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To what extent has the Earth's surface been geologically mapped?
For instance, what fraction has been surveyed at 1:10 000, 1:100 000 or 1:1 000 000 scale?
Edit: By 'surface' I mean land surface, not covered by water. By 'geologically mapped' I mean 'described by a conventional geological map which shows the exposed surface lithologies and their structures'.
As far as I know there are no official data on this. All I can do is give a rough estimate, based upon work and personal interest in about 30 countries, over 40 years.
We can say with certainty that everywhere has been mapped at the 1: million scale although, at this 'reconnaissance scale', at least half of the land surface is only sketchily known, with sparse ground-truth in areas covered by ice, dense forest or drifting sand. In such areas the best detail is probably from geophysics rather than from actual surface mapping.
For example, gravity anomaly by satellite, or aeromagnetics.
At 1:100,000 scale most areas (I'm guessing 80 to 90%) are mapped wherever there is reasonable bedrock exposure. Certainly in Europe, the America's and most of the world's mountain chains you would be hard-pressed to find areas that haven't been mapped at this scale, although the maps are not all available ('commercial, in-confidence', etc.).
The only exception I can think of is parts of the less accessible Himalayas, where the scales of around 1/2 million seem to be the best on offer.
That's not to say that there isn't much scope for further mapping at 1:100,000 scale. Rock exposure changes over time, and many of the existing maps could use a more detailed upgrade, especially in respect of microfossil zonation, facies zonation and structural analysis.
1:10,000 scale or even finer detail is only for the 'very interesting' bits, such as ore deposits, mining tenements, reservoirs and engineering site investigations. There are thousands, if not millions of such maps, most of which are not publicly available. Your guess is as good as mine about what percentage of land is covered by such maps, but I would guess <<1%.
I'd like to add to Gordon's answer by emphasising that a geological map (in the classical case of mapping rock outcrops in colour) is an interpretation of the surface, occasionally with the aid of other methods such as drilling or geophysics.
Being an interpretation, the amount of detail varies. One map can say that one area is just a huge block of granite, while another will say that this area is composed of various rock types and features. Would you consider the first one as a map? Or the second? There is no absolute answer to your question.
