Porphyritic & Vitrophyric Textures in Rocks | UPSC Mains GEOGRAPHY-PAPER-II 2019

What are 'porphyritic' and 'vitrophyric' textures? Describe with the help of suitable sketches. Comment on petrogenetic significance of vitrophyric texture.

How to Approach

This question requires a descriptive and analytical answer focusing on igneous petrology. The approach should involve defining porphyritic and vitrophyric textures, explaining their formation processes, and illustrating them with sketches. Crucially, the petrogenetic significance of vitrophyric texture needs to be discussed, linking it to rapid cooling and specific magma compositions. The answer should demonstrate understanding of the relationship between texture and cooling history of igneous rocks. A clear, well-labeled sketch is essential for both textures.

Igneous rocks, formed from the cooling and solidification of magma or lava, exhibit a wide range of textures that provide valuable insights into their origin and cooling history. These textures are primarily determined by the size, shape, and arrangement of mineral grains within the rock. Two significant textures observed in igneous rocks are porphyritic and vitrophyric. Porphyritic texture indicates a two-stage cooling history, while vitrophyric texture signifies extremely rapid cooling. Understanding these textures is fundamental to deciphering the petrogenetic processes involved in the formation of igneous rocks. This answer will detail these textures with sketches and elaborate on the petrogenetic significance of vitrophyric texture.

Porphyritic Texture

Porphyritic texture is characterized by large, well-formed crystals (phenocrysts) embedded in a fine-grained or glassy matrix (groundmass). This texture indicates that the magma underwent two stages of cooling: slow cooling at depth, allowing for the growth of phenocrysts, followed by rapid cooling near or at the surface, resulting in the fine-grained or glassy groundmass.

Formation: The phenocrysts represent the initial phase of crystallization in a deep-seated magma chamber. Changes in physical conditions (e.g., pressure release, temperature decrease, water content) can trigger the rapid crystallization of the groundmass as the magma ascends towards the surface.

Examples: Granite porphyry, Andesite porphyry.

Vitrophyric Texture

Vitrophyric texture is characterized by abundant glass (vitreous material) containing a few, small, isolated crystals (microlites). This texture forms due to extremely rapid cooling of lava, preventing the formation of significant crystalline structures. The lava cools so quickly that atoms do not have enough time to arrange themselves into a crystalline lattice.

Formation: Typically occurs in lavas erupted from volcanoes, especially those with high silica content (rhyolitic or dacitic lavas) and low viscosity. The rapid cooling is often facilitated by contact with air or water.

Examples: Obsidian, Pumice, Rhyolite.

Petrogenetic Significance of Vitrophyric Texture

The vitrophyric texture holds significant petrogenetic information:

Rapid Cooling Rate: The presence of glass unequivocally indicates an extremely rapid cooling rate. This suggests a shallow depth of formation, typically at or near the Earth's surface.
High Silica Content: Lavas that form vitrophyric textures are generally silica-rich. High silica content increases the viscosity of the lava, hindering crystal growth even with relatively faster cooling. The complex tetrahedral structure of silicates requires more time to organize into a crystalline structure.
Low Water Content: While not always the case, lavas forming vitrophyric textures often have relatively low water content. Water can act as a flux, promoting crystallization. Its absence contributes to the glassy nature.
Eruptive Style: Vitrophyric textures are commonly associated with effusive volcanic eruptions, where lava flows readily onto the surface.
Magma Composition: The composition of the microlites present in a vitrophyric rock can provide clues about the overall magma composition and the sequence of crystallization.

Furthermore, the study of melt inclusions trapped within the glass can provide direct information about the composition and volatile content of the original magma. This is a powerful tool for understanding magma evolution and the processes occurring during volcanic eruptions.

Additional Resources

Key Definitions

Phenocryst

A large, well-formed crystal embedded in a fine-grained or glassy matrix of an igneous rock. They represent the first generation of crystals to form in a magma.

Microlite

A tiny, microscopic crystal found within a glassy matrix, typically in vitrophyric igneous rocks. They represent attempted crystallization during extremely rapid cooling.

Key Statistics

Obsidian, a volcanic glass with vitrophyric texture, constitutes approximately 5% of the total volume of volcanic rock globally.

Source: US Geological Survey (as of 2023 knowledge cutoff)

Approximately 70% of all volcanic eruptions are effusive, producing lava flows that often exhibit vitrophyric textures.

Source: Global Volcanism Program, Smithsonian Institution (as of 2023 knowledge cutoff)

Examples

Mount St. Helens Eruption (1980)

The 1980 eruption of Mount St. Helens produced significant amounts of dacitic lava with vitrophyric texture, forming obsidian flows and pumice deposits. This provided valuable data on the rapid cooling rates and magma composition during the eruption.

Frequently Asked Questions

▶What is the difference between obsidian and pumice?

Both are volcanic glasses with vitrophyric texture, but obsidian is dense and typically dark-colored due to a lower gas content. Pumice is lightweight and frothy due to the presence of numerous gas bubbles (vesicles) formed during rapid degassing of the lava.

What are 'porphyritic' and 'vitrophyric' textures? Describe with the help of suitable sketches. Comment on petrogenetic significance of vitrophyric texture.

Model Answer

Introduction