Suppose if I can drill a hole to mantle at arbitrary point of crust, will magma start rise from the hole and finally become a volcano?

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    $\begingroup$ Just to clarify - the mantle is not liquid magma. It would probably melt a bit if you did this though due to the depressurization. $\endgroup$ – bon Jun 8 '16 at 6:45

As Bon says, the mantle is solid, albeit somewhat viscous / plastic under the prevailing pressure and temperature conditions, so your hypothetical borehole isn't going to puncture any liquid magma chamber. Could it liquify if suddenly depressurized in a borehole? I doubt it, although it might do so if there was a local concentration of volatiles to depress the melting temperature.

But the question is forever going to be hypothetical because there is no drill string capable of cutting into the lower crust, let alone the upper mantle. The heat would soften the drilling bit to the point where it was like putty. Also, the lateral rock pressure would be so high that the borehole walls would implode.

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  • $\begingroup$ If we really wanted to dig down deep, could we detonate bombs and create a whole with a not-as-steep gradient in order to combat that lateral pressure problem? $\endgroup$ – Ovi Jul 4 '16 at 6:53
  • $\begingroup$ @Ovi The deepest hole ever drilled, the Kola Superdeep Borehole (smithsonianmag.com/smithsonian-institution/…) reached 7.5 miles before the 356-degree temperatures caused too many problems with the equipment. It's only 9 inches in diameter, so you couldn't fit much of an explosive in it, and any explosion in the hole would ruin the hole for further drilling. (If you used a small nuke, you'd also make the area too radioactive for a while. See en.wikipedia.org/wiki/Project_Plowshare) $\endgroup$ – jeffronicus Jan 31 '18 at 19:55

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