Back To CourseEarth Science 101: Earth Science
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Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.
In this lesson, we will talk about Bowen's reaction series and magmatic differentiation, which are basically two attempts to explain or predict how igneous rocks form. We recall that rocks made from the cooling and solidifying of magma are called igneous rocks. So the Bowen's reaction series and magmatic differentiation pertain to these types of rocks.
So who was Bowen and why do people who know a lot about igneous rocks know his name? Well, Norman Bowen is well known in geological circles because of some experimenting he did back in the 1920's and 30's. Through his experiments, he discovered that minerals crystallize differently as they cool. The result of his research gave us what we call the Bowen's reaction series, which we can define as the crystallization sequence from magma as cooling occurs.
As Bowen dug into his understanding of the crystallizing process, he realized that there are two sequences that minerals can follow. These are the discontinuous series and continuous series, as we see here on this diagram of the Bowen's reaction series. The discontinuous series is seen on the left and it contains minerals high in iron and magnesium. We also see that the series progresses with a drop in temperature.
When we follow this branch, we see that at very high temperatures, olivine is the first mineral to form. In other words, olivine minerals, which are high in iron and magnesium, tend to crystallize at very high temperatures. Then, as the magma begins to cool, some of the olivine becomes pyroxene. As we progress in the sequence with more cooling, the pyroxene turns into amphibole and finally the amphibole turns into biotite.
You might want to use an acronym to remember the steps of the sequence, such as 'Olive Pits Are Bitter.' Each step of the discontinuous series represents a very distinct change with the creation of a new mineral, so the change is not a smooth continuous flow, but instead a discontinuous process, hence the name. With the formation of biotite, the discontinuous series officially ends, but there can be more to it if the magma has not completely cooled and depending on the chemical characteristics of the magma. For instance, the hot liquid magma can continue to cool and form potassium feldspar, muscovite or quartz.
You might want to use a technique to remember these final minerals as well. For example, you could use the acronym 'P.M. Quiet.' The 'P.M.' is useful because these minerals are formed late in the sequence, just like the p.m. hours are late in the day. And, 'Quiet' is useful as a memory jogger because these minerals are formed during the coolest or 'quietest' temperatures of the sequence.
The continuous reaction series is going on at the same time as the discontinuous series and we see it here as the right branch. With the continuous branch we see the reaction has more of a flow or 'continual' reaction taking place, hence the name for this series. With the continuous series we see plagioclase minerals. It starts with the highest temperature mineral, which is calcium-rich plagioclase.
As the magma cools down, the calcium is replaced with sodium. But this happens in somewhat of a flow with the calcium and sodium mixing in a continual series, so a plagioclase in the middle of the series could be thought to have about 50% calcium and 50% sodium. At the bottom of the series we see sodium-rich plagioclase.
By remembering that the 'c' in 'calcium' comes before the 's' in 'sodium,' you can recall that calcium-rich plagioclase is at the top of this series and sodium-rich is at the bottom. Then, as we saw with the other branch, as the temperatures continue to cool and the chemical characteristics continue to change, we see the formation of potassium feldspar, muscovite or quartz - 'PM Quiet.'
Now let's switch gears and take a look at the second process for predicting how igneous rocks form that we mentioned at the beginning of this lesson, which is known as magmatic differentiation. This can be defined as the process that explains how different igneous rocks can form from a single magma melt.
So we already learned that as magma starts to cool, crystals form out of the magma. But what we must also consider is that when these solid crystals form, they become denser. Dense things are heavier, so they tend to sink; just as a dense rock sinks if you drop it into a pond of water. These heavier crystals sink to the bottom of the liquid magma and take with them some of the available minerals that were in the original magma.
This changes the chemical composition of the remaining magma. This process continues with more crystals forming and settling out of the magma, further changing the composition of the remaining magma. After a while, we are left with layers of chemically-different igneous rocks that have settled out of the original magma. So with magmatic differentiation we can have many different igneous rocks forming from the same initial magma melt.
Let's review. Bowen's reaction series can be defined as the crystallization sequence of magma as cooling occurs. It has two parts, the discontinuous series and the continuous series. Both branches progress with a drop in temperature. With the discontinuous series, we see that olivine is the first mineral to form, and it forms at a very high temperature. As the magma cools we see the formation of pyroxene, amphibole and finally biotite. You can use the acronym 'Olive Pits Are Bitter' to recall this sequence.
With the continuous series we see calcium-rich plagioclase forming at the highest temperatures, then as the magma cools the calcium is continually replaced with sodium until we have sodium-rich plagioclase. As the magma continues to cool and the chemical characteristics continue to change after both series, we see the formation of potassium feldspar, muscovite or quartz. Using the acronym 'P.M. Quiet' may help you recall these minerals.
Magmatic differentiation is a process that explains how different igneous rocks can form from a single magma melt. As crystals solidify in the magma, they sink and settle out of the liquid magma. This changes the composition of the remaining magma and leaves us with layers of chemically-different igneous rocks that have settled out of the original magma melt.
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Back To CourseEarth Science 101: Earth Science
23 chapters | 151 lessons