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Viewing Inside the Earth
Roland Piquepaille writes "Last week, a study released by Princeton University said that geoscientists have captured images of the interior of the Earth by using techniques similar to body scanning by physicians. This study also revealed in-depth structures which might explain how our planet is changing -- and aging. They studied more than 86,000 earthquakes which occurred since 1964. And they found 32 "mantle plumes" which are believed to cause island chains, such as the Hawaiian Islands and Iceland. They have been conjectured thirty years ago, but this is the first visual evidence they exist. This overview contains more details and references, including a rendering of mantle plumes in action."
How do we know how hot the core of the Earth
is?
We derive our primary estimate of the temperature of the deep earth from the melting behavior of iron at ultrahigh pressures. We know that the earth's core depths from 2,886 kilometers to the center at 6,371 kilometers (1,794 to 3,960 miles), is predominantly iron, with some contaminants. How? The speed of sound through the core (as measured from the velocity at which seismic waves travel across it) and the density of the core are quite similar to those seen in of iron at high pressures and temperatures, as measured in the laboratory. Iron is the only element that closely matches the seismic properties of the earth's core and is also sufficiently abundant present in sufficient abundance in the universe to make up the approximately 35 percent of the mass of the planet present in the core.
The earth's core is divided into two separate regions: the liquid outer core and the solid inner core, with the transition between the two lying at a depth of 5,156 kilometers (3,204 miles). Therefore, If we can measure the melting temperature of iron at the extreme pressure of the boundary between the inner and outer cores, then this lab temperature should reasonably closely approximate the real temperature at this liquid-solid interface. Scientists in mineral physics laboratories use lasers and high-pressure devices called diamond-anvil cells to re-create these hellish pressures and temperatures as closely as possible.
Yes, the inversion problem is potentially ambiguous. I haven't my copy of the article yet, but I will be interested in seeing how they address these criticisms. Plumes originating above the thermal boundary may have originally originated at the boundary and are no longer being fed. The base of the column continues to rise due to its lower density. Yellowstone may be in this category, given its age. The issues appear similar to those in atmospheric and hydrospheric modeling that involve chaotic dynamics.
Actually it is believed that Iceland _does_ sit on top of a mantle plume as well as being on a plate boundary - the evidence is that there is apparently just too much magma being produced there to be just explained by oceanic ridge magmatism. The online article on "VOLCANOES, MANTLE PLUMES, and HOT SPOTS" linked to in the weblog mentions that hotspots can occur on plate boundaries.
Dropping a few thousand nukes in a nice pattern will also work.
And no, solid rock is fine for wave propagation (even better as it will transmit both P and S waves).
The Apollo missions deployed seismometers on the moon and recorded over 12000 "events" from 1969-1977. There's some info in this abstract, which I found using, you guessed it, google (search term "moonquakes study interior"). Apparently most of these events are moonquakes caused by tidal forces, as opposed to plate tectonics on Earth.
I hold it, that a little rebellion, now and then, is a good thing. -- Thomas Jefferson
The Core and Real Science should never ever appear within the same train of thought. Not only was it a horrendous movie, but the physics they used to explain it was complete garbage.
Sig (appended to the end of comments you post, 120 chars)
There really is some debate over wether this is so. 3He/4He ratios are indeed high, and this is normally attributed to a lower mantle source (possibly as deep as the core mantle boundary). HOWEVER four recent (and independent) tomography experiments image a major, strong, low-wave-speed anomaly in the upper mantle beneath Iceland that does not continue down into the lower mantle. These studies strongly suggest that iceland is fed by an upwelling originating in (and confined to) the upper mantle.
The suggested explanations for this are somewhat tenuous - that the plume gains strength in the upper mantle and is not imaged at depth runs counter to the improved tomographic resolution with depth, and besides, doesn't make much sense chemically.
Speaking as a geochemist, the real interest lies in tieing in the 3He origin with tomographic images. The evidence is rapidly shrinking for a chemically seperate 'upper' and 'lower' mantle, but rather for a 'marble cake' type mantle with small regions of primitive material mixed in with more recent 'evolved' mantle material.
geologically, uranium doesn't 'like' to be in the metallic form, but rather exists on earth mainly as a sulfide or oxide. Given the redox state of the earth (and ignoring the hydrosphere - the wet bit at the top), it is very hard to get *any* uranium to mix in with the iron phase. Since we know approximately how much U we should have in the earth from solar condensation type experiments, meteorites and cosmic abundances, we still have more (by many orders of magnitude) Fe than U. So, given that uranium likes to be with the rock (its a strongly lithophile - 'rock loving' element ), it is very unlikely that there is a extant reactor in the core. The ONLY possible way i can get ANY U into the metallic phase is by making the core predominantly iron sulfide, but that messes with the density profiles given by seismic waves....
the best heat sources for the core appears to be latent heat of crystallisation, continued gravitational settling and the decay of radioactive potassium dissolved in the metal.
A few random comments on this subject.
/. story is posted on the first day of the fall AGU (American Geophysical Union) meeting. I'm quite sure that there will be discussions and debate about this.
So-called hot spots (fixed volcanic positions) was first proposed by the great Canadian geophysist J. Tuzo Wilson almost 40 years ago.
The mantle plume origin of these hot spots was proposed by W. Jason Morgan (as mentioned in the Princeton link). Morgan also the put forth the first model for global plate tectonics on a sphere (spring AGU meeting, 1967). This extended the work of Wilson that looked only at transform faults.
I've known Jason for 25 years and can truly say that he is one of the nicest guy you could ever meet. He is also an incredible smart scientist.
Ironically, this
Seismic tomography has been around for over two decades. Global body-wave seismic tomography has been performed primarily since the 1990's. I did seismic tomography work about twenty years ago albeit using surface waves covering only a portion of the Earth.
I hope that they publish a resolution map of their inversion model. An error map would also be good. Many times only the final model is presented.