[grobdriver]

weakened by 15 percent over the last 200 years

So... someone recorded this data in 1818 to get a baseline?

[VA40]

Good question!

[blam]

"So... someone recorded this data in 1818 to get a baseline? "

It's being recorded in the rocks every second of every day....all lava that is solidifying this minute records the data as it hardens.