New Hampshire Geology New Hampshire Bedrock Map

Precambrian Period page 2
(4600-540 million years ago)

The Earth Passes Gas

During the next 1.3 billion years, known as the Archean Eon (28.26 yards long on the "100 Yard Model"),  the  Earth separated itself into layers while it was still liquified.   Denser material settled toward the center of the planet and lighter material rose towards the surface.  This has resulted in the Earth having four major layers, as shown in the diagram on the right.

During this time, the various gases that would make our early atmosphere and oceans were mixed into this molten mess.  These gases now had the opportunity to bubble up and pass through the liquid rock to escape to the surface.  Once the gases passed through, they were kept from floating off into space by Earth's gravity.  Today, our atmosphere, our oceans and the crust are recycled and renewed through volcanic activity, both on land and under the oceans.  This helps keep our atmosphere from getting 'stale'.

After a time the Earth's exterior began to cool and form a hard crust.  However, due to the convection (warmer materials rising and cooler materials sinking in a circular motion) of the earth's interior, the crust cracked into many pieces known as tectonic plates.  These plates are like giant puzzle pieces of the Earth's crust. The convection also moves the plates around as the top of the convecting molten rock - with a consistency of thick, sticky taffy - 'grabs' onto the underside of the plates and puls them along in whatever direction that particular bit of convection is going. As tectonic plates collide, several things can happen:

1    One plate can slide under another and dive back into the mantle, melt and be recycled in a future volcanic event, a process known as subduction. Subduction occurs, among other places, as the edges of the Pacific plate go under all of the continental plates surrounding the Pacific creating what geologists call the 'Ring of Fire'. Mount Saint Helens in the Cascade Range of Washington State is part of this 'Ring of Fire'; or

2    Both plates can push into each other causing mountain chains to rise along their colliding boundaries, which is how the ancient Appalachian Mountains were formed (and today's Himalayas are continuing to be built); or

3    Plates can diverge, or move apart from each other, creating a gap where magma (liquid rock in, or below, the crust) rises up to form volcanoes. This process is occuring at the Mid-Atlantic Ridge on the seafloor of the Atlantic Ocean; or

4    Plates can slide past each other in what is called a 'Strike-Slip Fault' which is occuring along the San Andreas Fault in California.

Separation Anxiety

As two plates move away from each other they open up a gap known as a rift.  This rift allows molten magma to seep up from the mantle to fill in the gap.  The Mid Atlantic Ridge is a well known example of a rift zone

Like Shrek, The Earth Has Layers

Image Credit: USGS

Plate Tectonics

The theory of plate tectonics was first developed by the German scientist, Alfred Wegener.  He saw the continents on each side of the Atlantic Ocean as giant puzzle pieces.  He figured (correctly) that they must have been joined together at one time and somehow drifted apart. More on plate tectonics.

Image Credit: USGS



Next Page