Earth's crust

[RedFox]

I wonder if Earth having continental and oceanic crust was the result of the collision that created the Moon. Otherwise, it'd have a shell of one kind of crust. Did anyone figure out anything about the origins of the crust's present makeup? Could it have both kinds of crusts without such collision?

[Godless Dave]

I wasn't aware there were two kinds of crusts.

[The Lone Ranger]

Probably not. Venus and Mars both appear to have "oceanic" and "continental" crusts similar to the Earth's, but neither planet has a moon remotely like ours. (Venus has no moon, and Mars' two small moons are almost certainly captured asteroids.)

That having been said, that the continental crusts (indirectly) resulted from collision events isn't an outrageous claim. Chemically, the continental crusts appear to be derived from material that erupted from within the Earth's mantle. One leading hypothesis is that early in the Earth's history, when it was still almost completely molten, asteroid strikes punched through the thin crust and the material that erupted outward from the mantle and spread over the surface formed the continental crust.

Cheers,

Michael

[RedFox]

Thanks. :D
I read somewhere about structures like spreading ridges being found somewhere on Venus. Venus's crust is thick and may melt once in a while. Does the lack of oceans have anything to do with it? Does the water lubricate Earth's plate tectonics?
Mars is smaller and cooled down sooner and doesn't have plates. The hot spots keep piling up stuff to make big volcanoes where Earth has chains because of plate movement over hot spots. Do the hot spots themselves have their own movements? :chin:

[The Lone Ranger]

Thanks. :D
I read somewhere about structures like spreading ridges being found somewhere on Venus. Venus's crust is thick and may melt once in a while. Does the lack of oceans have anything to do with it? Does the water lubricate Earth's plate tectonics?
Mars is smaller and cooled down sooner and doesn't have plates. The hot spots keep piling up stuff to make big volcanoes where Earth has chains because of plate movement over hot spots. Do the hot spots themselves have their own movements? :chin:

I'm reasonably certain that Earth's oceans have nothing to do with the phenomenon of plate tectonics. The Moon does, though. Earth wouldn't be quite so geologically active if it weren't for tidal interactions with the Moon generating a little extra internal heat. (An extreme example of this phenomenon is Jupiter's moon Io.) If Venus had a moon the size of ours, chances are good that it'd still be geologically active too.

I was taught that the magma plume beneath a "hot spot" seems to be relatively stable, and that the reason for island chains is that as the oceanic plate moves over the stationary plume, magma occasionally punches through and erupts to make a volcano -- which may grow to sufficient size to form an island. By the next time the magma punches through, the plate may have moved some distance, so the next eruption may make a new island. And so on, and so on.

There are "hot spots" beneath continental crust here and there too. (There's one under Yellowstone, for instance.) But since continental crust is much thicker than oceanic crust, breakthroughs are much less common.

Cheers,

Michael

[Bluenose]

.... Mars is smaller and cooled down sooner and doesn't have plates.

As of our present knowledge. :D

The "facts" about Mars and other bodies in the Solar System keep accumulating and the data yields ever better views of the "big" picture.

Plate techtonics is less than a hundred years old and I figure that we will modify much of our present view, just as we have had to modify so much of what we assumed in the last 50 years. :yup:

[Dragoon]

Although I majored in Geology and am presently taking geology courses at the local college (post retirement), I claim no expertise in plate techtonics. Still, I would like to say a couple of things pretty much off the top of my head.

First, there is a profound difference between continental and oceanic crust. Oceanic crust is generally much younger and is made of the much more dense ferromagnesiumsilicate rocks such as olivene basalt. The continental crust is made of accumulations of much lighter aluminosilicate rocks.

Because of their density, continental rocks "float" much higher on the fluid part of the mantle (called the athenosphere) than does the heaver oceanic crust. Since water has to go somewhere, it will naturally accumulate above the lowest crustal members, the ocean crust, leaving the continents "high and dry."

Like the stitching on a baseball, ridges circle the globe. It is no accident that most of these ridges are in the middle of oceans because these are where new crust is constantly upwelled by convection currents driven by heat within the earth.

The new, heavy ocean crust, rich in ferromagnesiumsilicates spread out from these areas of forced upwelling and spread out on either side of the ridges. This new crust pushes older ocean crust outward and the whole thing moves as a huge plate.

Since there is new ocean floor constantly being created, old ocean floor must somehow be recycled, so at the boundary of plates and especially at the edges of lighter continental rocks, the old plates plunge down underneath carrying with them water and some lighter materials with them. That, by the way, is why the ocean crust is younger (<60 MY) than most continental crust (some of which goes way, way back to long before the Cambrian).

Some of the lighter material gets scraped off the plunging plate and accumulates at the margins of the continents while some of the lighter aluminosilicate stuff carried down along with water melts at a great depth. This hot water impregnated material migrates up (being hotter and lighter than the surrounding rock) to form rhyolitic and andecitic mountain ranges (like the Andes and Cascades) and the granite "plutons" that form the cores of new mountain ranges inland from the margin of the continent. Thus is new continental material produced and the continents kept above sea level.

There is no need to involve the moon in the formation of either the continental crust or the oceanic crusts. As a matter of fact, the leading theory seems to indicate that the moon was formed by a mars-sized object (not mars itself) hitting the very early earth and causing a blob of material to be ejected later to form the moon.

OK, that, in a brief nutshell and without illustrations, is how I remember it.

By the way, If you have a heat source within a planet to cause convective currents resulting in spreading centers and plates, you don't need water to have the equivalent of "oceanic basins," Without water, geologic processes like mountain building by rhyolitic volcanism and intrusion of granitic batholyths would not work so well if at all, I would think. Still you very well could have young active basins of heavy density rock and higher, older rocks of lighter aluminosilicates forming continents surrounded by dry "seas."

:popcorn: