In this article, we will learn about different Magma textures and types of igneous compositions.
The volcanoes running at the top are not very explosive. So the material slides down on either side and creates these very gentle slopes and it should say usually have steep calderas at the summit that will then collapse and create these massive holes within the middle.
If you exclude extraterrestrial things like the Sun, Moon, or an asteroid then the list becomes pretty small and if we talk about changes that have an observable impact in our lifetime while excluding ourselves, the only things leftover is volcanoes.
The heart of all volcanoes is their magma.
As a kid, you may have wondered if a volcano would pop up randomly nearby where you live and maybe question why that doesn’t happen after all magma clearly comes from inside the earth. So what’s stopping it from coming to the surface?
Well, magma isn’t actually abundant in the interior of the earth. So where does magma come from? The collection of tectonic plates on which we live comprises what we call the lithosphere.
These solid structures sit on the lower more viscous asthenosphere or the upper layer of the mantle. As these plates slowly move around, they bump and force themselves into each other.
If one plate is pushed underneath the less dense one, there are two main compositions of plates. Those with denser oceanic crust and those with less dense continental crust.
Oceanic crust is mostly mafic in composition which means there’s a lot of magnesium and iron which forms a nicely organized array with other silicate structures.
Continental crusts are more felsic in composition which means a lot of silicon is presently forming large complexes of tetrahedral networks because of their different compositions and densities. If these plates collide in the ocean, the plate will be pushed down underneath.
Below are some Magma textures
A phonetic is a fine grain Magma texture that is formed from a rapid rate of the coin of lava or magma. So you can’t see the individual grains without a microscope. It may contain what we call vesicles or voids and the most common rock is basalt.
Phaneritic
Next, we have phaneritic. Phaneritic is what we consider coarse-grained Magma texture. So these slow cooling crystals can be identified without a microscope and grains are commonly equal in size.
Porphyritic
Minerals form at different temperatures as well as different rates. So large crystals are what we call FINA crust and will be embedded in a matrix. Smaller crystals are called groundmass.
Glassy
Glassy is the rapid cooling of molten rock. It’s mainly from viscous granitic magma. The most common rock is obsidian.
Pyroclastic
These are fragments ejected into a violent eruption.
Pegmatitic rock
This is an exceptionally coarse grain Magma texture. It forms in the late stages of crystallization and most have compositions that are pretty similar to granite. The most common Rock is a pegmatite.
Vesicular
Spherical voids or vesicles are left by escaping gases and the most common rock is pumice but we’ll also see this in scoria. There’s a different type of environment of formation.
Read to know more about Magma textures.
There are two primary types of igneous compositions.
We have dark or our ferrule magnesium silicates which make up our mafic rocks. The typical rock is salt and it compositionally has no quartz. Mostly calcium feldspar and our dark silicates. Some of those would be olivine pyroxene in purple and biotite mica.
Granitic is our light-colored silicates. It’s referred to as being felsic because it has feldspar and silica. It’s rich in silica and it mostly makes up the continental crust. Basaltic compositions are dark silicates and calcium-rich feldspars.
It’s referred to as mafic because it has a lot of magnesium and iron. It’s darker and denser than our granita cracks and it makes mostly our oceanic crust. We have our intermediate group which is also referred to as acidic in composition. So it contains all these 25% of dark silicate minerals.
Silica content and crystal rocks can range anywhere from about 45 percent to 70 percent. Silica influences a Magma’s behavior because granitic magma which has high silica content or viscosity means they’re really resistant to flow.
Basaltic Magmas are low in silica; they’re much more fluid. Also, they’re less viscous. It flows a lot like water and it’s gonna crystallize at much higher temperatures than what our granitic magma is.
The naming of different igneous rocks
Felsic rocks
These are our light-colored ones. It’s 20 to 25 percent quartz and about 50 to 65% of feldspars. It can sometimes have a porphyritic texture. It’s typically a byproduct of mountain building. It’s very abundant. It’s formed from large intrusive masses. So this is intrusive igneous rock.
Rhyolite
It is an extrusive rock. It may contain glass fragments. but because that volcano never contained it and it was extruded, it became rhyolite.
Obsidian
This might be an exception to the rule of looking at them and determining whether it’s felsic or if it’s mafic. Because obsidian is black but it’s actually a felsic rock. So it can vary from black to red and it has that glassy texture.
Pumice
pumice is a volcanic extrusive rock. it’s glassy vesicular, which means it has all those voids from escaping gases. it’s a frothy mass and can actually float on water. It’s a typically light gray color.
Different types of intermediate rocks
Andesite
It is a fine grade or a phonetic which means you can’t see it with your eyes. Volcanic is another term for extrusive rocks. It often can resemble rhyolite and it’s medium gray in color.
Gabbro
Gabbro is the equivalent of basalt. So again if this was extruded it would create basalt. It’s composed primarily of a Pyrrhic scene and calcium-rich plagioclase feldspar and it makes up a significant portion of the ocean unit crust.
Pyroclastic
These are fragments that were ejected during a volcanic eruption. So some of our different varieties. We have ash-sized fragments, we have volcanic breccia which are particles that are larger than ash.
How can you recognize igneous processes that are occurring at the surface?
Most igneous processes aren’t visible. we can’t see magma chambers. So intrusive intrusions can have a variety of shapes and sizes. Many important accumulations of metals are produced by igneous processes.
We can have mineral resources forming from magma segregation. Heavy minerals crystallize early. They settle to the lower portion of the magma chamber and we often get things produced from late stages of magnetic processes and we’ll get pegmatite. We also get hydrothermal solutions.