Boogardie Station is a small settlement in the outback of
Western Australia. With the nearest city, Perth, over 550 km away,
it’s safe to say that it’s fairly remote. Yet it also has great
geological significance, for this region of Australia contains
ancient parts of earth’s crust. In fact some material mined from
the quarry there has been dated at over 2.6 billion years old.
And now a superb sample of rock from Boogardie Station has found
its way to Edinburgh. The rock, a rare specimen known as orbicular
granodiorite, stands 1.7m high, weighs a hefty 870kg, and is
believed to be the largest of its kind in the UK.
Where will the orbicular granodiorite be displayed?
The orbicular granodiorite is set to become one of the key
monoliths on the origin of rocks in the Restless Earth gallery,
which will explain the structure of the earth and its complex and
varied history.
Various different types of rocks will be displayed to emphasise
their wide variation. These variations can be split into three
different types: igneous (rocks formed by complete melting),
metamorphic (rocks formed by transforming an existing rock type by
exposing it to heat and pressure) and sedimentary (rocks formed by
deposition and consolidation of mineral and organic material and
from precipitation of minerals from solution). The orbicular
granodiorite will represent one of the many types of igneous rock
found in the earth.
What’s so special about orbicular granodiorite?
Both granodiorite and orbicular granodiorite are formed when
magma (molten rock beneath the earth’s surface) intrudes into the
earth’s crust deep underground then cools down, forming
crystals.
The crystals that make up granodiorite and orbicular
granodiorite are made of the same material. However, if you compare
the images in the diagram below, they look remarkably different.
Somehow the orbicular granodiorite has formed large concentric
orbs, whereas the granodiorite hasn’t.
The diagram above shows the differences between a micro
granodiorite, a granodiorite and an orb of granodioritic
composition. Despite the apparent visual differences, they are
comprised of identical materials.
How is orbicular granodiorite formed?
Scientists have yet to agree on how the orbs are formed. Usually
the slower a magma cools, the larger the crystals formed,
suggesting that the orbs take a long time to form. However, they
are surrounded by a finer grained layer that is chemically
identical to the orbs. This suggests two phases of cooling: one
slow phase needed for orb formation and one rapid phase for the
surrounding layer. The problem with this theory is various elements
would be removed from the mix during the slow cooling phase,
changing its composition and altering the final surrounding
layer.
However, some researchers suggest that the orbs form quite
quickly, when water enters the magma chamber, altering the ability
of the magma to remain molten. Orbs would then form rapidly and
start to drift around in the hot magma chamber. This means that the
orbs remain plastic and pliable, which explains why they are not
all perfect spheres.
For the orbs to remain fully formed they have to settle in a
region of the magma chamber that has just the right set of
environmental conditions. If they move into a part of the magma
chamber that is not ideal for them, they will be re-melted into the
magma. This would probably explain why orbicular rocks tend to be
found in small areas.
Other scientists suggest the formation of the orbs is due to a
shortage of material in the magma for the crystals to form around
(known as a nucleation point), so the magma wants to form a solid
mass of crystals as it cools but cannot. Eventually the magma
cannot remain in this liquid state and quickly forms the centres of
the orbs. Other minerals would then form around these centres
fairly rapidly, building concentric shells.
Other more complex ideas about the rocks’ formation have been
put forward; but however the orbs are formed, there’s no denying
that this is a beautiful and fascinating specimen.
The diagram above shows a cross section of a typical
subduction plate boundary showing orbs forming in the base of a
magma chamber. Click on the image to see a larger version.
Where can I see the orbicular granodiorite?
This spectacular rock is on display in the Restless Earth gallery, which explores how our
planet has changed over billions of years.
Above: Installing the orbicular granodiorite in the gallery. The
rock is so heavy it had to be moved into position using a
crane.
Above: The Restless Earth gallery. Photo © Jenni Sophia
Fuchs.