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.