Unit 2: Composition of Earth Chapter 5: Igneous Rocks     Igneous Rocks Billions of years ago, Earth's molten surface began to cool and solidify. The first rocks to form on the young planet were igneous rocks. The other major rock types, sedimentary and metamorphic, formed as these original rocks slowly began to weather, erode, and undergo changes in temperature and pressure. The processes that cause one rock type to change into another are known collectively as the rock cycle. The rock cycle did not end in Earth's early geologic history; it is an ongoing process that continues today. To understand the rock cycle, however, geologists first had to understand how igneous rocks form. Formation of Igneous Rocks Early geologists could easily observe the formation of igneous rocks on Earth's surface by watching lava cool and solidify. However, they knew that not all molten material reached Earth's surface, and they wondered what type of igneous rock formed when magma cooled and crystallized underground. Where were these subsurface igneous rocks and what did they look like? The answers to these questions were locked in samples of granite, one of the most common rocks found on continents. Clues from Granite In the 17th century, geologist James Hutton, working in Scotland, observed that formations of granite cut across beds of sedimentary rocks. The sedimentary rocks were fractured. Hutton hypothesized that these fractures occurred when molten granite was intruded, or forced into, the sedimentary rocks. This was the first clue that granite originated from molten material beneath Earth's surface. Upon additional study, Hutton observed that the minerals of the sedimentary rocks that bordered the granite were different from the minerals of nearby sedimentary rocks that did not touch the granite. Also, the contact or surface separating the granite and sedimentary rocks appeared "baked." Hutton proposed that the baked contacts and mineral differences could be explained by the high temperatures associated with the molten granite. This was the second, crucial piece of evidence that led geologists to conclude that granite was an igneous rock that formed beneath Earth's surface. One other piece of evidence rounded out the mystery of subsurface igneous rocks-the large-grained texture of granite indicated a slow cooling rate, which is consistent with conditions that exist underground. Hutton's discoveries showed conclusively that igneous rocks form both above and below ground. Following the publication of his work, geologists began to divide igneous rocks into two major groups: intrusive and extrusive. As you learned in your textbook, intrusive rocks form beneath Earth's surface and extrusive rocks form on Earth's surface. LINK-UP: Find out more information about igneous rock formation. Ongoing Geologic Studies Hutton's discoveries led to a major breakthrough in the scientific understanding of geologic processes. In fact, he is often called the father of modern geology for the contributions he made during his lifetime. Although much has been learned about intrusive igneous rocks since the 17th century, geologists have yet to directly observe the formation of igneous rocks in a magma chamber deep underground. Laboratory experiments attempt to replicate the conditions that exist at great depths beneath Earth's surface. However, these experiments can only estimate what might happen over long periods of time to the chemical composition and the behavior of molten material. Geologists must be detectives and use the evidence left behind-igneous rocks-to solve these mysteries. Activity Research and write a report about the contributions of James Hutton to geology. For example, Hutton proposed an important principle known as uniformitarianism. Explain how this principle affected the modern view of Earth's geologic history.