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In a porphyritic volcanic rock, the mineral grains that are last to crystallize are typically the finer-grained groundmass minerals, which may include plagioclase, pyroxene, or quartz, depending on the specific composition of the rock. The larger, more visible crystals, known as phenocrysts, form first during the initial stages of cooling, while the smaller crystals in the groundmass crystallize later as the magma continues to cool and solidify. This texture reflects the complex cooling history of the magma, with rapid cooling at first followed by slower cooling.
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What texture does a volcanic rock have?
Textures of volcanic rocks include aphantitic (mineral grains are present but microscopic) , porphyritic (some grains are visible to the naked eye), glassy (all or much of the rock lacks a crystalline structure), and vesicular (solidified gas bubbles are present).
Does obsidian rock contain few or no mineral grains?
Obsidian is a type of volcanic glass that lacks a crystalline structure, so it does not contain mineral grains. It forms from quickly cooling lava rich in silica, resulting in a smooth, glassy texture without distinct mineral crystals.
What are the different types of grains in a rock?
Grains in a rock can vary in type based on their mineral composition and formation process. Common types of grains include quartz, feldspar, mica, and calcite, each contributing to the rock's overall characteristics. Grains can also differ in size, shape, and texture, leading to classifications such as coarse-grained, fine-grained, or porphyritic. Additionally, the presence of sedimentary, igneous, or metamorphic processes can further influence the type and arrangement of grains within a rock.
When water combines with mineral grains do the grains become smaller or larger?
When water combines with mineral grains, the grains can become smaller due to mechanical weathering processes like abrasion and attrition, where the movements of water cause the grains to break down into smaller pieces. However, in some cases, water can also facilitate processes like dissolution or precipitation that can lead to mineral grains growing larger.
When igneous rocks from a volcanic eruption cools what does it form?
When igneous rocks from a volcanic eruption cool, they can form either intrusive or extrusive rocks. Intrusive igneous rocks form when magma cools slowly beneath the Earth's surface, resulting in larger mineral grains. Examples include granite and diorite. Extrusive igneous rocks form when lava cools quickly on the Earth's surface, resulting in smaller mineral grains. Examples include basalt and pumice.
What term denotes the larger mineral grains in a porphyritic igneous rock?
Phenocrysts are the larger mineral grains in a porphyritic igneous rock. These crystals are surrounded by a finer-grained matrix known as the groundmass. The presence of phenocrysts helps in understanding the cooling history of the rock.
What texture does a volcanic rock have?
Textures of volcanic rocks include aphantitic (mineral grains are present but microscopic) , porphyritic (some grains are visible to the naked eye), glassy (all or much of the rock lacks a crystalline structure), and vesicular (solidified gas bubbles are present).
Does a porphyritic texture include two different sizes of mineral grains?
Yes, a porphyritic texture consists of two distinct sizes of mineral grains - larger crystals called phenocrysts surrounded by a finer-grained matrix called groundmass. This texture typically forms in magmas that experience two stages of cooling, resulting in the different sizes of crystals.
Volcanic particles about the size of a rice grains are called?
Volcanic particles about the size of rice grains are called volcanic ash. Ash is made up of small rock and mineral fragments that are expelled during a volcanic eruption.
Does obsidian rock contain few or no mineral grains?
Obsidian is a type of volcanic glass that lacks a crystalline structure, so it does not contain mineral grains. It forms from quickly cooling lava rich in silica, resulting in a smooth, glassy texture without distinct mineral crystals.
Which rocks has two different size grains of the same mineral?
Pegmatite is a type of igneous rock that can have two different grain sizes of the same mineral present. This is due to the slow cooling process of the magma, allowing for the growth of large crystals (phenocrysts) within a finer-grained matrix.
Does pumic have mineral grains?
Pumice does not have grains.
How do porphyritic rocks form?
I believe that porphyritic andesite is formed from volcanic ash and bits and pieces of minerals that eventually become ejected from the volcano. How the andesite becomes porphyritic is that it formed inside the volcano before being blown out of along with the magma and hot gases. You can find silver and gold specs as well as a few other metals encased by a smooth texture of solidifyed ash. Lee Durst (WVC 08)
Bowen's reaction series predicts the sizes of the different mineral grains that grow from crystallizing magmas true or false?
True. Bowen's reaction series is a concept in geology that predicts the order in which certain minerals crystallize from a cooling magma based on their melting points. This series helps to explain how different mineral grains of varying sizes form as the magma cools.
Does scoria have mineral grains?
Scoria is a type of volcanic rock that is typically composed of small mineral grains, such as olivine, pyroxene, and plagioclase. However, the grains are generally not easily distinguishable due to the rock's vesicular (full of holes) and frothy texture, which is a result of gas bubbles trapped during the rock's formation.
Rocks with large mineral grains are formed from what?
Rocks with large mineral grains are usually formed from slow cooling of magma beneath the Earth's surface. When magma cools slowly, crystals have more time to grow, resulting in larger grains in the final rock. This slow cooling process allows minerals to crystallize and accumulate, forming rocks like granite.
Why does obsidian contain few or no mineral grains?
The material that would normally form mineral crystals does not have time to form a crystalline structure because of very rapid cooling after volcanically erupting into the air or onto the surface. Obsidian is a volcanic glass.
