How does a rock form a 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.
How would you describe a porphyritic texture?
A porphyritic texture displays minerals in two distinct size populations: one or more minerals are consistently larger than the rest of the minerals in a rock. The extra large mineral grains are called phenocrysts.
What is the best description of porphyritic?
Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all types of igneous rocks can display some degree of porphyritic texture.
What can you conclude about a rock with porphyritic texture?
A rock has a porphyritic texture. What can you conclude about the rock? A porphyritic texture indicates that the rock underwent at least two different episodes of cooling.
What is an example of porphyritic texture?
Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals. Examples of porphyritic rocks are: andesite porphyry and rhyolite porphyry.
What does porphyritic mean in science terms?
porphyritic. / (ˌpɔːfɪˈrɪtɪk) / adjective. (of rocks) having large crystals in a fine groundmass of minerals.
How do phaneritic rocks form?
Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. As magma cools slowly the minerals have time to grow and form large crystals.
What kind of igneous rock has a porphyritic texture?
extrusive igneous rock 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 kind of rock is porphyritic?
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.
What is an example of a porphyritic rock?
Examples of porphyritic rocks are: andesite porphyry and rhyolite porphyry. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice.
What is phaneritic texture and how does it form?
PHANERITIC TEXTURE – Igneous rocks with large, visible crystals because the rock formed slowly in an underground magma chamber. PORPHYRITIC TEXTURE – an igneous rock in which PHENOCRYSTS (large crystals) are surrounded by a fine groundmass (very small crystals).
Why does porphyritic texture always go along with aphanitic or Phaneritic textures?
C) porphyritic- texture is combination of aphanitic and phaneritic textures. magma cools slowly beneath surface, magma is forced to the surface and those crystals that have already started to form are foced up with it. as lava cools quickly at surface, the larger crystals are trapped between smaller crystals.
What does porphyritic texture tell you about the history of an 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). The classification of igneous rocks is based largely on two criteria.
What does porphyritic texture indicate about the history of an 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). The classification of igneous rocks is based largely on two criteria.
What does a phaneritic texture tell us about how that igneous rock formed?
Phaneritic rocks indicate slow cooling of magma deep beneath the Earth's surface. With slow cooling, the rate of growth of mineral grains exceeds the rate of nuclei formation within the minerals of the rocks. As a result, relatively large mineral grains are formed, yielding a phaneritic texture.
How are phaneritic rocks formed?
Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface. As magma cools slowly the minerals have time to grow and form large crystals.
Why is porphyritic common in extrusive rocks?
The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.
What do igneous rock textures say about the rock that formed?
The igneous texture tells us how the magma cooled and solidified. Magma can solidify into igneous rock in several different ways, each way resulting in a different igneous texture. Magma may stay within the earth, far below ground level, and crystallize into plutonic igneous rock (also known as intrusive igneous rock).
What does texture usually tell us about an igneous rock?
Geologists like igneous textures because they reveal so much about how a rock formed. The first set of textures focuses on the size of mineral crystals. Crystal size primarily reflects the rate of cooling, but is also often strongly affected by rock composition (especially water or gas content).
What does a fragmental texture tell us about how an igneous rock formed?
Pyroclastic (pyro = igneous, clastic = fragment) textures occur when explosive eruptions blast the lava into the air resulting in fragmental, typically glassy material which fall as volcanic ash, lapilli and volcanic bombs. Figure 1. Different cooling rate and gas content resulted in these different textures.
What does a porphyritic texture indicate about the cooling history of an 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).
How would you describe the texture of this igneous rock?
The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling yields small crystals.
Why do porphyritic igneous rocks have the most complex cooling history?
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.
What distinguishes a porphyritic texture in igneous rocks?
The larger crystals will flow out with the lava. The lava will then cool rapidly, and the larger crystals will be surrounded by much smaller ones. An igneous rock with crystals of distinctly different size (Figure 7.14) is said to have a porphyritic texture, or might be referred to as a porphyry.
How do you describe the texture of a rock?
The texture of a rock is the size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (for igneous and metamorphic rocks). Also of importance are the rock's extent of homogeneity (i.e., uniformity of composition throughout) and the degree of isotropy. The…
What is the cooling history of an igneous rock with 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 kind of cooling history do porphyritic textures indicate?
Porphyritic texture indicates the magma body underwent a multi-stage cooling history, cooling slowly while deep under the surface and later rising to a shallower depth or the surface where it cooled more quickly.
What does the texture of an igneous rock tell you about its cooling history?
The texture of an igneous rock is determined by the cooling history of the magma from which it formed. Magmas that cool slowly form large crystals, and those that cool quickly form small crystals.
What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history?
What does the mixture of grain size in a porphyritic igneous rock indicate about its cooling history? This indicates that some cooled slowly at depth, so that large phenocrysts form, then the melt erupted and the remainder cools quickly.
What is a porphyritic texture quizlet?
porphyritic texture. composed of large crystals embedded in a matrix of smaller crystals. The large crystals are called phenocrysts, while the matrix crystals are called groundmass.