IGNEOUS ROCKS: SOLIDS FROM MELTS

• How Do Igneous Rocks Differ from One Another? 92
• How Do Magmas Form? 97
• Magmatic Differentiation 99
• Forms of Igneous Intrusions 102
• Igneous Processes and Plate Tectonics 105

MORE THAN 2000 YEARS AGO, the Greek scientist and geographer Strabo traveled to Sicily to view the eruptions of Mount Etna. He observed that the hot liquid lava spilling down from the volcano onto Earth’s surface cooled and hardened into solid rock within a few hours. By the eighteenth century, geologists began to understand that some sheets of rock that cut across other rock formations had also been formed by the cooling and solidification of molten rock. In these cases, the magma had cooled much more slowly because it had remained buried in Earth’s crust.

Today we know that deep in Earth’s crust and mantle, rock melts and rises toward Earth’s surface. Some magmas solidify before they reach the surface, and some break through and solidify on the surface. Both processes produce igneous rocks.

Understanding the processes that melt and resolidify rock is a key to understanding how Earth’s crust forms. Although we still have much to learn about the exact mechanisms of melting and solidification, we do have good answers to some fundamental questions: How do types of igneous rock differ from one another? Where and how do magmas form? How do rocks solidify from those magmas?

In answering these questions, we will focus on the central role of igneous processes in the Earth system. Observations of igneous rocks by geologists from Strabo to today make sense only in light of plate tectonic theory. Specifically, igneous rocks form at spreading centers where plates move apart, along convergent boundaries where one plate descends beneath another, and at “hot spots” where hot mantle material ascends to the crust.

In this chapter, we will examine the wide range of igneous rock types, both intrusive and extrusive, and the processes by which they form. We will explore the forces that cause rock to melt and form magmas and the ways in which those magmas reach the locations at and below Earth’s surface where they solidify. We will then take a more detailed look at the igneous processes associated with specific plate tectonic settings.