[Sacajaweau]

imminent in the next 200k years.

And they know that it flipped before how?

[Red Badger]

Iron particles in rocks.................

[rktman]

Charts. They have charts.

[Hoosier-Daddy]

The direction of the poles shows in rock strata. As magma cools, anything magnetic will line up and show the direction.

[katana]

Sediment cores taken from the ocean bottom show that the Earth's magnetic polarity has reversed regularly over geologic history. That this happens has been common knowledge for decades.

But I think the effect on Earth's magnetic field shielding effect and the degree and duration of any exposure to solar winds and radiation is debatable. The evidence seems to show the "flips" happen quickly. One effect might be Auroras suddenly occurring for a time at temperate and tropical latitudes. Wouldn't that send folks scurrying to temples, churches, and the nearest shrink.

[Hoosier-Daddy]

The direction of the poles shows in rock strata. As magma cools, anything magnetic will line up and show the direction.

[gr8eman]

The same way they know global temperatures from 2,000 BC!

[Vermont Lt]

Actually its an interesting process.

They take samples of the bed at the Mid Atlantic ridge and measure the direction of the iron in the minerals. They noticed that the direction changed along the earth as it came up from the mantle and it pushed east and west.

So, it looks like stripes going in opposite directions if you were to stand over it and look through a microscope.

Thats how they know. (You probably think the answer was something silly!) My college professor would be proud of my retaining this knowledge.

[Bloody Sam Roberts]

And they know that it flipped before how?

Exactly. We have found no archaeological evidence that Neanderthal society had any problems with their power grids or cell phone networks. I mean...c'mon.

[Rebelbase]

Before she died Helen Thomas told them about it.