What is a flow texture and where do you commonly find igneous rocks with such texture?
Answers
Answer:
Introduction to Igneous Rocks
An igneous rock is any crystalline or glassy rock that forms from cooling of a magma.
A magma consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase.
Magma can cool to form an igneous rock either on the surface of the Earth - in which case it produces a volcanic or extrusive igneous rock, or beneath the surface of the Earth, - in which case it produces a plutonic or intrusive igneous rock.
Characteristics of Magma
Types of Magma
Types of magma are determined by chemical composition of the magma. Three general types are recognized, but we will look at other types later in the course:
Basaltic magma -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
Andesitic magma -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
Rhyolitic magma -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na
Gases in Magmas
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:
Mostly H2O (water vapor) with some CO2 (carbon dioxide)
Minor amounts of Sulfur, Chlorine, and Fluorine gases
The amount of gas in a magma is also related to the chemical composition of the magma. Rhyolitic magmas usually have higher dissolved gas contents than basaltic magmas.
Temperature of Magmas
Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows:
Basaltic magma - 1000 to 1200oC
Andesitic magma - 800 to 1000oC
Rhyolitic magma - 650 to 800oC.
Viscosity of Magmas
Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature.
Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma).
Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma).
Thus, basaltic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to 100,0000 times more viscous than water. Rhyolitic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water. (Note that solids, even though they appear solid have a viscosity, but it is very high, measured as trillions time the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas.
I hope it helps you.