A number of the people coming through have been asking about the geology of the area and after talking to a couple of school groups and guests on some of my hikes I thought I would put my passion into words. Now the White Mountains have a very complex geologic history and it will take me a few posts to explain it. I’ll start out with the main geologic principles in the first post, then get into the various mountain building events and the volcanic history in the second post, and finish off with the glacial and surficial history in the last post. With the basics I’ll get into the main rock types, and plate tectonics.
Geology? Why do I want to know about rocks?
Geology is pretty cool and there’s more to it then looking at minerals and fossils. Like all sciences geology is constantly changing and affects our daily lives. It plays into beach erosion and what we can do to keep houses from being lost to winter storms, how high flood waters will get if we continue to build in flood plains, where our oil comes from, if a stream will go dry when a water distributer builds a bottling plant near by, why the early settlers had so much trouble farming in the North East and endless other examples.
Geology is the study of the Earth. This includes the Earths processes, composition, history and so much more. Early naturalists believed that volcanic eruptions and earthquakes were caused by violent winds escaping the Earth. Da Vinci saw shell fossils high in the mountains and recognized that they were from once living organisms and that there had been fluctuations in water levels. Geological breakthroughs regarding sedimentation, the intrusion of igneous rock, and that the geologic processes (such as erosion, weathering and plate tectonics) were introduced during the seventeenth and eighteenth centuries. James Hutton founded the theory of uniformitarianism. Uniformitarianism states that the processes that occurred in the past are the same as the processes occurring in the present. This has been abbreviated to “the present is the key to the past”. So ripples in the sand that are found in coastal environments are formed in the same way as ripples found in hardened sandstone. The theory of plate tectonics is a relatively new theory (established during the 1960’s) that now encompasses the previous theory of continental drift. I’ve gone through my mother’s geology text books from the early seventies and plate tectonics wasn’t the popular theory yet. The reason I enjoy geology so much is because the land and rock around us tell stories and once you learn how to read it the story of the Earth’s past is an open book.
The Rock Types
Rocks fall into three main categories these include sedimentary rocks (layered rocks), metamorphic rocks (changed rocks), and igneous rocks (volcanic rocks). These rock types are constantly being recycled through the rock cycle.
Mountains are being weathered and worn down into sediment that later hardens into sedimentary rock such as sandstone or shale. Sedimentary rocks are not usually found in the White Mountain Region. Eventually sandstones, conglomerates, and mudstones will form along old river beds.
When rock undergoes extreme pressures or temperatures it is morphed to form metamorphic rock like gneiss or schist. At times during it’s formation it is deformed and forms beautiful folds in the rock. You can find gneiss and beautifully folded schist in the Presidential Range.
Igneous rock originates as melted rock that either hardens after it comes out of a volcano (extrusive igneous rock) and forms rock like ryolite, andesite, or basalt. Or it hardens before it reaches the surface intrusive igneous rock) and forms rocks like granite(New Hampshire is the Granite State). If you’re looking for extrusive igneous rock take a hike up Kearsage North (off Hurricane Mountain Road, Intervale, NH) and you’ll find debris flows that carry pieces of rock now found primarily on top of Mount Washington.
These different rock types make up the Earth’s crust which is made up of individual plates. There are seven major plates and about twenty smaller ones. These plates move and interact with each other above the mantle. Within the mantle there are convection currents which drive plate interactions. The mantle is heated by the core. The hot material rises through the crust pushing plates apart. As the hot material cools it pushes plates away from the spreading center and towards other plates creating subduction zones. This now cool material then settles back towards the core to be reheated. Depending on what stage the convection current is in will govern what is happening to the plate and the surface features found in that area. This process reminds me of heating a bowl of tomato soup. The stove is the core which heats the soup, or mantle, eventually the soup heats up and the bubbles of hot soup (convection currents) rises to the surface creating breaks in the film (crust) that inevitably forms if you don’t stir the soup.
The Plate Boundaries
There are two main types of crust that compose the plates. These are continental and oceanic crust. Oceanic crust is denser and is usually composed of basalt, while continental crust is composed of ‘lighter’ rocks. During a collision between these two crusts oceanic crust will slide underneath the continental crust because of that density difference. There are three main types of plate boundaries where these plates interact with each other.
The first are spreading centers or rift zones. This is where one of the convection currents is breaking the crust’s surface forcing the two plates on either side to gradually move away from each other. An example of this would be the Mid Atlantic Ridge which runs down the center of the Atlantic Ocean. Iceland is where this molten material, which breaks through the crust in the form of volcanoes, has broken the ocean’s surface. Volcanic activity is what drives the thermal vents and hot springs.
When two plates are moving past each other they form a transform boundary. Since rocks are not completely smooth they build up friction and will lock. Over time pressure builds until the two plates slit and move past each other. This usually results in an earthquake. One common example of this is the San Andreas Fault in California. The last type of plate boundary is the collision zone.
There are two types of plate collision boundaries. These are subduction zones and continent-continent collision zones. In subduction zones you have an oceanic plate colliding with a continental plate. Due to the density differences between the compositions of the plates the oceanic plate slides under the continental plate and is subducted. On the surface you will generally find deep trenches where the two plates collide. Further inland you’ll generally find a chain of volcanic mountains. These volcanoes are found inland after a subduction zone because as the oceanic plate is pushed further and further down into the mantle it starts to melt and some of this melt rises to the crusts surface forming volcanoes. In continent-continent collisions you have two continental plates of equal densities colliding and forming tall mountain ranges. The majority of the continental crust does not get subducted and instead compresses against the other colliding plate. Presently this is happening in the Himalayans and occurred along the Appalachian Mountains millions of years ago.
These basic principles will play a large role in the next post when I get into the Mountain building events that went into shaping the Appalachian Mountains and the White Mountains. Check back soon for more information on the geology and other fascinating natural observations.
The plate tectonic figures can be found at: http://pubs.usgs.gov/gip/dynamic/understanding.html