Francis J. Degnan
Rocks are for formed in three ways. Areas representative of each of these methods are easily located in the New Haven area. East and west Rocks are examples of lava or igneous rock that is of volcanic origin. Molten or liquid rock is called magma below the ground and lava once on the surface. In geologic terms the lava in our area that is commonly called basalt cooled quickly. Rapid cooling doesn’t’ allow time for any minerals to crystallize. These rocks are hard and massive. Granite is an igneous rock that cooled slowly deep below the earth’s surface. Rocks formed this way have coarse mineral grains and crystals that can be easily seen. We see granites because of the erosion of the earth’s surface or because of the activity of the continental plate activity.
Sedimentary rock was once loose material that was deposited in layers (strata) millions of years ago. The materials include older rock, plants and animals. The passage of time caused these materials to form solid rock. As layers of clay were deposited, the water is squeezed out and shale is formed. Natural chemical substances bind the aggregate together in sandstones and conglomerate rock. These are common and found throughout the Connecticut River Valley. There are also organic sediments that turn into rock. In the northwest corner of our state are the limestone deposits that were coral reefs and shell deposits from the Iapatos sea. Coal is also considered a rock formed from organic material.
The final type of rock is called metamorphic. This is rock that has changed in appearance and perhaps even in mineral composition. These changes may take place because of the heat and pressure from the movement of the earth’s crust as mountains are built or by nearby igneous (lava) intrusions. Such extremes of pressure and temperature often make these rocks look crunched. They may be irregularly banded with light or dark minerals. This happened when the crevices formed under great pressure and were filled with chemically active fluids that cooled slowly forming crystals. Sedimentary rock may be metamorphosed, under such conditions shale becomes slate, limestone becomes marble and sandstone quartzite. The two most common metamorphic rocks are schists and gneiss. Schists glisten with mica and other minerals and are metamorphosed from impure sandstones, limestones and shale. Gneiss is a rock banded with light and dark minerals.
Perhaps you have noticed the word mineral repeatedly used in the above paragraphs? Minerals are the ingredients that rock is made of. All rock contains minerals. The term mineral refers to a substance that meet the following criteria: 1) it is found in nature, 2) the substances making up the mineral were never alive, 3) no matter where on earth it is found it is chemically the same, 4) they can form crystals, that is they also can be arranged in a regular pattern. Oil and coal are not minerals because they were formed from the remains of plants and animals. The elements gold, silver, copper, graphite, and sulfur are minerals, but most are combinations of elements.
Glaciers and their deposits pose a problem for us. We find rocks dropped here that have traveled thousands of miles. The identification of a student’s finds becomes more difficult with this variable introduced. Students should first observe identified samples of rocks noting the similarities and differences. Then they can apply what they have learned to the samples that they find. An individual’s ability to observe the different characteristics of the rocks is the key.
