Magmas which have different chemical compositions have different temperatures i.e basaltic magmas erupt at about 1200 °C whereas rhyolitic magmas erupt at about 700 °C. what is the difference in chemical composition between these two? also why can the composition of magma change from mafic to felsic with decreasing temperature? thanx
I have figured it out, finally.
The difference in chemical composition between basaltic and rhyolitic magmas lies primarily in the relative proportions of different elements and minerals present.
Basaltic magmas are characterized by a higher proportion of magnesium (Mg) and iron (Fe) compared to silicon (Si) and aluminum (Al). They also contain lesser amounts of potassium (K), sodium (Na), and aluminum silicate minerals. This composition leads to a relatively low viscosity and fluid nature of basaltic magmas.
On the other hand, rhyolitic magmas have a higher proportion of silicon, aluminum, potassium, and sodium compared to magnesium and iron. This results in a higher viscous nature and a greater propensity for explosive eruptions. Rhyolitic magmas also contain an abundance of aluminum silicate minerals and have a lower temperature of eruption.
The change in magma composition from mafic (basaltic) to felsic (rhyolitic) with decreasing temperature is primarily due to the process of fractional crystallization. As a mafic magma cools, certain minerals, such as olivine and pyroxene, start to crystallize and separate from the remaining liquid magma. These minerals are rich in magnesium and iron, and their removal decreases the proportion of these elements in the remaining magma.
As the magma continues to cool, it can become progressively enriched in elements like silicon, aluminum, potassium, and sodium, leading to a felsic composition. This is because minerals like feldspars and quartz, which are rich in these elements, are more stable and crystallize at lower temperatures.
Therefore, the changing composition of magma with decreasing temperature is a result of selective crystallization of minerals, leading to the separation and removal of certain elements from the remaining liquid magma.