What does amygdaloidal mean?
Definition of amygdaloidal : of, being, or containing small cavities in igneous rock that are filled with deposits of different minerals (such as chalcedony)
Where is amygdaloidal basalt found?
Amygdaloidal basalt is an igneous rock which is extrusive in nature, these specimens formed over 60 million years ago during the Tertiary period and come from Ireland. They are part of the Antrim lava group and exhibit a dark colouration with occurrences of amygdales containing quartz and calcite crystals.
What is amygdaloidal texture?
A related texture is amygdaloidal in which the volcanic rock, usually basalt or andesite, has cavities, or vesicles, that are filled with secondary minerals, such as zeolites, calcite, quartz, or chalcedony. Individual cavity fillings are termed amygdules (American usage) or amygdales (British usage).
How are vesicles formed in basalt?
Basalts are most commonly vesicular. The drop in pressure that a magma experiences as it flows from underground to the Earth’s surface allows water and gases in the lava to form bubbles. If the bubbles do not get large enough to pop, they are frozen in the lava as vesicles.
What causes columnar jointing?
Why do columnar joints form? As a lava flow cools from the outside edges toward its center, molten lava slowly solidifies into rock in the same direction—the solid-liquid interfaces move toward each other from all sides.
What is Ophitic texture in geology?
Ophitic texture is the association of lath-shaped euhedral crystals of plagioclase, grouped radially or in an irregular mesh, with surrounding or interstitial large anhedral crystals of pyroxene; it is characteristic of the common rock type known as diabase.
What is Phaneritic and aphanitic?
APHANITIC TEXTURE – Igneous rocks that form on the earth’s surface have very fine-grained texture because the crystals are too small to see without magnification. PHANERITIC TEXTURE – Igneous rocks with large, visible crystals because the rock formed slowly in an underground magma chamber.
What does Aphanitic texture mean?
Aphanitic – This texture describes very fine grained rock where individual crystals can be seen only with the aid of a microscope, i.e. the rock is mostly groundmass. An aphanitic texture is developed when magma is erupted at the Earth’s surface and cools too quickly for large crystals to grow.
What is an amygdule?
Unsourced material may be challenged and removed. Amygdules or amygdales ( / əˈmɪɡ.djulz, – deɪlz /) form when the vesicles (pores from gas bubbles in lava) of a volcanic rock or other extrusive igneous rock are infilled with a secondary mineral, such as calcite, quartz, chlorite, or one of the zeolites.
What is amygdule in igneous rock?
Amygdule. Amygdule, secondary deposit of minerals found in a rounded, elongated, or almond-shaped cavity in igneous rock. The cavities (vesicles) were created by the expansion of gas bubbles or steam within lava. Some amygdules consist partially of lava, which indicates their formation during solidification of the rock.
What causes amygdules to form?
Amygdules usually form after the rock has been emplaced, and are often associated with low-temperature alteration. Amygdules may often be concentrically zoned. Rocks containing amygdules can be described as amygdaloidal .
Where are amygdules found in lava?
Since gas bubbles tend to rise through the lava, amygdules are most common near the tops of flows. The secondary minerals may form after the solidification of the lava; the gases may help to form the filling minerals. A great variety of minerals have been found as amygdules, including some spectacular museum specimens of zeolites.