File:Basalt (Newark Supergroup, Lower Jurassic; Somerset County, New Jersey, USA) 19.jpg

English: Basalt from the Jurassic of New Jersey, USA.

Igneous rocks form by the cooling and crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Basalt is a common extrusive igneous rock. It is the dominant rock in Earth’s upper 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. This refers to igneous rocks that are generally dark-colored, have 45-55% silica (= SiO2 chemistry) (it has also been defined as 45 to 52% silica), 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 or almost all crystals are less than 1 millimeter in size), which is the result of relatively quick cooling of lava. Some basalts 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).

This New Jersey sample comes from the Newark Basin, from one of three basalt units in the Upper Triassic to Lower Jurassic Newark Supergroup, a succession of siliciclastics-dominated rocks filling a series of rift basins. Rift valleys formed during the Triassic, in what is now eastern North America, as the ancient Pangaea supercontinent attempted to break apart. The initial rifting failed, but a subsequent rifting event in the Jurassic was successful.

Stratigraphy: Newark Supergroup, Lower Jurassic

Locality: unrecorded/undisclosed site in Somerset County, New Jersey, USA

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For more info., see:

en.wikipedia.org/wiki/Newark_Supergroup