Book - Sustainable Aggregates

4 Exploring The national forest
the Carboniferous and Precambrian. Some
rocks can be dated in millions or thousands
of years, measured by the radioactive decay
of certain elements in the minerals that they
contain.
Plate tectonics
Throughout geological time, continents
have moved across the Earth’s surface by
a mechanism known as plate tectonics.
Since the Precambrian rocks of Charnwood
Forest were formed over 600 million
years ago, plate movements have shifted
England from the southern to the northern
hemisphere. In doing so, it has passed
though every major climatic zone we see
today, from equatorial to arctic. Throughout
this journey, the land has been submerged
under the sea on many occasions, but has
also been raised above sea level, washed by
major rivers and covered by tropical rain
forest or desert. Most recently, it has been in
the grip of ice ages. These changes in climate
and environment have helped to create a
great diversity of landscape and geology.
Tectonic plates are rigid portions of our
crust that are continually moving. These
movements are so slow, rarely
much more than a few millimetres
per year, that we cannot see
them happening. However, they
generate earthquakes which are
sometimes felt along the zones
where the plates meet. Where
plates move apart (diverge), such
as in the middle of the Atlantic,
molten rock (magma) rises and
solidifies to form new oceanic
crust. In England we are a long
way from a plate boundary and
so the incidence of earthquakes
is very low; when they do occur,
they are generally insignificant,
causing little damage.
Oceanic crust
Lithosphere
Precambrian — volcanoes, seas and the
dawn of life
The Precambrian rocks of Charnwood
Forest are some of the oldest in England.
They were formed when England was in
the southern hemisphere, located along
an immense structure called a subduction
zone. This was the meeting point between
two colliding tectonic plates, one of
which was forced down beneath the
other. This caused the rocks to melt at
depth; the resulting magma rose to the
surface, forming a chain of active volcanoes
surrounded by the sea — known as an
island arc. These volcanoes were very
violent and explosive. They produced
virtually no lava flows that we can see,
but spewed out huge amounts of ash,
accompanied by solid but red-hot debris
that avalanched down the slopes of the
volcanoes and into the sea as pyroclastic
flows. The fine-grained ash settled on the
sea floor after falling out from the air,
amassing a great thickness of volcanic (or
volcaniclastic) material (at least 3.5 km)
forming a sequence of rocks known as
the Charnian Supergroup. The active
volcano on the island of Montserrat in the
Caribbean is thought to be a modern-day
Trench
SUBDUCTION ZONE
Charnian
volcanoes
Magma
rises
Rocks melt
Island arc
Continental
crust
Lithosphere
Reconstruction of the Precambrian subduction zone
beneath England and Wales.