Types of Rock II

Here is some food for thought.
Have you ever noticed that the smaller features of rock (hand/footholds and smaller) almost always mimic the large scale gross features of the rock (ledges or even the entire rock itself)?
I’m not certain, but I’ll bet geologists have a term for this occurrence.
One of the inherently interesting things about rock climbing is the variance of the types of stone we encounter.
Granite chickenheads, the “eyebrows” of Looking Glass, Indian Creek splitters, the cobbles of Maple Canyon, etc.
I’ve been asked what my favorite rock type is for climbing and this is a question that I’ve never been able to fully answer.
I once thought that Yosemite cracks were the best until my first trip to the Colorado Plateau in Utah. At one time I thought that the edges of sandstone/limestone were the ultimate until I climbed the granite of City of Rocks or Devils Head.
I’ve always loved the features of Devils Lake metamorphic stone.
I guess that my favorite climbing medium is the next different and unique rock that I have not previously encountered.

I learned that the reason for the vertical splitters in Indian Creek has to do with how the rock is formed. Sandstone is a sedimentary rock, which means that it is made from sediment. In even simpler terms, it means sand settling to the bottom of a body of water. Because this settling occurs in horizontal layers, the strongest bonds are formed in a horizontal plane. Over time, the sediment becomes rock. When the rock is stressed, it will fault and crack. Because the strong bonds are in a horizontal plane, the cracks form along the vertical plane. That is why the cliffs are vertical and why the splitter cracks are too.

Devil's tower geology is pretty cool too. The tower was the core of a volcano that cooled and solidified. Then the cone of the volcano eroded away faster than the core leaving the tower.

Ah yes, The Tower. An igneous/basaltic intrusion of phonolite porphyry.
Since it is a solidified core of an ancient volcano, I guess you could call it a Laton Kor (latent core).
lol.

Tom Hanson wrote:Here is some food for thought. Have you ever noticed that the smaller features of rock (hand/footholds and smaller) almost always mimic the large scale gross features of the rock (ledges or even the entire rock itself)?

Is that because of the crystal structure of the rock? I.e. if you look at a table salt crystal at the single atom level it is a cube, then if you stick a bunch of those together it forms a (roughly) cubic shape too. My guess is that some rock is formed the same way - if the underlying structure is cubic the gross features like handholds would also be roughly cubic, etc...

If only there were a geophysicist around...

AJS,

Yes, this is exactly what I am trying to describe.
The micro simulates the macro.
Your analogy to table salt is spot on.
This is why you can get a gut feeling that a distant crag will be climbable even though you are viewing it from a long distance away.

'Rock' is fractal-like.

Shorelines, clouds, mountains, trees ... Benoit Mandelbrot mentions these as examples and motivation for studying fractals, a term he coined. Fractals are self-similar, that is they have the same structure on different scales. Zoom in or zoom out, you see the same kind of features.

Yes, rock can be fractal like, and you can see the same features at differing scales. It is incorrect to say that this is always the case.

In the example of your salt cube, that is a mineral, rocks are made up of a mixture of minerals that have different cooling rates and because of their chemical makeup, they weather and erode differently in different climate environments. The fracturing that they exhibit also can be very different based on the structural stresses that are applied to them.

Therefore, a limestone in a desert environment in a tectonically quiet basin setting is going to have fractures and erosional handholds that are quite different from those that it will have in a highly humid, tectonically active area.

John, sort of a simple rule of thumb for igneous rocks is that the bigger the phenocrysts are in the porphory, the deeper it was emplaced. When molten rock cools quickly the crystals don't have as much time to grow and the crystals are small. The extreme version of this is when it is vented directly to the air or water and obsidian (glass) is formed. When it cools slowly, down deep in Pluto's workshop, the crystals have time to grow. All minerals have different pressure and temperature ranges that they live in their different phases (liquid - solid - gas). Because of this, different minerals will crystalize at different pressure and temperature settings, i.e. the depth and temperature gradient of their area of emplacement. That is why you see those big crystals in a matrix of finer material that had already crystalized out.

Strangely, the minerals that are most stable at deep depths are the least stable at surface pressures and temperatures. In other words, rocks that live in the deep mantle erode very quickly on the surface. There is a list of these mineral assemblages in stability order called the Bowen reaction series. Wikipedia this for more info on the subject.

Hey Scott -

Am I translating correctly?

phenocrysts: inhomogeneities in the rock that cool/form at different temps/pressures than the bulk of the rock

porphory: the main bulk material of the rock

So instead of "those big crystals in a matrix of finer material that had already crystalized out" could be "phenocrysts in a matrix of porphory that had already crystalized out"?

Scott M. Mossman wrote:John, sort of a simple rule of thumb for igneous rocks is that the bigger the phenocrysts are in the porphory, the deeper it was emplaced. When molten rock cools quickly the crystals don't have as much time to grow and the crystals are small. The extreme version of this is when it is vented directly to the air or water and obsidian (glass) is formed. When it cools slowly, down deep in Pluto's workshop, the crystals have time to grow. All minerals have different pressure and temperature ranges that they live in their different phases (liquid - solid - gas). Because of this, different minerals will crystalize at different pressure and temperature settings, i.e. the depth and temperature gradient of their area of emplacement. That is why you see those big crystals in a matrix of finer material that had already crystalized out. Strangely, the minerals that are most stable at deep depths are the least stable at surface pressures and temperatures. In other words, rocks that live in the deep mantle erode very quickly on the surface. There is a list of these mineral assemblages in stability order called the Bowen reaction series. Wikipedia this for more info on the subject.

Try this link for more info.
jersey.uoregon.edu/~mstrick…

Tom Hanson wrote:I’m not certain, but I’ll bet geologists have a term for this occurrence.

-cleavage (?)

...seriously

AJS wrote:Devil's tower geology is pretty cool too. The tower was the core of a volcano that cooled and solidified. Then the cone of the volcano eroded away faster than the core leaving the tower.

A volcanic plug is formed when a volcano becomes extinct and the molten rock in tube that carried the magma from deep in the earth to the crater of the mountain cools and becomes solid igneous rock. Usually the rock in the tube is much tougher than the rest of the mountain and as the wind, rain and snow erode the mountain away, the plug becomes exposed. One well-known example of a volcanic plug is Ship Rock in New Mexico which towers 1,700 feet above the surrounding plain.

Most of the evidence suggests that Devil's Tower isn't the remains of an extinct volcano, however. There is no trace in the surrounding countryside of other geological phenomena that might be associated with a volcano such as ash or lava flows.

A more likely theory is that the strangely-shaped mountain is a laccolith. A laccolith is an intrusion of hot magma from deep within the earth that never reaches the surface. It pushes up a bulge of sedimentary rock above it, but no caldera or crater is formed. As the molten rock cools and the soft sedimentary rock of the bulge is worn away, the harder igneous rock is exposed. If this is the case the top of the tower probably became visible between one and two million years ago.
unmuseum.org/devtowergeo.htm

It's more fun to believe its a volcano when your climbing it.

AJS wrote:Hey Scott - Am I translating correctly? phenocrysts: inhomogeneities in the rock that cool/form at different temps/pressures than the bulk of the rock porphory: the main bulk material of the rock So instead of "those big crystals in a matrix of finer material that had already crystalized out" could be "phenocrysts in a matrix of porphory that had already crystalized out"?

Phenocrysts are the bigger crystals you can see that have grown in the slushy matrix of the cooling magma. A porphory is the generic rock name for a rock that has these phenocrysts in it.

Tom Hanson wrote: I’ve always loved the features of Devils Lake metamorphic stone.

I've found that metamorphic rock is often either gneiss or schist.

Ryan Brough wrote: I learned that the reason for the vertical splitters in Indian Creek has to do with how the rock is formed. Sandstone is a sedimentary rock, which means that it is made from sediment. In even simpler terms, it means sand settling to the bottom of a body of water. Because this settling occurs in horizontal layers, the strongest bonds are formed in a horizontal plane. Over time, the sediment becomes rock. When the rock is stressed, it will fault and crack. Because the strong bonds are in a horizontal plane, the cracks form along the vertical plane. That is why the cliffs are vertical and why the splitter cracks are too.

The Wingate Sandstone is the remnants of sand dunes from 200+ million years ago. Since dunes don’t have distinct bedding planes, the formation is isotropic. It is also homogenous, meaning all individual particles are roughly the same. This means that if you have a cube of Wingate, you could apply a force in any direction and it would behave the same way. When the stress of the overlying weight of rock was removed from the erosion of the upper members of the Colorado Plateau, the fractures propagated vertically when the Wingate reacted to this stress relief.

What a fascinating thread.
Anyone remember that book about the geology of the 50 most popular crags in America or something like that? Always wanted a copy of that but never got one.  

Mike Mu. wrote: Reviving an almost 12 year old thread--that has to be a record on the MP.  My favorite type of rock--Adirondack anorthosite. Shit is only found in a few areas of the Earth and it is on the Moon.  So it has that going for it, which is nice. And CA Needles granite--gets me hard

Where in the adirondacks can you find anorthosite? Is it the tops of the high peaks? I was always curious about the Snowy Mountain boulders as the granite huecos that formed there are beautiful. 

edit: found this researchgate.net/figure/Map…;

Wow, the resurrection of this thread

Rocrates wrote: Wow, the resurrection of this thread

Geology is cool

Just goes to show that you should never take gneiss schist for granite.