Iíve been thinking about granite a lot recently; primarily because we have been covering plate tectonics in my AP Environmental Science class. Granite figures prominently in this discussion because the plates that constitute the moveable portion of the Earthís crust are composed of two fundamentally different types of rock - granite and basalt. Continental crust is largely granite, oceanic crust is largely basalt. Both are igneous rocks (igneous rocks are those that have formed directly from magma-molten rock). Basalt is whatís known as an extrusive igneous rock - it is volcanic, formed from molten magma that extruded onto the Earthís surface and hardened. It is dark in color and heavy and dense. Floating on top of this dense basalt is granite, a type of intrusive igneous rock. Intrusive means that the granite formed deep within the Earthís surface, when molten magma solidified under the surface rather than being extruded by volcanic activity. The granite "floats" on top of the basalt because it is made up mostly of aluminum silicates which are less dense than the magnesium silicates that comprise basalt.
The second reason that granite is on my mind comes from a recent hike looking for porcupine dens among some cliffs on Mt Agamenticus. I didnít find any porcupines but saw lots of magnificent granite cliffs draped with ice and snow, glistening in the late winter sun. Granite is all over New England. Granite is the state rock of New Hampshire and the state "building and monument stone" for Massachusetts (the Massachusetts state rock is the fancifully-named Roxbury puddingstone). I looked up but couldnít find a state rock for Maine (however, in case you are curious, the Maine state gemstone is tourmaline). According to some accounts, the White Mountains are so named because they were first sighted (by Europeans) from the Piscataqua estuary - their bare mica-laced granite faces glistening in the sun. An alternative theory about the name involves their snow-capped peaks garnering similar attention.
So, while looking for porcupines I climbed among the granite slabs of some eroded, exposed cliffs on the south side of Mount Agamenticus. I saw trees ferociously clinging to the rock. Lichen and moss-crusted rock. Rock split, millimeter by millimeter, by ice that poured from cracks in the rock and spilled down the cliffs. This was granite at its finest, granite that had formed deep within the crust and over the ages had been uplifted and exposed by erosion of overlying layers.
How do you recognize granite, tell it apart from all the other rocks out there? Granite is a relatively (compared to basalt for instance) light-colored rock. I was recently in Iceland which is largely basalt. Light-colored granitic rocks stuck out like a sore thumb. If you pick up a piece you will see that the grains are large enough to be visible to the unaided eye. This is because granite forms from slow cooling (or crystallization) of magma - the slower the cooling the larger the crystals. Next time you look at some volcanic rock, which cooled quickly, this will be obvious: volcanic rock is typically very fine-grained - you canít make out the different minerals by eye.
Granite is primarily composed of quartz and feldspar with small amounts of mica, and other minerals. This variable composition is what gives granite its different hues of pink, red, gray and white. Iím sure I learned this back in middle school, but recently re-learned the difference between a rock and a mineral. A mineral like feldspar or quartz is an inorganic solid, with a definite chemical composition, and an ordered atomic arrangement - this means we know exactly what chemicals are in the mineral and how they are arranged. A rock like granite, on the other hand, is an aggregate of more than one mineral and can have variable composition. In addition to the feldspar and quartz, there is a lot of mica in the granite around here, which contributes the shine to all those granite outcrops of the White Mountains.
When we see that exposed ledge in New England, it is like we are seeing the ancient bones of the Earth. Of course there are other types of rocks to be found in New England, but much of what you see is granite: the sand grains on the beach - most of it granite brought here by the last glaciers, the moss and lichen-covered rocky outcroppings in the woods, and the cliffs and exposed summits of our mountains.
Susan Pike, a researcher and an environmental sciences and biology teacher at St. Thomas Aquinas High School, welcomes your ideas for future column topics. She may be reached at firstname.lastname@example.org. Read more of her Nature News columns online.