The Basics:
Sidewalk cracks may not seem like a geological process. They are often very annoying, causing one to trip or have an uneven bike ride. But this Earthcache is developed to explain WHY these cracks form, and maybe provide some added interest to a morning walk, bike ride, or any other commute by sidewalk.
Geology:
Not only are these and many other sidewalk cracks formed by geologic processes, but there are many different types of processes that can form cracks. Your task will be to deduct which formed these specific cracks. Cracks are formed by ground movement, but ground movement is attributed to a number of things. The three we will study today are faulting, clay soil expansion and contraction, and mass wasting.
--Faulting: faulting (or a fault) is a fracture found on the crust of the earth that results from the uneven displacement of one side in respect to the other. The pressures against the two sides form a gap, often with one side higher than the other. The displacement can range from a centimeter long to many kilometers along the fault plane. Faults can be formed horizontally or vertically. Naturally, with cracks in the earth's surface forming, this will crack sidewalks as well. Many cities list in their records that faulting is their number one priority to repair. It's damage is of the highest severity.
--Clay Soil Expansion and Contraction: soil with clay contained in it can expand and contract. This is also called "shrink-swell" soil due to the "heaving" and "collapsing" of the soil. These fluctuations are determined by the percentage of clay in the soil. The clay is prone to large volume changes that are directly related to the water content in the soil. Studies have shown the volume can increase as much as 30%, and this is bad news for foundations of buildings and, of course, sidewalks. High-moisture soils "swell," while low-moisture soils "shrink." Now this is only a concern if there are variations in the moisture levels. If the moisture content is held constant, there will be no problem.
--Mass Wasting: mass wasting is a very simple process. It is the downward movement of rocks caused by one thing: gravity. Eventually, when the rocks or soil can no longer support themselves, gravity takes over and they fall or settle. There are two types; high speed rock fall, and slow soil creep. Rock fall is the detaching of rocks from a steep slope or cliff, falling at high velocity down to the ground.
The shattered rocks are known as talus (so if you are hiking in the mountains or camping and see a talus cliff, be sure to avoid it! But it is one way to tell sites of rock fall, which would be interesting). This version of mass wasting is not plausible here or course, since we are dealing with a paved surface with the moment happening underground (or at least under the sidewalk). So if mass wasting were occurring (that's for you to figure out
), it would be the latter of the two types; soil creep.
Soil creep is known as the "gentle landslide" because it is hardly noticeable. The symptoms, if you will, can be things such as cracked foundations, sloping fences or telephone poles, or grassy hillsides that seem to "ooze" because of small ripples formed on the hill by the creep. It is simply the settling of the rocks and dirt, slowly shifting the ground.
Review and Logging Requirements:
These sidewalk cracks are not like some others that are caused by tree roots or wear and tear. These (not to say others are not, but these are just a good example) sidewalk cracks have been formed by geological processes. The cracks can be attributed to three main processes; faulting, clay soil expansion, or mass wasting. To log this Earthcache as a find, e-mail with the following:
•the name of this Earthcache
•pick a crack on the featured sidewalk (preferably the biggest you see, but you can pick any ypu desire) and measure the width of it
•explain which process(es) you think caused the sidewalk cracks
Sources:
-http://www.crystalcovestatepark.com/crystal-cove-beach-geology/beach-geo1-8.pdf
-http://www.stalbert.ca/sidewalks
-http://www.britannica.com/EBchecked/topic/202708/fault
-http://en.m.wikipedia.org/wiki/Expansive_clay
-http://www.foundation-repair-guide.com/expansive-soil.html
-http://geology.campus.ad.csulb.edu/people/bperry/Mass%20Wasting/Types_of_Mass_Wasting.htm
-http://earthsci.org/flooding/unit3/u3-03-03a.html
-http://earthsci.org/flooding/unit3/u3-03-03.html