2) Natural cracks and present
in hard rocks causes their breaking.
*
Answers
Answer:
true
Explanation:
true
mark as the brainliest plsss
Answer:
TRUE
The nature of cracks
(from a geologic perspective)
Harmon D. Maher Jr., Dept. of Geography and Geology, University of Nebraska at Omaha, 68182-0199
2007
This page was created for students engaged in early undergraduate research (EUR) on chalcedony veins in western Nebraska and South Dakota. The research is funded by a NSF STEM grant to the University of Nebraska at Omaha.
Introduction: I am using the term crack here simply because it is widely understood. This is in essence a research project about cracks. It turns out that cracks are quite interesting geologically (more on that below). Most people know a crack when they see one, but that doesn't mean they understand or have thought much about cracks - not, at least, in the way we will. For our purposes a crack is a break (a brittle discontinuity) where the sides pull apart as the crack opens, breaking bonds to create the crack. It is a tensile fracture. This can be distinguished from a break where the sides slide past each other, a shear fracture. Faults are shear fractures. Tensile and shear fractures are fundamentally different features, although they commonly occur together.
Simplified sketches of tensile versus shear fractures.
If you develop expertise in any field you develop vocabulary to go along with it. 'tensile' and 'shear' are words we will use a lot. Types of tensile features found in geology include joints, veins, dikes, sills, fissures, crevices, and crevasses. For the Inuit there are many words for snow and ice, and for the geologists many words for cracks and fractures. Again, more words that may not mean much at this point, but through experience, discussion and reading they can take on more and more meaning.
The earth is all cracked up. Any outcrop shows rocks to be fractured. For a road cut or a mine some of the fractures may have be caused by the excavation process, but many of the fractures were there before - they are of geologic and human origin. Below are some images to demonstrate the ubiquity and diversity of cracks in rocks. It will be helpful in this research project and in learning geology to train your eye to start seeing crack patterns, and your mind to start thinking about them.
This is a peak in the Snowy Mountains of Wyoming. It is composed of very old quartzites (once quartz rich sandstones) that have been deformed and metamorphosed. The quartzite layers form the large relatively flat cliff faces steeply dipping toward the viewer. They are intensely cracked by relatively straight fracture sets known as joints. Climbers are especially attuned to these joint sets. The form of this mountain is due to fracture controlled erosion.
To the south and east of Laramie, Wyoming is a terrain underlain by granite. The granite is fractured in a semi-regular pattern as can be seen here. The orientation of the joints is highly non-random. Erosion of the homogeneous, but jointed granite results in a very distinctive landscape.
Above are some jointed basalts (volcanic flows) from Giant's Causeway, Ireland. Classic columnar jointing is developed here. Note that not all the columns are perfectly hexagonal. Also note how subhorizontal fractures segment the columns along their length. These fractures have a distinctive disk shape, and are both convex upward and downward. The mechanics of the hexagonal columns are well studied, but those of the dish shaped fractures are which segment the columns are to my knowledge not.
This is the river bed of the Niobrara river near the Norden bridge, and the field of view is several feet. These are Tertiary sediments somewhat similar to those we will be conducting research on. The pattern of cracking shown here is distinctly different that shown above. It is on a much smaller scale and of a much more complex and irregular manner. It could be considered polygonal. Note also how there are fracture patterns within fracture patterns.