Crystal Size and Weathering
Name: Elizabeth M.
Date: Winter 2013-2014
Do crystals make rocks more resistant to weathering?
The reason there are so many exceptions to this statement is that it (weathering) is a complex process. I may over explain this answer so, take what you want and leave the rest.
When we talk about resistance to weathering (or erosion) there are two types: chemical and physical. Rocks with interlocking crystals do well against physical attach (glaciers, getting stuck in a fast flowing stream, geologists with hammers and your average junior high student) unless the crystals themselves chemically break down.
The resistance of the minerals in a rock to chemical breakdown is defined by Bowen's Reaction Series. The series represents the minerals the solidify in magmas as the magma cools. The first minerals to solidify in an igneous melt are simple and single or double chain silicates (SiO2 based) minerals. These minerals are stable (happy?) at high temperature and pressures. As such, they weather quickly at the surface. Rocks composed of these minerals weather quickly.
The last two minerals to solidify are mica and quartz. and they are happier at low temperatures and pressures found at the surface. That is why there is so much sand lying around. That is also why sandstone is such a common sedimentary rock.
For sedimentary rocks the same two types of weathering are active. The grains in most sandstone are held together by a "cement". If the cement weathers easily, like limestone (calcium carbonate), the sandstone weathers quickly. If the cement is silica (SiO2) then it will last well enough to be a valuable building stone.
That is a basic look at things and, hopefully is more than your students will want. If you would like more input or have a question, feel free to contact me directly at firstname.lastname@example.org.
Hope this helps.
Bob Avakian, BS-Earth Science, MS-Geophysics
Oklahoma State Institute of Technology
The chemical and biochemical decay, and physical fragmentation of rocks, at the surface of Earth is known as weathering.
Chemical weathering, commonly associated with acid rain, disintegrates carbonate minerals. It is a surficial process, driven by acid rain, organism, and organics.
Rocks are also be subjected to mechanical weathering (physical fragmentation) through freeze-thaw, flowing water, wind, humans, root pry, thermal expansion and burrowing animals.
All rocks are affected by physical weathering, yet chemical weathering, primarily from acid rain, predominantly affects carbonate minerals. Calcite is a common carbonate mineral and it is the mineral that makes the rock limestone. When limestone is metamorphosed it becomes marble. Limestone (a sedimentary rock) and marble (a metamorphic rock) are easily weathered by acid rain.
Go to the local cemetery and look around. You will see that rocks that have silicate minerals (for example, granite) have held up over the years, whereas those rocks made of carbonate minerals (or carbonate cements), are highly weathered.
Leslie Kanat, Ph.D.
It might be easy to think of this as the difference between sand and a rock. Sand metemorphizes into sandstone, is more "granular" and less "concrete-like" whereas igneous rock (like granite) is more solid (solider?)
Erosion of different rock types depends on the constituency of the rocks and the eroding mechanism (rain, ice, wind) in the environment. So specific numbers that apply to all types of igneous or metamorphic rock would be hard to come by.
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