BASALT
Colour: When fresh it is black or greyish black; often weathers
to a reddish or greenish crust.
Grain size: Fine.
Texture: Usually dense with no minerals identifiable in hand specimen;
a freshly broken surface is dull in appearance. May be porphyrithic.
Structure: Often vesicular and/or amygdaloidal. Xenoliths are
relatively common and usually consists of olivine and pyroxene;
they have a green colour. Columnar jointing is common and often
spectacular.
Mineralogy: Phenocrysts are usually olivine (green, glassy), pyroxene
(black, shiny) or plagioclase (white-grey, tabular). If olivine
is present the rock is called olivine basalt. Microscopic examination
show the groundmass to consist of plagioclase (usually labradorite),
pyroxene, olivine and magnetite, with a wide range of accessory
minerals. Amygdales may be filled, or partly filled with zeolites,
carbonates or silica, usually in the form of chalcedony or agate.
Field relations: Lava flows and narrow dykes and sills. The edges
of dykes or sills are often finer grained than the centers or
even glassy, due to rapid cooling on intrusion. Most basalts occur
as lava flows either in volcanoes or as extensive sheets building
up a lava plateau, which may cover hundreds of thousands of square
kilometres, and may be fed by numerous fissures. The surface forms
of lavas are of two principal types; smooth or ropy (the surface
looks like a rope) which is known by the Hawaiian term of pahoehoe,
and scoriaceous which is rough and clinkery and has the Hawaiian
name aa. Another common form is pillow lava which consists of
pillows or balloon-like masses of basalt - usually with a very
fine-grained or glassy outer layer. They are formed by the eruption
of lava into water." (Hamilton
et al 1976, 170)
