Weathering

One of the reasons a geologist wants to lay hands on a sample before she can identify it is because most rocks you see on the ground are weathered: the surface you see is not the pristine, original form of the rock, but has been altered with time and exposure to the elements. A dark green rock can look gray or even light brown on the outside, and you will rarely see individual minerals in a weathered rock surface. A geologist thus routinely carries a rock hammer to bypass this rock disguise - to break open any rock she wants to investigate so she can get at the raw, "clean" form hidden inside.

Q:

Subject: mrs.dunns 7th grade science class please reply
hello my name is kylie, i was wondering if any type of rock could be
weathered or eroded by heat and preassure such as being impacted under
a large rock or the suns u-v rays.
Kylie R.


A:
All rocks will erode, some faster than others. If you see a stack of sedimentary rocks in the desert (where there is little vegetation to hide it), you will see some layers that are "eaten out" deeper into the outcrop than other layers. This happens because those layers are softer. The one theoretical exception to weathering and erosion is a diamond; it is the hardest mineral on the Earth's crust, so nothing else can dent it. Heat and pressure are usually associated with deep burial and metamorphism (as I indicated earlier, the rocks if buried deep enough and toasted hot enough will deform plastically).

Weathering in a few cases can be accelerated by exposure to UV light (I'm thinking of a mercury-based mineral in that I've seen in Nevada: it changed color from green to brown within a few seconds of exposure to sunlight). By far the most important causes of weathering and erosion are water - and freeze-thaw cycles. To a lesser extent wind contributes - not usually just the wind by itself, but by the sand and dust entrained in it that then chisel the rock outcrops and slowly break them down. If you've ever seen a sand-blaster, you will understand immediately how this works.

Another process in the gradual destruction of rocks is chemical weathering. Many of the constituent grains of rocks will slowly be converted (by water, oxygen, and heat) to clays. For this reason weathered rocks look notably "grungy" - that's a technical term for ugly rock. OK, I made that one up. But it's common for a rock to have a "rind" or shell of more weathered rock on the outside. If you bust it open, you will generally find the original crystalline form of the rock... unless the weathering has progressed all the way through it.

That's why you see geologists in the field almost always carrying rock hammers. It's also why you don't want to be anywhere nearby when their eyes light up and they start flailing all over a rock with their "g-pick" as it's sometimes called (one end is a hammer, and the other end is a pick for chiseling out small pieces of interest). Rock chips fly extremely fast when a rock is struck, and can blind you if you don't use protective eye-wear. I still have scars on the backs of my hands and forearms from bashing volcanic rocks to get at their cores. When I've forgotten my g-pick, I have been known to pick up a nearby rock and bash the one I'm interested in. That's when you really need to be wearing glasses, and probably gloves too!

~~~~~