How Many Miles to the Core of the Earth
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What is the deepest man has penetrated below the earth's surface, and what proportion does this constitute of the distance to the core?
Tony Stern, London W2
- Since the earth's radius is about 6500km, and the average continental crust is about 35km the maximum depth one could dig would appear to be about 0.5% of the radius (the oceanic crust is about 6km thick (I think), so if you dug through the ocean floor you could get about 0.1% of the way to the core). Below the crust is the more ductile asthenosphere which, if the overlying rock were released suddenly, would melt and erupt. In reality the temperature of the crust (I think the base of the continental crust is about 1500degC, and the curvilinear relationship between temperature and depth is almost straight through the crust, such that at a depth of 8km you might expect the rock temperature to be well in excess of 300 degrees centigrade) and length of drill shaft would mean that you couldn't get anywhere near the asthenosphere. I vaguely remember hearing that in the early part of last century someone had tried to pentrate the crust to study the mantle and that he failed for the above reason, but I can't remember his name or how far he got. I would guess that the deepest digs around today are diamond mines such as Kimberley in South Africa.
A Thompson, Norwich, England
- The deepest is 3,777 metres at the Western Deep Levels gold mine in the Transvaal, South Africa. The earth is not spherical and the best approximation is an ellipse rotated about its minor axis, so distance to the centre of the earth varies with latitude. The earth's equatorial radius is 6,378km; assuming this is reasonably accurate for the Western Deep Levels, the depth represents little more than 0.05% of the distance to the centre of the earth, although a latitude correction should be made. The depth to the core, as opposed to the centre of the earth, is approximately 2,898km, so the mine depth is marginally over 0.1% of this figure.
Dave Haynes, Lancaster
- Further to Dave Haynes' reply, the deepest penetrations have been by boreholes, not mines. There is a 9.1km borehole in Germany, part of the Continental Deep Drilling Project (KTB), but deeper still is the 12.25km "Superdeep" hole in the Kola Peninsula near the Arctic Circle in Russia, drilled between 1977 and 1989. The latter is roughly 0.4% of the distance (2,900km) to the core of the earth.
Leib Wolofsky, Ontario Canada
- The Mohole project in the early 1960s drilled to 124 miles (122 miles of ocean floor beneath two miles of ocean off the coast of California). The purpose of this project was to determine the nature of the Mohorovicic discontinuity, which is the boundary between the earth's crust and the earth's mantle, on top of which it floats.
Philip Sills, Plainsboro, NJ US
- Regarding the Mohole project, you may want to see http://www.nationalacademies.org/history/mohole/ (link) which suggests that the project drilled to a depth more similar to that of the South African mines cited before.
Robert Jordan, Cherry Hill USA
- Some of the earlier answers have been misleading. Project Mohole was funded by the US government in 1957, stoked by the belief that the USSR was planning deep drilling. The pilot project succeeded in drilling 197m -not 197km, as Philip Sills stated (Notes & Queries, March 30) - into the bed of the Pacific ocean from a drilling barge on the surface nearly 3km above. The project was abandoned in 1966 when unsavoury financial links between the Texan oil-drilling company running the project and the Democratic Party came to light. The USSR had indeed planned since about 1960 to drill deep holes onshore. The Superdeep Number 3 in Kola, Russia, remains the world's deepest hole into the earth, at 12,261m. It took from 1970 to 1984 to get this far. One of the scientists in charge told me that they were keen to license their world-leading drilling technology to the west for hard currency. But the government decreed that since the west wanted the knowledge, then it should be classified as a state secret - so no deal. The project was put on hold due to lack of funds, and the Russians' capability to drill to 15,000m will probably never be tested. However, the results disproved outdated concepts held by influential conservative USSR geologists, and the plate tectonic paradigm came to be officially accepted. The German deep drill hole, the KTB in Bavaria, was planned to drill to 10,000m, but drilling was abandoned in 1994 at a depth of 9,100m, as the temperature of 280C (536F) at the bottom turned out to be hotter than expected. Every time the drill-bit was withdrawn for replacement, the lower portion of the hole closed up by plastic flow of the rocks. Thus it will probably never be possible to drill much deeper unless you choose a region of cold crust, as the Russians did.
Prof David Smythe, Glasgow
- Some of the earlier answers have been misleading. Project Mohole was funded by the US government in 1957, stoked by the belief that the USSR was planning deep drilling. The pilot project succeeded in drilling 197m -not 197km, as Philip Sills stated (Notes & Queries, March 30) - into the bed of the Pacific ocean from a drilling barge on the surface nearly 3km above. The project was abandoned in 1966 when unsavoury financial links between the Texan oil-drilling company running the project and the Democratic Party came to light.
The USSR had indeed planned since about 1960 to drill deep holes onshore. The Superdeep Number 3 in Kola, Russia, remains the world's deepest hole into the earth, at 12,261m. It took from 1970 to 1984 to get this far. One of the scientists in charge told me that they were keen to license their world-leading drilling technology to the west for hard currency. But the government decreed that since the west wanted the knowledge, then it should be classified as a state secret - so no deal. The project was put on hold due to lack of funds, and the Russians' capability to drill to 15,000m will probably never be tested. However, the results disproved outdated concepts held by influential conservative USSR geologists, and the plate tectonic paradigm came to be officially accepted. The German deep drill hole, the KTB in Bavaria, was planned to drill to 10,000m, but drilling was abandoned in 1994 at a depth of 9,100m, as the temperature of 280C (536F) at the bottom turned out to be hotter than expected. Every time the drill-bit was withdrawn for replacement, the lower portion of the hole closed up by plastic flow of the rocks. Thus it will probably never be possible to drill much deeper unless you choose a region of cold crust, as the Russians did.
Prof David Smythe, Glasgow
- Jeremiah 31:37
Tim D, Michigan USA
How Many Miles to the Core of the Earth
Source: https://www.theguardian.com/notesandqueries/query/0,5753,-5830,00.html