscosity and dissolved gas content of the magma varies from volcano to volcano, and differs from the composition of the mantle, as some mantle components melt more easily than others. These components are over-represented in the mamga resulting from the partial melting.

Much of the magma produced on Earth is generated relatively close to the top of the mantle, often being formed from the melting of water-rich old oceanic crust as it plunges down into the hot mantle at ocean-edge subduction zones, but it is possible that some serious melting may also occur deep down, in the region of the mantle-core boundary.

If you want to learn *any* Earth science, I'd recommend that you steer well clear of children's books, as the ones that aren't oversimplified are probably wrong anyway.

In the interests of trying to restart this discussion, I'd like to quey a couple of points that Potholer made.

Geologists are unsure as to the origin of some magmas, that is, they are not all dehydration melts of subducted oceanic crust e.g rift systems and flood basalts.

With regard to books, be wary of books older than 5 years without revision as the information WILL have changed (as an example, for some of my University research assignments, articles more than 12 months old are out of date!).
One book worth looking at for the interested beginner is "Earth's Dynamic Systems" by Hamblin and Christiansen. It covers most areas in an easy to follow way and is a gentle intro to geological terminology (a separate language in its own right)

I thought there was a general acceptance that flood basalts were caused by large convecting plumes arising from hotspots close to the mantle-core boundary, and that the magma reacing the surface in rift systems at spreading plate boundaries was generated to a significant extent by decompressive melting as the plates on either side move apart (though as the movement of the plates themselves is driven largely by convection, which requires an upflow of (hotter) material along the boundary, it is rather a chicken-and-egg situation.)

However, I suppose we have to remember that it took a long while for Wegener's ideas of continental drift to overcome the orthodox views of the time, so we must always be open to new ideas. What are the current competing theories to the ones above?

Stepping Stones is very recent (late 1999, I think). I noticed a couple of minor errors in diagram labelling, etc early on, but such things are terribly easy to forgive in such a wonderful book.
I guess it might be a little overwhelming for a first-time reader, but would be excellent once someone's got background knowledge from a few more typically popular-science style books.

OK, I think we're both comfortable with the production of basaltic/rhyolitic magmas through decompression melts at divergent plate boundaries; and with the production of andesitic/rhyolitic magmas through the introduction of volatiles (mostly water) at convergent plate boundaries.

*deep intake of breath*

As to flood basalts, shield volcanoes and mantle plumes generally, what their origins are is still vague to say the least. Either way plumes are caused when less dense material starts to rise, and partially melts through a drop in pressure. Why there is this difference in composition, or from how deep it originates is unclear, though possibly from the core/mantle boundary. It is possible that mantle plumes are what drive Plate Tectonics, but that still doesn't solve the chicken and egg scenario. It has been hinted that the Atlantic Ocean might have opened up because a mantle plume found a weakness in continental lithosphere which started forcing plates apart.

*phew*

Ever thought of working on this an assignment within the Faculty of Science at the University?

A good beginner's guide to Earth's interior is "Naked Earth: the New Geophysics" by Shawna Vogel.

Dear Potholer,

Perhaps you can answer a question (probably very stupid) to which I have never heard the solution. When mountains such as the Alps are formed, can you see it happening? How long does it take? If continental drift is slower than human fingernail growth, did the Matterhorn go up a centimetre a year or whatever? Or was there an immense amount of pressure built up and it suddenly shot up in an earthquake? If not, why are the mountains so pointy?

The 'pointiness' is pricipally a result of erosion, rather than being caused directly by folding or uneven uplift of the earth's surface. If I remember correctly, a very large thickness of material (something of the order of a couple of kilometres) has been removed from the alps by erosion since their formation.

You can't really *see* uplift happening, but I believe it is possible to measure the rising of mountain ranges via satellite-based radar. I don't know the exact figures for the rate of uplift, but I would suppose they are in the same ballpark as those for sideways continental drift. There can be some more sudden localised effects, when faults slip, but they could just as easily result in some areas going down as lifting up

In addition, in Europe at least, there are still some significant vertical movements resulting from the end of the last Ice Age, with land in Scandinavia rising, and land in Southern England and similar latitudes falling, as mantle rock which was forced from under the glaciers by the weight of ice slowly flows back to its original position. I believe in some areas that the resulting uplift from this process can be several centimetres per year, and could be seen, at least over the course of a human lifetime, by observing the change of sea-level relative to the land.

A new study published in the journal "Geology" suggested that plumes were rising from only 200-600 kilometers below the lithosphere, rather than from the mantle/core boundary.

I would have thought that erosion by wind would make mountains more rounded and smooth, not pointy. Why would this not be so?

Eventually erosion wears mountains down to rounded hills, but early erosion of up-lifted blocks tends to make sharp cuts in the rock. If you pour water on damp sand, the water will make a v-shaped cut into the sand; rivers do the same work on mountains. Rivers of ice, or glaciers, tend to make sharp cuts rock as well. Take a sharp ridge, for examlple. If glacial ice moved through the valleys on both sides of the ridge, then the rock was cut away on both sides, leaving a knife-like ridge called an arete. A horn (like the Matterhorn in Switzerland or Triple Divide Peak in Montana) is formed when glaciers cut away at several sides of a mountain.