Every rock has a story to tell.
No matter what kind of rock, or where you find it,
it preserves evidence of its history. And we can learn to decipher that history.
Take for example, the rock in the Science Center lobby.
This piece of mica schist, now serving as a wall decoration,
started its life long before the dinosaurs. It has undergone many changes
as it has traveled many miles, both horizontally and vertically
over hundreds of millions of years. Its story starts about 450 million years
ago,
on the East coast of the proto-North American continent, where rivers flowed
into the ocean.
The muddy sediment carried by the rivers
slowly settled out in the quiet water beyond the waves.
Over hundreds of thousands of years,
more mud was deposited every year, gradually accumulating to form a thick
layer.
During this time, the Iapetus Ocean was gradually decreasing in size,
because the tectonic plate carrying the proto African continent
was converging with the proto-North American plate. The western, leading-edge
the proto- African plate bend down and subducted underneath the eastern edge of
the proto-North American plate.
The presence of a deep oceanic trench along this subduction zone
allowed a great thickness sediments eroded from the proto-North American
continent
to accumulate.
The increasing weight overlying younger sediments caused the mud to be compacted
and cemented
into a fine-grained sedimentary rock called shale.
Eventually the Iapetus Ocean floor was completely subducted,
allowing proto-Africa to collide with proto-North America,
forming the super continent of Pangea. Continents have too low a density to be
subducted,
so continent-continent collisions cause intense deformation of
the sedimentary rocks caught between. Our shale layer was buried even deeper
and folded like an accordion. This compression cause the clay minerals in
the shale to be rotated into alignment
perpendicular to the compression, and the higher temperature at this depth
caused the clay minerals to re crystallize into somewhat larger
mica grains,
forming a metamorphic rock called slate. Clay minerals and micas
all have a crystal structure made up of thin 'sheets' of atoms
that are strongly bonded within each sheet, but have very weak bonds between
each sheet. Graphite has a very similar crystal structure,
which is why it is used in pencils. When you write on a page with one,
you brush off the sheets of carbon atoms.
When clay minerals and micas become strongly aligned in a rock that is squeezed,
the rock itself develops a fine-scale layering
or foliation along which it tends to break most easily,
much like the pages in a book
Thus, the orientation of the foliation in an
ancient metamorphic rock preserves a record of the plate compression
direction.
Continued compression between the two continents further deformed the rock.
It formed the very high ancestral Appalachian Mountains,
and also formed correspondingly deep 'roots'. This deformation
caused the slate to be buried somewhat deeper, where the temperature was even
greater.
This in turn caused the mica grains to grow even larger in size,
form this sparkly silver colored muscovite we see today,
and transforming the rock from a slate into a mica schist
After the Appalachian Mountains reached about as high as the present Himalayas,
the two continents could not converge anymore, and so the mountains
began slowly erode. About 200 million years ago,
there was a reversal a plate spreading directions, which caused the
supercontinent Pangea to rift apart,
and the present Atlantic Ocean began to open. The rifting created valleys like
the African rift valley of
today, and dinosaurs left their footprints in the mud along lakeshores in
present day Connecticut Massachusetts.
the Appalachian Mountains continued to be eroded during this time;
glaciers and rivers and landslides gradually wore them down,
lower and lower, transporting sediments downhill and ultimately depositing them
in the growing
Atlantic Ocean. As the mountain peaks were lowered by erosion
the total mass above ground decreased. To compensate for this,
the mountain roots rose buoyantly upward, just like an iceberg
that melts on the top is replenished by the buoyant rise of it root.
Thus over tens of millions of years, the once deeply buried mica schist
was passively uplifted closer and closer to the Earth's surface.
The schist was finally exposed at the surface
within the recent Pleistocene lce Age, when ice sheets originating in northern Canada
scraped south across New England multiple times. And then only very
recently,
a geologist found the mica schist in New Hampshire, took it back to Brown,
and put together its amazing story.