BASALT
Basalt
is a very common extrusive igneous rock. Basalt is the dominant rock in
Earth’s oceanic crust. Black lava rocks seen at famous volcanoes
such as Mt. Kilauea in Hawaii and Mt. Etna in Sicily are composed of
basalt. Basalt has a mafic chemistry. Mafic igneous rocks
are generally dark-colored, have 45-52% silica (= SiO2 chemistry),
are rich in iron (Fe), magnesium (Mg), & calcium (Ca), and are dominated by
the minerals plagioclase feldspar and pyroxene.
Basalts have an aphanitic texture (finely
crystalline; all crystals <1 mm in size), but some are porphyritic or
coarsely crystalline if the lava flow took time to cool & solidify (e.g.,
some Proterozoic basalt lavas in the Keweenaw Peninsula of Michigan's Upper
Peninsula).
Basalt
Olivine basalt (4.0 cm across) - some basalts have obvious
yellowish-green olivine crystals (= ((Mg,Fe)2SiO4). This olivine basalt is from the Neogene
near Keeler, western Inyo County, eastern California, USA.