File:Porphyritic diabase (Cape Ann, Massachusetts, USA) 2.jpg

English: Porphyritic diabase from Massachusetts, USA.

Igneous rocks form by the cooling and crystallization of hot, molten rock (magma and 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. Basalt 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. Mafic igneous rocks are generally dark-colored, have 45-55% silica (= SiO2 chemistry), are rich in iron (Fe), magnesium (Mg), & calcium (Ca), and are dominated by the minerals plagioclase feldspar and pyroxene.

Many basalts have an aphanitic texture (finely crystalline; all or almost all crystals are <1 mm in size). Porphyritic basalt has the same chemistry & mineralogy as ordinary basalt, but differs in the size range of its crystals. Porphyritic basalts have a mix of large & small crystals. Large crystals in a porphyritic rock are called phenocrysts. The smaller crystals make up the groundmass.

The rock seen here is a porphyritic diabase, the intrusive version of a porphyritic basalt. The light-colored masses are single, large plagioclase feldspar phenocrysts.

Porphyritic diabase is sometimes referred to by the horrid term "diabase porphyry".

Geologic unit & age: unrecorded, but possibly from the Cape Ann Plutonic Suite (Cape Ann Plutonic Complex), Middle Ordovician to Devonian

Locality: unrecorded site at Cape Ann, eastern Essex County, far-northeastern Massachusetts, USA