During the Cambrian
Period, the early North American and European tectonic plates were
colliding. These early plates were called cratons. A
craton is the original building block of a tectonic plate that gets added
on to or has sections separated from it later in Earth's geologic
As the two plates
approached each other, the edge of the North American craton started to
slide under the European craton. This is a process called subduction.
As subduction occurs, the diving plate plunges into the Earth's upper mantle, where
temperatures are much hotter. The higher temperatures melt the
subducted plate, creating magma. Hot
magma is less dense than the cooler tectonic plate and it starts to
rise to the surface. Warmer things like to rise and cooler things like to fall. This is called convection. When magma reaches the surface in an eruption,
geologists call this molten rock lava.
As the North
American plate subducted under the European plate, a chain of volcanic
islands formed from the rising magma. The islands formed in an
arc in the middle of the fast closing ocean between the two land
masses. This tropical volcanic island arc would closely resemble
the Caribbean Islands of today.
The Big Squeeze
Even while this
volcanic arc of islands was forming, the tectonic plates continued to
plow towards each other. Eventually, the ocean would close and
the islands would be crushed between the plates. These
'pasted-on' smaller land masses are known as exotic terranes.
Exotic means 'from somewhere else'. Almost all of New Hampshire is composed of exotic terranes. In fact, the only 'native' rock is from the volcanic rock created in the Cretaceous period. The eroded
remnants of these once tropical volcanic islands are among the first
parts of New Hampshire to appear on North America.
These islands still
exist as the hills of the Bronson Hill Complex.
This chain of hills stretches from the Connecticut Coast, up the
eastern side of the Connecticut River Valley to about Littleton, NH
where it curves off to the east and crosses over to the Maine border
Elsewhere on the
North American craton, this same collision pushed up the Taconic Mountains
and the Early Appalachian Mountains.
Looking at this Bedrock Geology Map of New Hampshire, developed by the
New Hampshire Geological Survey, the Bronson Hills can be seen as a
narrow band of purple splotches and streaks visible running from
the Connecticut River Valley in the southwestern part of the state,
running up to Littleton and hooking north and east into Maine.