Lecture 7 – Igneous Rocks and Magmas

 


Continuing with igneous rocks…

As said Wednesday, we subdivide igneous rocks based on texture into the Extrusive/volcanic and Intrusive/plutonic categories. In each of these categories, rocks are then distinguished based on their chemical composition. Different chemical compositions lead to specific silicate minerals occurring together. This in turn leads to the general characteristic of changes in the color of the rock as the chemical composition changes. (Recall how silicate minerals vary in color and structure from mineralogy lecture and the first two labs).

The two terms that are often used to describe general variations in chemical composition are:

  1. Felsic: rich in chemical components such as SiO2, Al2O3, Na2O and K2O. FELSIC igneous rocks contain an abundance of FELdspars (K-feldspar = pink or white, Plagioclase feldspar = white with striations) and Quartz (SILica) — also micas (muscovite and/or biotite). Therefore, these rocks are generally light in color (pink, red, gray, or white).
  2. Mafic: rich in MgO, FeO and CaO. MAFIC rocks contain minerals rich in MAgnesium and iron (Fe), such as olivine, pyroxene, and amphibole. These rocks are usually dark in color (green, dark gray, or black).

Rocks with compositions between these are referred to as INTERMEDIATE between felsic and mafic, and contain minerals like plagioclase, hornblende (an amphibole), and biotite ( a mica).

There are also extremely mafic rocks called ULTRAMAFIC rocks, which are almost completely composed of iron-magnesium minerals. These types of rocks, which make up the mantle of the earth, are called PERIDOTITE.

The different types of rocks can be categorized simply by distinguishing based on their silica content (SiO2 in weight %). Here is a table of the different types of igneous rocks separated by their composition and textures:

 

  Felsic (high silica) Intermediate (medium silica) Intermediate (lower silica) Mafic (low silica)
Coarse-grained Granite Granodiorite Diorite Gabbro
Fine-grained Rhyolite Dacite Andesite Basalt
SiO2 % >65% 60-65% 55-60% 45-55%

 

Silica increases to the left

Sodium increases to the left

Potassium increases to the left

Calcium increases to the right

Magnesium increases to the right

Iron increases to the right

 

Also VISCOSITY (resistance to flow) increases to the left. This is important because it influences which type of magmas get to the surface-more viscous liquids are more likely to crystallize as plutonic rocks, granites, than erupt as rhyoliltes.

Magma temperature and crystallization temperature increases to the right.

Thus, rocks are given different names, depending on their texture, even though they have the exact some chemical composition.

A. MELTING

What causes melting to occur?

Pressure and temperature increase as the depth below the earth's surface increases (heat from the core, pressure from overlying rocks, etc.). But, because pressure increases along with temperature, the rocks in the mantle remain solid. For example, a rock's melting temperature on the surface might be 1000 ºC, but 200 km below the surface under much higher pressure, the melting temperature of the rock might be 1300 ºC. So, one process which encourages melting is a decrease in pressure.

Water and volatile content promote melting by lowering the melting temperature of rocks. Thus, a dry rock would have a higher melting point than a rock with water percolating through it or bound up in the minerals. Example includes subduction zones and heating up sedimentary rocks.

Increased temperature-increasing the temperature of a rock will cause melting-the above two cases may be more important for the mantle but there crustal rocks are often thought to be melted by the passage of basaltic magmas (and heat from them) through the crust.

 

How do magmas form?

Melting rocks located deep in the earth's crust or mantle produces magmas. Melting experiments in the laboratory setting show that materials that are mixtures, like rocks, do not melt at one set temperature - rather they melt over a range in temperature with some minerals melting more easily than others. To completely melt a rock requires much more of a temperature increase than melting only partially. Thus, the most common form of melting is PARTIAL MELTING (usually from <1 % to 30?% melt + the rest mineral residue). Since different minerals (quartz, micas, olivine, etc.) have different compositions, bond strengths, and structures, they will have different melting points.

 

Where do magmas form?

Divergent plate boundaries - mostly basaltic lava, partial melts of the upper mantle. Thus oceanic crust is dominantly basaltic in composition.

Subduction zones/convergent plate boundaries - mafic, intermediate, and felsic intrusive and extrusive rocks. Mafic rocks are partial melts of the upper mantle (mantle wedge) caused by influx of water from the subducting plate. Intermediate rocks are evolved or mixed in composition (see next topic - differentiation). Felsic rocks are either evolved or, commonly, melted crust (sediments, other felsic rocks) due to emplaced mafic rocks.

Mantle plumes/hot spots - located away from plate boundaries, magma punches through the crust (originating in the lower mantle?) and is generally basaltic and erupted in large quantities.

 

 

 

 

 

 

 

VISCOSITY

Silicic/felsic magmas crystallize primarily into feldspars and quartz, which are 3-D network silicate minerals in which all of the silicon-oxygen tetrahedra are connected to one another. Silicic magmas also display a significant amount of "connectiveness" at the atomic scale, which acts to resist flow, which in turn results in magmas with a high viscosity. Because of their high viscosity, most silicic magmas do not reach the surface of the Earth, but crystallize within the Earth as intrusive rocks. Therefore, granite is more abundant than rhyolite.

Considering those silicic magmas that do reach the surface, the high viscosity of the magma does not allow dissolved gases to escape easily, which produces high pressures (similar to shaking-up a warm container of soda), which in turn leads to explosive eruptions. Therefore, rhyolitic pyroclastic explosions (ashflows, ashfalls) are more common than rhyolite lava flows.

We will cover volcanoes and volcanism in the next few lectures, so I want to briefly go over the different kinds of magmatic intrusions that can form.

 

Large igneous intrusions are called plutons, so the rocks that make up these intrusions are called plutonic rocks – see intrusive rock names in the earlier table. Funnel-shaped intrusions that cover over 100 km2 are called BATHOLITHS and are common in exposed convergent plate margins, representing the plumbing systems for numerous volcanoes – examples West coast of North America, the Andes of South America.

 

Large volumes of magma are emplaced and cool slowly. Often, the magma moves upward by invading cracks in the overlying country rock (wedging), by breaking off chunks (which then sink into the magma, called stoping), or by consuming the surrounding rock (thereby contributing new elements and volatiles, "crustal contamination").

 

Smaller and flatter intrusions also occur. Depending on their relationship with surrounding rocks, they are either called SILLS or DIKES.

 

A sill is a tabular sheet that was emplaced parallel to the layering of the country rock (but not necessarily flat). Thus these are called concordant intrusions and may be mistaken for lava flows (but are coarse grained, no bubbles, rocks heated above and below sill).

 

A dike is a cross-cutting intrusion, cuts across bedding or igneous interlocking textures. Dikes can be small or continuous over hundreds of kilometers (South Africa). Often, hundreds or dikes are localized in an area of volcanism or splitting of the crust – called swarms.

 

Veins are much smaller than dikes and sills, only a few mm to meters across but meters to km in length. Veins are often located on the periphery of plutons, dikes, and sills where magma has invaded cracks in the country rock. Sometimes these veins (or dikes) cool extremely slowly and form very large crystals = PEGMATITES.

 

Sometimes pegmatite dikes and veins formed from magma enriched in unusual, rare elements, forming minerals rich in Li, Be, gold, and gems. Single crystals in pegmatites can weigh thousands of pounds – examples include mica crystals over 10 feet across, or a beryl crystal 25 feet long that weighed >25 tons!

 

So, now that we have covered the different types of igneous ROCKS, we need to step back a bit and understand the igneous processes responsible for the magma that crystallized to form these rocks.