During the Second World War, ships hunting for submarines detected weird magnetic anomalies (oddities, differences) in the ocean floor that occurred in long tiger-like stripes running roughly North and South. At the time no one thought much about it, until the 1960s when the research vessel the Glomar Challenger (this ship is as famous to Geologists as the Beagle, which Darwin sailed on when he "discovered" evolution, is to Biologists) set out to map the ocean floor. The Glomar set out sailing West to East, and those old anomalies were something they were interested in. What they discovered was that starting, say at Newfoundland and moving toward Europe, the iron-rich rock in the sea bed is magnetically aligned to the magnetic North Pole. As molten (liquid) rock comes to the surface it hardens, but within it small crystals of iron like the needle of a compass align (point) themselves towards the magnetic North Pole. However, the magnetic North Pole moves because a portion of the Earth's core is liquid, and spins around in convection cells (rather like a gerbil in a treadmill), and that generates the Earth's magnetic field. If the "wheel" spins one way North is at the "North Pole", and if it spins the other way (for reasons we don't know), the magnetic North Pole flips down to the physical South Pole. Weird but true!
Therefore, if you start at Newfoundland, you will see North-South bands of rock (our tiger stripes) with their iron crystals either pointing to the current North or to the South, i.e., (Newfoundland) N S N S N S N S N S N (Europe). Further, if you age the rock, it will look like, (Newfoundland) Oldest-Newest-Oldest (Europe) The youngest rock has been newly erupted to the surface along the Mid-Atlantic Ridge (in the middle of the ocean), and spreads out left and right as new material bubbles to the surface. This was the ultimate proof of Alfred Wegner's original theory of Continental Drift, which has been refined, and eventually became known as Plate Tectonics, and includes information on why we have earthquakes, volcanoes, and more.