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maline
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Given that both the continents and the oceans are "floating" on magma, and the continents are much heavier, why doesn't the pressure created by the continents force the ocean floor upward?
It does. Continental plates are thicker than oceanic plates; oceanic plates are denser than continental plates.maline said:pressure created by the continents force the ocean floor upward?
Can you constrain your question --- there are libraries full of information on the topic.maline said:more details?
Continental crust is considerably less dense than the upper mantle. Oceanic crust is only slightly less dense than the upper mantle. In fact, at subduction zones, the older, cooler oceanic crust is more dense than asthenosphere rock. That's why it subducts.maline said:Given that both the continents and the oceans are "floating" on magma, and the continents are much heavier, why doesn't the pressure created by the continents force the ocean floor upward?
It doesn't. You're hung up on the oceans. Forget the oceans.maline said:So the overall crust mass per square mile, including the water of the oceans, is approximately constant over the Earth's surface?
How does this occur?
Water causes the crust to be less dense. The rock carried down into the Earth at a subduction zone is heavily saturated with water. This water is squeezed out down inside the Earth. Most of it escapes as steam in a volcano, but some of it changes the chemistry of the rock. The result is granitic rock. The lava emerging along the rim of fire is mostly granitic while the lava emerging at the mid-ocean ridges is basaltic.what causes continental crust to be thick and light?
No. Let's go back to my thick cork vs thin oak analogy. While the top of the cork is higher than the top of the oak, the bottom of the cork is lower than the bottom of the oak. The same applies to the continents and oceans.maline said:Is the lower boundary of the crust at the same height for continental & oceanic crust?
Plate tectonics is perhaps a better explanation. The continents get pushed and pulled around in a cycle that forms and then breaks apart supercontinents. Convergent collisions between two continental crust plates results in mountain building; the Himalayas and the Alps, for example. This makes continental crust thick and old. Mountains are where the crust is thickest. Convergent collisions between an oceanic crust plate and some other plate results in subduction, with the subducting plate being oceanic crust. This keeps oceanic crust young and thin.Does the lightness of continental crust also explain why it's thicker?
A key concept that you are perhaps missing is that the rock that form oceanic crust and continental crust are a bit different from one another. That said, geologists have found oceanic crust thousands of meters above sea level. These are "ophiolites". Google that term for more info. In fact, a lot more info. Ophiolites are one of the keys that matured Wegener's concept of continental drift into plate tectonics.maline said:If the lower boundaries are different, then shouldn't some of the continental crust soften from the temperature and become part of the asthenosphere, or maybe some asthenosphere rock solidify onto the oceanic crust?
This is incorrect.D H said:Water causes the crust to be less dense. The rock carried down into the Earth at a subduction zone is heavily saturated with water. This water is squeezed out down inside the Earth. Most of it escapes as steam in a volcano, but some of it changes the chemistry of the rock. The result is granitic rock. The lava emerging along the rim of fire is mostly granitic while the lava emerging at the mid-ocean ridges is basaltic.
What I wrote is not incorrect. Granite most definitely is less dense than basalt, and granite would not occur without water.Ophiolite said:This is incorrect.
1. The principal consequence of the water is to lower the melting point of minerals within the subducting slab and adjacent mantle. i.e. the physics of the rock are changed, rather than the chemistry.
2. This generates andesitic magma, not granitic magma. Granite is much more silicious (and less dense) than andesite.
3. Most granite is generated through metamorphism and selective melting of sedimentary and igneous rocks within the crust.
D H said:What I wrote is not incorrect. Granite most definitely is less dense than basalt, and granite would not occur without water.
D H said:The lava emerging along the rim of fire is mostly granitic
I did not dispute the lower density of granite and I specifically noted the role of water in lowering the melting temperature of rocks. However the other points you made were incorrect. I fully acknowledge the importance of simplification (my own response was grossly oversimplified) in answers, but the simplification should not include actual errors.D H said:What I wrote is not incorrect. Granite most definitely is less dense than basalt, and granite would not occur without water.
I did not want to get into the details of selective melting with a novice.
The continents do not actually float on the surface of the Earth like a boat on water. Instead, they are supported by the Earth's mantle, which is a layer of hot, semi-solid rock beneath the crust. This layer is constantly in motion due to convection currents, causing the continents to move and shift over time.
The Earth's mantle is made up of several tectonic plates, which are constantly moving and colliding with each other. This movement is driven by the heat and pressure within the Earth's core. As the plates move, they carry the continents along with them, resulting in the shifting of landmasses over time.
Scientists can track the movement of the continents using various methods, including GPS technology, satellite imagery, and studying the patterns of earthquakes and volcanic eruptions. They have also found evidence in the form of matching geological features, fossils, and rock formations on different continents that were once connected.
No, the movement of the continents is a natural and ongoing process that has been happening for millions of years. While the rate of movement may vary over time, the convection currents within the Earth's mantle will continue to drive the movement of tectonic plates and the continents on top of them.
Yes, there have been instances in the Earth's history where continents have collided, resulting in the formation of mountain ranges, and others where continents have broken apart, leading to the formation of new oceans. However, these processes occur over long periods of time and are not a significant risk to human populations.