Why doesn't the earth's solid crust sink into the liquid mantle?

Question

Presumably the earth's crust is just cooled solidified mantle. Since a substance in its solid state is generally more dense than in its liquid state, shouldn't the crust just sink into the mantle? If so, then the new exposed mantle would cool, solidify, and sink. Eventually the earth would be entirely solid!
I once asked a geologist who visited our high school this question and she was unable to answer. I now know the answer but I thought I would ask the question here and see if yahoo answerers know more than some poorly educated geologists. (Apparently geophysics and geochemistry were not her specialties)

Answer 1

You have the right general idea...

The crust is not just solidified mantle, continental crust is actually less dense and oceanic crust is more dense. This is because the producing of melt in the mantle does not melt the mantle equally, some parts of it are more likely to melt.

Oceanic crust sinks because of its density and continental crust will not sink because it is too low density. In detail, new oceanic crust actually is still very hot so it is less dense than mantle, but after ~40 million years it cools enough to be denser than mantle. The longer it is at the surface, the more it cools and the more it gets denser. Some ocean crust can last more than 150 million years because it is stuck to continental crust.

It is true that the earth will be completely solid one day (like Mars), but not strictly because of subduction. Tectonics is caused by the heat of the earth and slowly releases this heat, but each subducting slab eventually does melt and return to the mantle. It is this process of tectonics that will eventually cool the earth to a solid state.

By the way, the mantle is not liquid, it is a slow-moving, plastic solid.

Answer 2

There might not be a liquid mantle. If the earth had a liquid mantle it would rip the earth into a million pieces because of centrifugal force. Also many earthquakes originate at the depths of where this liquid mantle is and earthquakes cannot form where there is no solid structure to resonate. In fact there are many cases of earthquakes as far down as 700 miles below the surface and i think the point at which all materials start to break down into flowing liquid matter or plastic basalt is 150-200 miles..the different wave lengths sent through the earth to determine how many times the waves refract (P and S or shearing wave) is meant to show that the material is becoming denser until it reaches the point where the density decreases so therefore the elasticity is increasing. In other words the waves are travelling around the inner core because its believed to be mostly liquid...Now this is the modern theory and even Albert Einstein himself formed an equation to try and determine the different densities of our planet by how many times a wave refracts.. Im not claiming to be Einstein, but the point is we dont know if the mantle is liquid or just empty space because the sound waves would behave similarly in both cases. One thing is for sure and i mentioned this before on a earlier post. We know very little about the interior and it is larglely guesswork. For example in the 1970's a project began in Northern Siberia called the "Kola well" or "Super deep well" project. by the early 1990s they had reached the depth of about 7 1/2 miles below the surface and this is as far as man has ever gotten. The scientists basically concluded that what they thought they would find at those depths was not and they also theorized that since the hot temperatures were not increasing that it had leveled off and at some point if they kept digging the temperature would begin to decrease. This is speculative in nature, but it makes a good point because 1.) We've only pricked the skin of the "apple" that is our planet 2.) Seismology and determining the nature of the earths interior is largely guess work 3.) No one can be for certain what is going on in the interior until were advanced enough to get there.

Answer 3

I'll brave the storm and try to answer. It may not be right, but how I have always thought of it is thus:


Although it is true a solid is generally more dense than a liquid this is not the case with the earths mantle. The sheer weight of the earths crust and upper mantle causes a large amount of pressure on the liquid mantle. As we know high pressure = more desensity. So in this case the liquid is more dense than the solid crust.

thus the crust floats as it were. Also helping this process is the small thought that heat radiates away from its source. The liquid is always moving a fairly predictable path. It is like studiying an ocean current. It rises and falls as it heats and cools. This causes the crust to be renewed continually.

Thats my answer and i'm stickign to it

Answer 4

It is a fluctuating continual process. If you think of the Earth as a living thing, it makes a little more sense. Think of the inside as a heart that is pumping the blood to the rest of the system. The outer layer is kind of like our outer layer. The Earth will not be entirely solid because the closer you get to the center, the hotter it is. Therefore, when the crust goes into the mantle, it turns back to liquid. And the reason it doesn't sink that often is do to the pressure that is in the mantle. Volcano erruptions. That is why they errupt. The only time that the crust goes into the mantle is during fault line activity. Hence, earthquakes, which I am sure that you knew. The edge of these fault lines collide and the pressure pushes one under the other. kinda like -/ then it is pushed into the mantle and recycled.

Answer 5

I would think your initial presumption is wrong. The surface is mostly silicon and iron while the core is more nickle and iron. The heavier stuff sank down deep and we're left floating on top. Also the stuff here on the surface is less compacted so it probably would not be as dense even if it was made of the same stuff as the core. But then I'm just a biologist by training so what would I know.

Answer 6

Why does Pumice float in water? well because it is less dense than water. The same principal applies to the earths crust.