Identification: Picrite is best
identified by its setting, its dark
green to black colour, its slightly
shiny, sugary look and its even
texture - not mottled like
peridotite nor with plagioclase
feldspar laths like dole rite
(diabase). which are
both often found
with picrite.
Picrites are dark, heavy rocks rather similar to peridotite.
Both form deep in the Earth's mantle, and both are rich in
dark green olivine and brown augite. But while peridotite is
often carried up into large intrusive masses, along with
gabbro, norite and
pyroxenite, picrite tends to be
found in sills and intrusive
sheets. Although picrite
magma forms only under
extreme pressures deep in the
mantle, it is the one
ultramafic rock that normally
erupts on the surface as lava,
as it did in the 1959 eruption
of Kilauea in Hawaii. In this
eruption, gigantic fire fountains
shot out picrite lavas containing as
much as 30 per cent olivine. Yet for
picrite to erupt as lava like this,
temperatures must be very high incleed -
which is why it is often linked to hotspot
volcanoes such as those in Hawaii.
Picrite is also often found in association with basalt as part of the ocean floor, which is
why many of the best known occurrences of picrite are in ophiolites - chunks of the ocean
floor brought to the surface by massive tectonic movements.
Occasionally, picrite can occur in substantial quantities in flood basalts, as it does in India's
Deccan and South Africa's Karoo, but most flood basalts have a fairly low picrite content.
Picrite is very rich in magnesium and iron. Indeed, one definition of picrite is that it has
at least 18 per cent magnesium oxide by weight. Komatiite is similar but has even less
sodium and potassium oxide. Picrite's iron content can be so high that it is actually
slightly magnetic. Some picrites, though,
are especially rich in hornblende
(see right). Others are
especially rich in augite, like
a few of those of Devon and
Cornwall in England, which
are sometimes called
palaeopicrites because they
formed well over half a billion
years ago in the Palaeozoic
era. Most picrites in this part
of the world date more
specifically to the Devonian
period, 408-360 million