To be frank, the answer to that is probably nothing - nothing on Earth would convince them because nothing on Earth convinced them it was just a few thousand years old in the first place. They were told that was the right thing to believe and that to be immune to reason and blind to contradictory evidence is something to be admired. The conclusion is sacred so facts must be ignored.
But imagine if there was some way Earth could have recorded its age in a way that is irrefutable, like a prisoner in a cell making a mark on the wall every day, and suppose this mark was not just marked on stone or even carved in stone but embedded forever in the structure of the stone itself, so no-one could have forged it or changed it and the only way to get rid of it was to destroy the stone itself.
What would it take to deny such evidence? Well, creationists, here's you chance to show the world just how creative you can be in your excuse to deny the evidence because there is just such evidence. It's not quite so regular as a prisoner marking his cell wall every day but with a record written in the fabric of Earth itself every few hundred thousand years or so, even the odd irregularity can be ignored especially when they are spread out in sequence averaging something like half a million years apart it doesn't take a mathematics genius to work out that it takes rather more than 6000 years to make dozens of these marks.
I'm talking about the magnetic anomalies in the volcanic basalt running parallel to the mid-Atlantic Ridge, of course.
The mid-Atlantic ridge was initiated some 180 million years ago when the supercontinent, Pangea, broke up, splitting present-day South America off from Africa followed soon after by North America splitting away from the other supercontinent to the north, Laurasia. Since then volcanic magma has been slowly welling up through this fissure as the land-masses move apart by massive convection currents below the mantle, at about the same rate as your fingernails grow, building up the mid-Atlantic ridge and spreading volcanic basalt out on the sea bed some two miles below the surface.
Superficially there is nothing remarkable about the basalt seabed, covered as it is by marine sediment and forming a system of parallel ridges and valleys such as you might expect for frequent period of volcanic activity followed by periods of relative tranquility. But in the 1950s scientists began to notice how there were strange magnetic anomalies in the Atlantic sea floor. Magnetic anomalies associated with basalt are well know because basalt contains magnetic particles called magnetite and these can influence compass readings, for example - something that Icelandic mariners have known about for a long time, Iceland sitting as it does on the northern end of the mid-Atlantic ridge.
However, when mapped in detail, not only were these magnetic anomalies regular and running parallel to the ridge, but when their polarity was mapped, they were found to form a pattern of stripes which were exactly mirrored either side of the ridge. Not only were the particles of magnetite aligned but their 'north' and 'south' poles regularly reversed, and they did so at the same time either side of the ridge.
|Geomagnetic reversals in the|
last five million years
And, as though to add insult to injury for creationists, this isn't a case of circular reasoning by assuming a timescale then using it to confirm itself. Geomagnetic timescale has been confirmed by measuring the polarity of magnetic particles in rocks whose ages have been independently confirmed by other means. The current estimate is that there have been 184 geomagnetic reversals in the last 83 million year, giving an average periodicity of about 450,000 years. Even if you are tempted to think it might have been very much faster to fit them all into the creationist loons' 6000 years for the age of Earth, bear in mind that compasses or lode-stones have been used by mankind for all of recorded history and there are no recorded instances of all the world's compasses suddenly going wrong and pointing in the wrong direction. There have been no recorded geomagnetic reversals in the last 6000 years, or, in creationists terms, since the world began.
Like a ticking clock, the mid-Atlantic Ridge has been slowly recording the age of the Atlantic Ocean bed and the last of dozens of mark was laid down way back before creationists like to imagine their special magic friend created the Universe just for them. It will take an especially creative creationists to explain that away and fit it into their prefered world view. My guess is that they'll just dismiss it and pretend it isn't there.