Margaret M. Loos
A.
Igneous Rock
The rocks of Forbes Bluff represent the first class of rocks in the rock cycle,
igneous
. Igneous rocks are formed from a molten state and solidification.
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(The concept that a rock can melt and flow can be impressed by the films or photographs of today’s active volcanoes. A lab demonstration of the changes of state, solid to liquid to gas should be given.) As the material cools the resultant rocks will vary in appearance from ash, glass with no granular arrangement (obsidian) to relatively coarse textures with crystals evident (pegmatite). High fluidity allows the chemicals in the molten material to circulate (as ions) and combine with each other to form larger crystals in time. The amount of the different chemicals, the cooling time, and the space available determine size of the crystals. Some igneous masses are found deep in the earth. If the masses are many miles thick they are called batholiths, (Greek: bathys-meaning deep and lithos-meaning stone). Smaller bodies of the deeply rooted rock (a few thousand feet to 10 miles across) are termed
stocks
. In addition, at a site of former volcanic activity the
necks
of the volcanoes may still exist as
igneous
deposits. Some of the characteristics of volcanic remains are present on both the East and West limits of our harbor area. Batholiths, stocks and necks are called
plutonic
because they are formed underground, and if they invade other materials as sills, dikes, etc they are termed
intrusive
rock. If ash or cinders are thrown into the atmosphere, the rock material is called
volcanic
or
extrusive.11
(figure available in print form)
A question not completely resolved by scientists is what is the source of the energy that heats up rock so it is fluid? A recent theory suggests that natural radioactive decay is capable of releasing tremendous energy that could produce such melts. Geologists also propose that “what is probably the essential mechanism of crustal deformation . . . (is) very slow plastic movements at about the level of the upper mantle”
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with rising and falling flows as well as horizontal ones where gravity and gradation of density of material could account for the vertical movement. These two theories are recent explanations for areas of geology that do not give surficial evidence.
At Forbes Bluff we may now infer that the rocks of the bluff are igneous, since they are highly resistant to erosion and the cracks in the rocks appear in regular patterns, a characteristic of cooling igneous rock. Also the columns of rock also break away where the cooling of the molten material has caused shrinkage in patterns like mud cracks or the top of a chocolate pudding. The process which accounts for the breaking off of the rocks can be demonstrated in the classroom. Students should prepare a glass soda bottle by completely filling it with water and replacing the cap. The bottle should then be placed in a plastic bag and put in the freezer of a refrigerator until the next class. (The weekend is a good time.) The bottle should be found to have cracked open, indicating that the water expanded in the process of freezing. This action is responsible for many changes that are caused by weathering.
Weathering
is defined as “the slow, but inevitable disintegration and decomposition of rock under the influence of air and moisture”,
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but it is implied that this is without transport.
B.
Sedimentary Rock
Good examples of the second form of rock are those we observed on the beach site.
Sedimentary rocks
are formed by the consolidation of weathered or eroded material of other rocks, or by chemical or organic processes. Some forms easily recognized are the multilayered shale produced by the depositions of clay particles. Silt produces siltstone, sand produces sandstone, and gravel in union with others of these produces conglomerate. All of these are produced from particles removed from preexisting rocks.
C.
Metamorphic Rock
is the third kind of rock in the rock cycle, and probably the hardest for students to understand, partly because it occurs deep in the ground and partly because the conditions are alien to the more familiar surface. Metamorphism means solid state changes in minerals and textures of earlier formed rocks which occur without melting. Heat pressure, and chemically active gases and fluids are the agents of metamorphism. If large masses of rock are affected without contact with batholiths and with the influence of high temperature and pressure it is called
regional metamorphism
.
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Geologists term the metamorphism that is caused either by the heat effect of intrusive igneous bodies or metamorphism caused at the site of a shearing action or granulation at the site of a fault
contact metamorphism
.
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A fault is a fracture (break) (in the earth’s crust) along which there is demonstrable slipping of one side past the other.
In metamorphism some of the actual changes may be
-
1. Enlarged grains in the minerals, because they unite under pressure, examples include quartzite and slate.
-
2. Recrystallization, as in marble.
-
3. The formation of new minerals because of chemical changes, as in garnets.
A characteristic weathering effect in metamorphic rocks is exfoliation where parallel layers of metamorphic rocks such as slate fall off the face of the rock due to the expansion of the water when it freezes and pushes the layers apart.
At this time our students should be able to make educated guesses about the types of rocks found at our different sites which we have observed and collected. They should even be able to tell something about the conditions that produced them. For instance, the granite in the breakwater at Lighthouse has large granules either produced by slow cooling in igneous or enlarged in metamorphism. They are indeed metamorphic. The boulders at Lighthouse might be classified as igneous if their resistance were considered, and John Rodgers, the outstanding expert on Connecticut’s geology says these boulders are “Granite, once molten rock that cooled slowly underground . . . As this exposure is close to the eastern border fault of the Mesozoic rift valley, it was cracked and shattered during the earthquakes along the fault. It was not then at the surface but has since been exposed by Cenezoic erosion.”
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Paleozoic, Mesozoic and Cenezoic refer to the three most recent eras of Geologic time.
Paleozoic
means ancient life and the era covered 350 million years to the
Mesozoic
era, which means the era of middle life and covered 140 million years up to the
Cenezoic
era, which means recent life and covers the last 60 million years until now.