File:Porphyritic diabase (Cape Ann, Massachusetts, USA) 1.jpg
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English: Porphyritic diabase from Massachusetts, USA. (5.3 centimeters across at its widest)
Igneous rocks form by the cooling & 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. 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 45-52% silica (= SiO2 chemistry) (mafic has also been defined as 45 to 55% silica), 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 shown above is a porphyritic diabase, the intrusive version of a porphyritic basalt. The light-colored masses are single, large plagioclase feldspar phenocrysts. The dark-colored, finely-crystalline groundmass consists of a mix of plagioclase feldspar, pyroxene, hornblende amphibole, and biotite mica. Porphyritic diabase is sometimes referred to by the horrid term "diabase porphyry". Geologic unit: 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 |
| Date | |
| Source | https://www.flickr.com/photos/47445767@N05/16746789261/ |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/16746789261. It was reviewed on 19 June 2022 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 08:27, 19 June 2022 | | 1,337 × 820 (1.82 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/16746789261/ with UploadWizard |
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