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"The earth's core is well above that temperature"

That's only an old theory. More recently, deep hole drilling has shown so far that unlike previously believed, temperature begins to decline after certain depths as does preasure, and highly concentrated salt water appears.

"Also, the earth's field is rapidly declining (about 8% since 1835 AD), which would be inconsistent with a permanent magnet. \"

No, it is not inconsistent with a permanent magnet model. They too loose strength over time. Because the material which makes our magnetic field is most likely within the outer layers of the earth, heat wouldn't be much of a factor except closer to the inner extremes of the magnetic layer.

How much faith can be put in the theories concerning the composition and density of rocks at different depths? The only place where the accuracy of scientific models can be tested directly is in the uppermost few kilometers of the crust. Although oil companies have drilled as deep as 8 km on land, they drill in sedimentary basins. The igneous and metamorphic basement, which averages 40 km thick and makes up most of the continental crust, has rarely been sampled deeper than 2 or 3 km.

The deepest borehole drilled for scientific purposes is located on the Kola Peninsula near Murmansk, Russia, in the northwestern part of the Baltic Shield. A final depth of 12,262 meters was reached in 1994. The drilling of this and other deep and super deep wells has produced one surprise after another, and the findings have been extremely embarrassing for earth scientists.

At the Kola hole, scientists expected to find 4.7 km of metamorphosed sedimentary and volcanic rock, then a granitic layer to a depth of 7 km (the 'Conrad discontinuity'), with a basaltic layer below it. The granite, however, appeared at 6.8 km and extends to more than 12 km; no basaltic layer was ever found! Seismic-reflection surveys, in which sound waves sent into the crust bounce back off contrasting rock types, have detected the Conrad discontinuity beneath all the continents, but the standard interpretation that it represents a change from granitic to basaltic rocks is clearly wrong. Metamorphic changes brought about by heat and pressure are now thought to be the most likely explanation.

The super deep borehole at Oberpfälz, Germany, was expected to pass through a 3-to-5-km-thick nappe complex into a suture zone formed by a supposed continental collision. The borehole reached a final depth of 9101 m in 1994, but no evidence supporting the nappe concept was found. What the scientists did find was a series of nearly vertical folds that had failed to show up on seismic-reflection profiles.

Rock density is generally expected to increase with depth, as pressures rise. Results from the Kola hole indicated that densities did increase with depth initially, but at 4.5 km the drill encountered a sudden decrease in density. The results also showed that increases in seismic velocity do not have to be caused by an increase in rock basicity. The Soviet Minister of Geology reported that 'with increasing depth in the Kola hole, the expected increase in rock densities was therefore not recorded. Neither was any increase in the speed of seismic waves nor any other changes in the physical properties of the rocks detected. Thus the traditional idea that geological data obtained from the surface can be directly correlated with geological materials in the deep crust must be reexamined.

he results of superdeep drilling show that seismic surveys of continental crust are being systematically misinterpreted. Much of the modelling of the earth's interior depends on the interpretation of seismic records. If these interpretations are wrong at depths of only a few kilometres, how much reliance can be placed on interpretations of the earth's structure at depths of hundreds or thousands of kilometres beneath the surface?

Contrary to expectations, signs of rock alteration and mineralization were found as deep as 7 km in the Kola well. The hole intercepted a copper-nickel ore body almost 2 km below the level at which ore bodies were thought to disappear. In addition, hydrogen, helium, methane, and other gases, together with strongly mineralized waters were found circulating throughout the Kola hole. The presence of fractures open to fluid circulation at pressures of more than 3000 bars was entirely unexpected. The drillers at Oberpfälz discovered hot fluids in open fractures at 3.4 km. The brine was rich in potassium and twice as salty as ocean water, and its origin is a mystery.

Another surprise at the Kola hole was that lifeforms and fossils were discovered several kilometres down. Microscopic fossils were found at depths of 6.7 km. 24 species were identified among these microfossils, representing the envelopes or coverings of single-cell marine plants known as plankton. Unlike conventional shells of limestone or silica, these coverings were found to consist of carbon and nitrogen and had remained remarkably unaltered despite the high pressures and temperatures to which they had been subjected.

It is generally assumed that temperature increases with depth, reaching 1000°C at a depth of about 80 km, 4800°C at the core-mantle boundary, and 6900°C at the earth's centre. It is certainly true that mine shafts and oil drilling operations have indicated significant increases of temperature with depth.
Indeed, superdeep drilling has shown that temperature increases with depth far more rapidly than predicted. In the Kola borehole, the temperature at 10-km depth was 180°C rather than the expected 100°C. Measurements revealed significant vertical variations in temperature gradient and heat-flux density along the borehole. Overall, the rate of temperature increase rose from 11° to 24°/km down to a depth of nearly 7 km, and then started to decline.

Clearly we know much less about the inner earth than we thought we did. When it comes to inner earth exploration, we've barely scratched the surface of the earth.