Igneous Rocks

Igneous rocks originated as magma or lava that solidified.   Magma is molten rock underground while lava is molten rock that has reached the earth's surface.

There are two main types of igneous rocks:   Light-colored and dark-colored rocks.  The light-colored rocks are composed of continental crust.  This rock is relatively light weight and the most common minerals are quartz and feldspars.  In New England, you may see a pretty pink GRANITE in places like Cadillac Mountain near Bar Harbor, Maine.  You may also see large crystals in PEGMATITES scattered throughout Connecticut.  Granite and pegmatite consist mainly of feldspar and quartz.

The dark-colored igneous rocks are composed of oceanic crust, which is relatively heavy.  The chemistry of oceanic crust may be very similar to that of the mantle inside the earth. The most common minerals are pyroxenes, amphiboles, micas and olivines.  The trap rock ridges of the Connecticut Valley are composed of BASALT, a dark-colored igneous rock with crystals that are too small to see. These rocks get rusty because of all the iron in them.  At one time the Connecticut Valley began to pull apart to form a new ocean and was flooded with HUGE lava flows.  If the basalt crystals were bigger, you would call the rock diabase, and if they were even bigger, you would call it gabbro. There are lots of other names for igneous rocks which depend on what type of minerals are in the rock and how big the crystals are.

Crystal size depends on how fast the magma or lava cooled.   The faster it cooled, the smaller the crystals. When lava cools super fast it hardens into glass, or obsidian.  The basalt of the trap rock ridges was lava that cooled very quickly.  Pegmatites in Connecticut, on the other hand, cooled very slowly because they never reached the surface of the earth.   Pegmatites can have crystals that are several inches, or even several feet, long.

When a blob of magma cools under the surface of the earth, it is called a batholith.  If the magma fills in a vertical crack in the surrounding rock, it is called a dike.  If it fills in a horizontal space between rock layers, it is called a sill.  These are all forms of intrusions, which are usually made of granite or pegmatite.  This rock may be harder than the surrounding rock.  If so, erosion may remove the surrounding rock and leave only the igneous intrusion, which forms a hill or mountain. An example is Cadillac Mountain in Maine.

Lava can also be very hard and resistant to erosion.   In Connecticut, the Trap Rock Ridges are made of basalt lava that is much harder than the red sandstone and shale that surrounds them.