What caused the large crystals in a porphyritic rock to form?
What caused the large crystals in a porphyritic rock to form?
Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix.
Do porphyritic rocks have crystals?
Porphyritic – This texture describes a rock that has well-formed crystals visible to the naked eye, called phenocrysts, set in a very fine grained or glassy matrix, called the groundmass.
How do you identify porphyritic rocks?
Rocks are said to be porphyritic if their groundmass is fine-grained ar aphanitic and porphyraceous if their groundmass in visible to the naked eye. Hence, rhyolite and basalt as fine-grained volcanic rocks are porphyritic and granite, syenite, etc. as coarse-grained plutonic rocks are porphyraceous.
What are the large crystals in porphyritic rocks?
igneous rocks The relatively large crystals in a porphyritic rock ordinarily occur as separate entities, known as phenocrysts, set in a groundmass or matrix of much finer-grained crystalline…
What does a porphyritic texture indicate about the history of igneous rock?
What does a porphyritic texture indicate about the cooling history of an igneous rock? It indicates that crystals were formed at depth (slow cooling) and then the magma moved to a shallow depth or erupted (fast cooling).
What are porphyritic crystals?
Porphyritic texture is an igneous rock texture in which large crystals are set in a finer-grained or glassy groundmass. Porphyritic textures occur in coarse, medium and fine-grained igneous rocks. Usually the larger crystals, known as phenocrysts, formed earlier in the crystallisation sequence of the magma.
What minerals are in porphyritic rocks?
Types of Porphyritic Rocks
- Quartz-porphyry. Quartz-porphyry has phenocrysts of quartz and usually feldspar, in a fine-grained ground mass.
- Feldspar-porphyry. Feldspar-porphyry is characterized by crystals of feldspar in a fine-grained or glassy ground mass; it has the composition of syenite.
- Lamprophyre.
Where are porphyritic rocks found?
Porphyritic textures are most common in volcanic rocks where one of the minerals will begin to crystallize prior to eruption (therefore producing larger grains), with the rest cooling more rapidly after eruption.
What does a porphyritic texture indicate?
What would a porphyritic texture indicate about a rock?
Many rocks with an overall fine-grained texture display scattered minerals that are more than 1 mm across. 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.
Where is porphyritic found?
Rhomb porphyry is found in continental rift areas, including the East African Rift (including Mount Kilimanjaro), Mount Erebus near the Ross Sea in Antarctica, the Oslo graben in Norway, and south-central British Columbia.
What is porphyritic rock used for?
Thus, “imperial”-grade porphyry was prized for monuments and building projects in Imperial Rome and thereafter. Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition.
