How Magmas Evolve

Because a large variety of igneous rocks exists, it is logical to assume that a wide variety of magmas must also exist. However, geologists have observed that, over time, a volcano may extrude lavas exhibiting quite different compositions. Data of this type led them to examine the possibility that magma might change (evolve) and thus become the parent to a variety of igneous rocks. To explore this idea, a pioneering investigation into the crystallization of magma was carried out by N. L. Bowen in the first quarter of the 20th century. In a laboratory setting Bowen and his co-workers demonstrated that as a basaltic magma cools, minerals tend to crystallize in a systematic fashion based on their melting points.

During the crystallization process, the composition of the remaining liquid portion of the magma also continually changes. Bowen also demonstrated that if the solid components of a magma remain in contact with the remaining melt, they will chemically react and change mineralogy. For this reason, this arrangement of minerals became known as Bowen’s reaction series. As you will see, in nature the earliest-formed minerals can be separated from the melt, thus halting any further chemical reaction. Minerals crystallize from a magma of basaltic composition under laboratory conditions. Evidence that this highly idealized crystallization model approximates what can happen in nature comes from the analysis of igneous rocks. In particular, we find that minerals that form in the same general temperature regime depicted on Bowen’s reaction series are found together in the same igneous rocks.

Bowen demonstrated that minerals crystallize from magma in a systematic fashion. But how do Bowen’s findings account for the great diversity of igneous rocks? It has been shown that, at one or more stages during the crystallization of magma, a separation of various components can occur. One mechanism that allows this to happen is called crystal settling. This process occurs when the earlier-formed minerals are more dense (heavier) than the liquid portion and sink toward the bottom of the magma chamber. When the remaining melt solidifies—either in place or in another location if it migrates into fractures in the surrounding rocks—it will form a rock with a mineralogy much different from the parent magma. The formation of one or more secondary magmas from a single parent magma is called magmatic differentiation.

Task 3. Look at Figure 4.2. Bowen’s reaction series shows the sequence in which minerals crystallize from a magma. Which is the first mineral to crystallize? Which minerals generate cooling further according to the diagram?

Task 4. Discuss the following questions:

1. N. L. Bowen’s investigation.

2. Bowen’s reaction series.

3. Magmatic differentiation.

Individual work

Task 1. Read the text and translate it into Ukrainian (in written form). Build up a list of key terms to the text.

Assimilation and Magma Mixing

Once a magma body forms its composition can also change through the incorporation of foreign material. For example, as magma migrates through the crust, it may incorporate some of the surrounding host rock, a process called assimilation. In a near surface environment where rocks are brittle, magma causes numerous cracks in the overlying rock as it pushes upward. The force of the injected magma is often sufficient to dislodge blocks of “foreign” rock, which melt and are incorporated into the magma body.

Another means by which the composition of magma can be altered is magma mixing. This process occurs when one magma body intrudes another that has a different composition. Once combined, convective flow may stir the two magmas and generate a mass having a composition that is a blend of the two. Magma mixing may occur during the ascent of two chemically distinct magma bodies as the more buoyant mass overtakes the more slowly moving mass.

In summary, Bowen successfully demonstrated that through magmatic differentiation, a single parent magma can generate several mineralogically different igneous rocks.