Like people, any individual rock can have its own story, but in aggregate, on mountains, their stories tend to follow a similar structure. Any given igneous rock could break in any number of ways–it could be dropped by an eagle, crushed by a tractor, or eaten by gastrolith.
Most igneous rocks will break in the same kinds of ways: frost, trickling water, acids, plants or animals. This is a good thing.
The sand that makes up the sandstone in Petra survived life as lava first. In fact, every rock on earth today was once broken down and melted–the earth is estimated to be 4.54 billion years old, but the oldest rock we have ever found is only 4.28 billion years old. That’s 260,000,000 years we don’t know anything about. 260,000,000 is longer than the amount of time it took for humans to evolve to our present state.
That quarter of a billion years we don’t have any information about pales in comparison to how little we know about the past of the vast majority of the rocks, those that weren’t lucky enough to hide out in Quebec for a few billion years. For example, the oldest rock yet found in Jordan is from the Precambrian period, which was only 570,000,000 years ago, leaving nearly 4 billion years of history of that region hidden in the molten rock beneath it or crushed to sand at the bottom of the sea, because all rocks break.
Most rocks will break in the same ways as other rocks like them in a process called weathering. On my trip to Gargoyles in the western Sierra Nevada mountains I caught pictures of some of the most common ways rocks break. A video is below. As always with posts like these, I used my Physical Geology by Charles Plummer, Diane Carlson and Lisa Hammersley as a reference.
Liquid water breaks down rocks I talked before a bit about volcanic mud and how weak it is when compared with other kinds of rock that form from lava. The beautiful cliffs at Gargoyles exist because of water. Water carves that weak rock into dramatic and suitable craggy faces as to appear almost like gargoyles on the buttresses of a cathedral. I’ve never been able to see them myself, but I’ve also never needed to analogize rocks to make them beautiful. This type of breaking down is easy to understand for anyone who’s played in a sandbox. The water runs over the volcanic mud, scooping up bits that are loose and carrying them downhill. This leaves only the parts that are strongest, in whatever shapes they formed, some of which are quite beautiful with or without the faces of gargoyles rising.
Frost breaks down rocks
See affect of frost-wedging on massive granite boulder.
Frost is the hardest kind of mechanical weathering to illustrate because it’s slow and often happens inside a rock. But everyone who’s ever exploded a bottle of Coke after putting it in the freezer knows that it looks like. Frost wedging happens because the same number of H2O molecules take up more space as ice than as water:
When that water is inside a rock, it in some ways becomes harder than rock and in little wiggles, it breaks the encompassing rock to pieces.
Plants and animals (the biosphere) break down rocks
A lot of the trees in the High Sierras are scrubby little things. There isn’t a lot of dirt, there isn’t a lot of room, and unlike lucky trees on sedimentary rock mountains, there aren’t any luscious aquifers under the surface to drink up. But while they’re scrappy, they can do some damage to survive. That tree just above is cleaving stone from stone with its tiny little roots. In a few centuries, it will have a pile of gravel and some nice nutrients for it/its grandbabies to eat. Trees play the long game.
That’s it for types of weathering igneous rocks go through. For folks who have been playing along since last fall’s Jordan Geology Project, you might remember some of the terms. If you have any questions, drop a question in the comments.
“This is it now Everybody get down This is all I can take This is how a heart breaks You take a hit now you feel it break down Make you stay wide awake” – Rob Thomas, “This is How a Heart Breaks”
(The original title for this post was “This is how a rock breaks!” but then I thought, nah)