Science Explorer. Multimedia Gallery. Park Passes. Technical Announcements. Employees in the News. Here some vesicles and olivine phenocrysts may be seen. The white spots are where the sample was scratched.
When you collect some special rock to display on your mantle piece, pack it carefully so that it doesn't rub against other rocks or hard objects! E-mail C. Jones with comments or corrections. Geology and Planetary Science Home Page. White Park Bay, Northern Ireland. Basaltic rock most likely basanite from Caldera de Taburiente, La Palma. Black is pyroxene augite, orange is olivine or more precisely what is left of it.
Orange patches are former olivine crystals that are now composed of a mixture of silicates and iron oxides which is known as iddingsite. Olivine is a common mineral in many basaltic rocks. Width of view 10 cm.
Another basalt chemically probably picrobasalt with lots of olivine fresh olivine is bright green, but it gets more and more yellow as it weathers. Width of sample 6 cm. Subaerial basalt forms lava flows or pyroclastic fields and cones. Two main types of basaltic lava flows are aa lava and pahoehoe lava. Aa lava has rough rubbly irregular crust while pahoehoe is smooth. Lava crust of aa type is broken into pieces while pahoehoe retains its continuity.
Both lava flow types are massive beneath the crust and this massive interior may be columnar. Columns are separated from each other by narrow cracks which form because cooling basaltic magma contracts.
Cracks start to form at the surface and propagate deeper as lava cools. Submarine basalt usually forms pillows. Pillow basalt forms as a result of very rapid cooling. Outer part of forming pillow cools very quickly in contact with cold seawater while the interior still fills with molten lava.
Basalt mostly forms lava flows because it is among the least viscous magma types and therefore does not generate explosive volcanic eruptions, but sometimes pyroclastic material is formed when magma contains more volcanic gases. Basaltic rocks can be thrown out of volcanic vents as lapilli singular: lapillus and volcanic bombs. Basaltic volcanoes are fed by dikes planar intrusive rock bodies when solidified that cut through other rocks and sills similar to dike but generally parallel to preexisting bedding planes.
Aa lava in the foreground. La Palma, Canary Islands. Pahoehoe lava ropy lava. Pillow lava near Fasoula, Troodos ophiolite, Cyprus.
Pillow lava is very common on Earth, but difficult to find because almost all of it is on the ocean floor. Examples can be found on land usually where former ocean floor is tectonically squeezed between two blocks of continental crust.
Scoriaceous lapillus from Etna, Italy. Despite being 5 cm in width it weighs only 15 grams because it is filled with gas bubbles vesicules. Similar rock type with a felsic composition is pumice. Sometimes dikes are so close to each other that the whole outcrop is composed of them. These sheeted dikes in Cyprus once fed volcanoes on the ocean floor.
What is a hole in a rock called? Pit is the general name for a hole in sedimentary rock that is produced by weathering. Small pits are typical of alveolar or honeycomb weathering, and large pits are called tafoni. Is scoria a sedimentary? Scoria is a highly vesicular, dark colored volcanic rock that may or may not contain crystals phenocrysts. It is typically dark in color generally dark brown, black or purplish red , and basaltic or andesitic in composition.
Most scoria is composed of glassy fragments, and may contain phenocrysts. What is a rock with a hole in it called? Hagstones are rocks that have naturally occurring holes in them. The names for the rocks vary by region, but hagstones have been viewed as magical across the world.
What is the texture of basalt? Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow; it is often porphyritic, containing larger crystals phenocrysts formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. Is Basalt mafic or felsic? Granite and rhyolite are considered felsic, while basalt and gabbro are mafic click here for more information on mafic and felsic.
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