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COROMANDEL LIFE LATE SPRING / HOLIDAY 2018
Left: 150 million-year-old Petrified Wood
logs in the Heaven & Earth gallery. Many
customers mistake them for real timber.
The rich brown shades of this polished piece
of petrified wood from Mill Creek in Whitianga
earned it the nickname ‘pumpkin rock’.
Of all the amazing stones of the Coromandel
Peninsula, petrified wood stands out as one
which bridges the gulf between the world of
living organisms and the mineral realm.
The formation of the Coromandel land
mass was almost completely volcanic, and
the many beautiful mineral and gemstone
treasures found here – quartz crystals,
carnelian, jasper, spherulitic rhyolite, andesite
and others – were all created through
volcanic activity.
FORMED BY PREHISTORIC ERUPTIONS
One of the most fascinating of these is
petrified wood, derived from the Greek root
‘petro’ meaning rock or stone – so literally
“wood turned into stone”. It is basically a
type of fossil in which the form of a tree
trunk or branch has evolved to stone in
a long, gradual process initiated by
violent volcanic action.
Uprooted by great flows of lava
or volcanic ash, trees were then
washed down from the highlands
and buried under cooling lava,
volcanic ash, and/or silt. This
sealed the wood in an oxygen-
free environment that prevented
normal decay. Groundwater, rich
in dissolved solids, gradually
seeped through the wood over
centuries, replacing the decaying
plant material cell by cell with silica
(quartz), calcite, pyrite, or other
inorganic materials such as opal.
Because the process is incredibly slow –
taking place literally one molecule at a time
– the rock formed often carries recognisable
features of the original tree or branch such as
bark, wood grain and cellular structures (such
as growth rings). Some specimens are such
accurate preservations that people do not
realise they are fossils until they pick them
up and are shocked by their weight. While its
rare to find these near-perfect preservations,
specimens that exhibit recognisable bark
and woody structures are common.
Recorded age may vary greatly universally,
with some pieces dating as far back as 385
million years. The famous Arizona Petrified
Forest in the USA is estimated at 225 million
years old. In specimens from Arizona, as well
as those from Madagascar, the silica often
appears in a vivid array of colours (see photo
below right).
Some NZ petrified wood has been verified
to be 6-18 million years old, with some
examples showing black edging, indicating
that the wood was charred by volcanic ash
or lava.
Right: Petrified Wood from Arizona like
this colourful piece, is estimated to be
over 225 million years old.
– Photo by Jon Sullivan Public Domain
By Robert Simmons
from Heaven & Earth Gallery
Petrified Wood