Is porphyritic mafic or felsic?
Igneous Rocks
| Atlas of Igneous Rocks | ||
|---|---|---|
| Rock Name / Image | Texture | Composition |
| Rhyolite Porphyry | Porphyritic | Felsic |
| Rhyolite Tuff | Pyroclastic | Felsic |
| Scoria | Scoreaceous | Mafic |
Is porphyritic texture intrusive or extrusive?
Porphyritic texture — andesite: This is an extrusive igneous rock. The magma from which it formed cooled slowly for a while deep below the surface (forming the large crystals), then finished cooling very quickly when it was ejected at the surface, forming the fine-grained groundmass.
What is the cooling history of porphyritic texture?
Porphyritic:a mixture of large and small crystals – perhaps a two-stage cooling history with the large crystals (the phenocrysts) cooling slowly and the small crystals (the groundmass) cooling more quickly. The rock to the right has a phaneritic texture; individual grains can be seen.
What are the 4 textures of igneous rocks?
Igneous Rock Textures
- COARSE GRAINED TEXTURE (PHANERITIC), mineral grains easily visible (grains several mm in size or larger)
- B) FINE GRAINED TEXTURE (APHANITIC), mineral grains smaller than 1mm (need hand lens or microscope to see minerals)
- C) PORPHYRITIC TEXTURE (MIXED FINE AND COARSE)
Is porphyry an intermediate?
Intrusive rocks in porphyry copper provinces are generally intermediate to felsic in composition, commonly granodiorite and quartz monzonite. They are almost invariably I-type and calc-alkaline, although some gold-rich deposits are associated with alkaline rocks.
Are porphyritic rocks intermediate?
Intermediate Composition Andesite is a fine crystalline intermediate extrusive rock. It is commonly grey and porphyritic.
What causes porphyritic texture?
A porphyritic texture is developed when magma that has been slowly cooling and crystallising within the Earth’s crust is suddenly erupted at the surface, causing the remaining uncrystallised magma to cool rapidly. This texture is characteristic of most volcanic rocks.
Is diorite a porphyritic?
Diorite has a phaneritic, often speckled, texture of coarse grain size and is occasionally porphyritic. Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from the partial melting of a mafic rock above a subduction zone.
What are the 5 types of textures of igneous rocks?
Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.
What are the 3 major texture categories?
Thus there are distinct igneous textures, distinct sedimentary texture, and distinct metamorphic textures. For the purposes of this exercise and routine classification, the kinds of minerals and their proportions, or MINERALOGY, are taken as the natural expression of composition.
What does porphyritic texture indicate?
This porphyritic texture indicates that the magma sat and cooled a bit below the Earth’s surface, thus giving time for the large crystals to grow, before erupting onto the surface and cooling very quickly. The large crystals are termed phenocrysts while the aphanitic rest of rock is called the groundmass.
What does a porphyritic texture indicate about an igneous rock?
Porphyry is an igneous rock characterized by porphyritic texture. Porphyritic texture is a very common texture in igneous rocks in which larger crystals (phenocrysts) are embedded in a fine-grained groundmass. Porphyry is an igneous rock that contains larger crystals (phenocrysts) in a fine-grained groundmass.
What is phaneritic texture?
Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth’s surface.
What is an example of porphyritic igneous rock?
Porphyritic is used as an adjective to modify the name of any fine-grained igneous rock that has less than 50% phenocrysts in it. Example: Porphyritic basalt, porphyritic andesite. Porphyry is used as a noun after the rock name if more than 50% of the rock is made of phenocrysts.
How are porphyritic rocks formed?
Formation. Porphyritic rocks are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano,…
