Instructions

Read Chapter 4 - Igneous Processes and Volcanoes (section 4.2 only), and the sections Partial Melting and The Origin of Magma Below. As you read, consider the learning objectives and take notes that will help you achieve these objectives. Read and connect the concepts to the learning objectives. The illustrations are instructive. Take the time to study each. Read the values of each axis on the graphs and consider how they relate to the reading and the learning objectives. As always, use active reading strategies and/or SQ3R to help you effectively read and retain information, and, save time!

Time Needed

About 1-1.5 hours.

Learning Objectives

Students can explain the rock cycle, differentiate among the three types of rocks, and use texture and composition to identify common rocks and their origins
Describe the processes that form common igneous rocks
Describe how the ratio between Fe/Mg and silica dictates the color and composition of igneous rock
Describe how and where magma forms and evolves

Partial Melting

The concepts of partial melting and magma evolution (how the composition of magma changes over time) can be tricky. Below, I've summarized the main points. Carefully read the main points and reflect on how you understand each as it relates to making magma and igneous rocks. Connecting each point to where it occurs in the illustration below.

Summary

Rocks are typically heterogeneous, a mix of different minerals; different minerals have different chemical compositions.
Different minerals melt at different temperatures because they have different chemical compositions, so an entire body of rock typically does not entirely melt but can partially melt.
The minerals with the lowest melting temperatures, those with the most silica, will melt first. In contrast, iron (Fe)/magnesium (Mg)-rich minerals (relatively low silica) commonly do not melt because temperatures don't get high enough to melt them.
For example, if the overall composition of a body of rock is mafic (Fe/Mg-rich), the magma produced by partial melting will be felsic and/or intermediate. This is because silica-rich minerals like quartz are easier (lower melting temperature) to melt than Fe/Mg-rich minerals, like augite, with higher melting temperatures.
Things work in reverse as a body of magma cools. Fe/Mg-rich minerals crystalize first at the highest temperatures. This causes the composition of a body of magma to evolve (change over time) through several processes. One of these processes, called magmatic differentiation, happens when the minerals that crystallize first, at the highest temperatures, utilize elements like iron and magnesium to make their crystals. This depletes the remaining magma of iron and magnesium, making it proportionally higher in silica; consequently, the minerals crystallizing at lower temperatures will contain more silica and less Fe/Mg. This results in an inverse relationship between Fe/Mg and silica in igneous rocks.

An illustration of magma forming and evolving

Subduction makes magma that rises and evolves from originally mafic magma to intermediate (not shown) and felsic magma.

The Origin of Magma

Igneous rocks form from the solidification of magma. So an important question related to igneous rocks is, "What is the origin of magma?" Contrary to what you may think, there isn't much magma inside Earth; in fact, it is difficult to melt rocks into magma. Let's discuss why this is the case. First off, the melting temperature of a rock is very high. For example, to melt quartz, the mineral with the lowest melting temperature among common minerals, temperatures must be greater than 1100^oF. To reach this temperature requires being deep within Earth - remember that temperature increases with depth. However, there is increasing pressure with increasing depth, and pressure is the enemy of melting. Why? Because going from solid to liquid requires a substance to expand, and a substance can't expand if there is too much pressure.

Examine the graphs below. Note on the left graph the red line delineating solid from liquid: the higher the pressure, the higher the temperature needed to melt rock.

Graph of melting temperature vs. pressure.
Graph from https://opentextbc.ca/

Now, you may ask yourself, "How does rock within Earth ever melt?" Take a close look at each graph for the answers. The graph on the left reveals that reducing pressure can allow rock to melt, i.e., be on the right side of the melting curve line. How does this happen? You already know the answer! Think of the mid-ocean ridge and seafloor spreading. As the seafloor spreads apart, pressure is relieved, triggering (1) decompression melting. The graph on the right considers what happens along convergent plate boundaries, where water is released from the subducting plate, lowering the melting temperature of rock. This is called (2) flux melting. A third setting for making magma is associated with hot spots, where the extra heat from the rising mantle plume causes (3) heat-induced melting in the upper asthenosphere. Below are graphs that convey the same information as above and show the places on Earth where magma is being made. The numbers coincide with the numbered melting processes above.

Graphs of pressure and temperature and an illustration showing a cross-section of the lithosphere and asthenosphere.

The bottom line is that there is very little magma inside Earth because the conditions needed to make magma are restricted to divergent and convergent plate boundaries and hot spots.