Question

An igneous texture characterized by large crystals embedded in a fine-grained background is called

Answer 1

A porphyritic texture in igneous rocks consists of large crystals embedded in a fine-grained matrix, forming when magma cools first slowly beneath the Earth's surface, then rapidly after erupting. These large crystals are called phenocrysts and are surrounded by a finer-grained material, making up less than 10% of the rock's volume.

Step-by-step explanation:

An igneous texture characterized by large crystals embedded in a fine-grained background is called porphyritic. This structure occurs when magma cools in two stages: it begins cooling within the Earth, allowing some crystals to grow to a coarse size, and then cools rapidly after erupting onto the surface, resulting in a fine-grained matrix surrounding the larger crystals. In porphyritic rocks, the large crystals, also known as phenocrysts, are often two to three times the size of the grains in the fine-grained background matrix, although they usually make up less than 10% of the rock's composition.

Igneous rocks are classified based on their texture and mineral composition, which are dictated by the cooling process and original molten material chemistry. Slow cooling, often deep within the Earth, leads to the formation of intrusive or plutonic rocks that are coarse-grained, like granite. Conversely, fast cooling at the surface results in fine-grained or extrusive rocks, like basalt. The specific minerals present in these rocks, such as quartz, feldspar, pyroxene, and amphibole determine the exact type of igneous rock.

Answer 2

A porphyritic texture in igneous rocks is characterized by large crystals within a fine-grained matrix, suggesting a complex cooling history. Intrusive rocks like granite with a coarse-grained texture form from slow cooling, while extrusive rocks are fine-grained due to rapid cooling.

Step-by-step explanation:

An igneous texture characterized by large crystals embedded in a fine-grained background is called porphyritic. This texture indicates a complex cooling history where large crystals formed deep within the Earth over long periods, resulting in a coarse-grained texture, before the remaining magma ascended and crystallized quickly at or near the surface, resulting in the encapsulation of these larger crystals within a fine-grained matrix. Porphyritic textures can often be observed in rocks such as diorite and andesite, depending on the composition and cooling rates of the magma.

Intrusive igneous rocks, like granite, are typically coarse-grained, consisting mainly of quartz, and feldspar, with minor hornblende and biotite, reflecting slower cooling within the Earth's crust. In contrast, extrusive igneous rocks cool rapidly on the surface and thus are typically fine-grained. The composition of the magma and the cooling process it undergoes are crucial in determining the final texture and classification of the igneous rock.