Lecture 2: Classification of Magma PDF
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University of Karachi
Dr. Adnan Khan
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Summary
This lecture covers the classification of magma based on silica saturation, including silica under-saturated, saturated, and over-saturated types. It details magma types, viscosity, temperature, and gas content, along with SiO2 content and related classifications. Supporting diagrams and descriptions are also included.
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# Classification of Magma ## Dr. Adnan Khan Assistant Professor Department of Geology University of Karachi ## Magma on the Basis of Silica Saturation - **Silica Under Saturated** - Rocks deficient in silica and form feldspathoidal mineral instead of feldspar - **Silica Saturated** - Rocks do no...
# Classification of Magma ## Dr. Adnan Khan Assistant Professor Department of Geology University of Karachi ## Magma on the Basis of Silica Saturation - **Silica Under Saturated** - Rocks deficient in silica and form feldspathoidal mineral instead of feldspar - **Silica Saturated** - Rocks do not contain free quartz or feldspathoidal minerals - **Silica Over Saturated** ## Magma Types - Each has a different mineral composition - All types of magma have a significant percentage of silicon dioxide - Basaltic magma is high in iron, magnesium and calcium but low in potassium and sodium. ### Magma Types | Magma Type | Viscosity | Temperature | Gas CONTENT | |--------------|--------------|-------------|-------------| | Basaltic | LOW | 1000-1200 °C | LOW | | Andesitic | INTERMEDIATE | 800-1000 °C | INTERMEDIATE | | Rhyolitic | HIGH | 650-800 °C | HIGH | ## SiO<sub>2</sub> content is related to classification | Magma type | SiO<sub>2</sub> content | Classification | General color | |------------|----------------------|---------------|---------------| | Rhyolitic | > 70% | Felsic | Light | | Andesitic | ~ 60% | Intermediate | 1/2 Light 1/2 Dark | | Basaltic | < 50% | Mafic | Dark | | | 0% | Ultramafic | Dark/Green | ## Silica Saturation-Undersaturation Shand (1927) proposed the following list of minerals, subdivided on the basis of silica saturation and/or undersaturation, i.e. those that coexist with quartz (+Q) and those that do not coexist with quartz (-Q) ### Saturated (+Q) - all feldspars - all pyroxenes - all amphiboles - micas - fayalite (Fe-rich olivine) - spessartine Mn3Al2(SiO4)3 - almandine Fe3Al2(SiO4)3 - sphene - zircon - topaz - magnetite - ilmenite - apatite ### Undersaturated (-Q) - leucite - nepheline - sodalite - cancrinite - analcite - forsterite (Mg-rich olivine) - melanite (Ti garnet) - andradite Ca3(Fe,Ti)2(SiO4)3 - pyrope Mg3Al2(SiO4)3 - perovskite - melilite - corundum - calcite Undersaturated and saturated minerals can coexist stably under magmatic conditions, but quartz, tridymite and christobalite can only coexist stably with saturated minerals. For example Q + ne is an impossible igneous assemblage, as is Q + Fo (Mg – rich Ol) but Q + Fa (Fe-rich Ol) is stable. ## Diagram description The image depicts a complex geological diagram showing various processes of volcanic rock formation and the different types of magma found in the Earth's crust. The diagram includes: - **Subduction zone:** Where an oceanic plate slides beneath a continental plate, causing melting of the mantle and generating magma. - **Continental crust:** The upper layer of the Earth's crust, composed of granite and other lighter rocks. - **Mantle plume:** A hot, rising column of mantle material that can melt the crust and provide a source of magma. - **Melts & fluids released from slab:** The result of the subduction process, contributing to the chemical composition of the magma. - **Metasomatised SCLM:** The upper part of the lower crust, subject to chemical alteration due to the influx of fluids from the mantle. - **Deep crustal recycling:** The process where the lower crust is recycled back into the mantle, influencing the composition of future magmas. - **Hypotheses of Low 834S:** This section explains potential explanations for the occurrence of low sulfur isotope ratios in alkaline complexes, including crustal contamination, SCLM metasomatism, and mantle plume involvement. The diagram illustrates, using arrows, the flow and interaction of materials during magma generation. Different magma types, such as **calc-alkaline** and **alkaline magmatism**, originate from distinct depths and processes within the Earth. ## Further Diagram description The image showcases a side-view perspective of the oceanic crust with a magnified section highlighting the features like ridge, abyssal hills, seamounts, and trough. - **A) Subduction zone:** depicts a subduction zone where oceanic crust slides beneath the continental crust. The process melts the mantle and generates magma that rises up to the surface. - **B) Mid-ocean spreading ridge:** demonstrates the formation of new oceanic crust from upwelling magma at a mid-ocean ridge. This process creates new seafloor, pushing older crust further away from the ridge. ## Image description The image shows a diver exploring a section of a deep underwater rift caused by tectonic plate separation. The water is crystal clear, creating a view straight down into the rock. The diver appears small compared to the size of the rift, highlighting the scale of the geological forces at play.
