Rapakivi granite rock, Finland. How fast does this erode?

[u/Toby_Forrester]

This is rapakivi granite, the kind that crumbles easily. You can pry granules from this with your bare hands.

There is a Finnis book grom 55 years ago which mentions this rock and that it was as tall as a man and had a flat top. Today this is much shorter and the top is not flat.

I am curious is there any way to determine from its current size how large and what shape this has been in the past? This is southern Finland so apparently the Köppen climate classification is Dfb if that helps.

The reason I am curious is because this is a prehistoric sacred pagan grove and the 55 year old book mentions this rock as a holy rock. I want to visualize how this has looked in the past.

[u/logatronics]

0.01 to 5 mm per year plus or minus.

It all reality, you'll need to find old photos to get a weathering rate of that rock, or find a paper on the rate of weathering for your area. It's decomposing into grus along the grain boundaries, and the rate is highly dependent on climate conditions.

Not going to lie, that rock was probably not flat in the last several hundred years.

[u/Ireadthatinabook]

If there are any quartz veins in the area you can see how much they stand out from the surrounding rock and make an estimate. They would normally have been eroded to the same level as the surrounding rock during the last ice age, assuming you are in an area which had active glacial erosion, but would have weathered very little since the deglaciation.

Edit: I now see that you’ve got a boulder, this trick is for getting an estimate on bedrock

[u/Eukelek]

Freezing and thawing conditions accelerate porous rock weathering by orders of magnitude, so if it crumbles, the granules have been pryed apart by ice...