Chapter 5 Igneous Rocks

Chapter 5 Igneous Rocks - 2009 Allan Ludman and Stephen...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
CHAPTER 5 © 2009 Allan Ludman and Stephen Marshak USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY PURPOSE • To become familiar with igneous textures and mineral assemblages; • To learn to use texture and mineral content to interpret the history of igneous rocks • To learn to identify igneous rocks MATERIALS NEEDED • A set of igneous rocks • A magnifying glass or hand lens and, ideally, a microscope and thin sections of igneous rocks. • Standard supplies for identifying minerals (streak plate, glass plate, etc.) 5.1 INTRODUCTION Every rock has a story to tell. The story of an igneous rock begins when rock in the lower crust or upper mantle melts to form molten material called magma which rises up through the crust. Some magma flows or spatters out on the surface as lava , or explodes into the air as tiny particles of volcanic ash or larger blocks. Igneous rock that forms from solidified lava or ash is called extrusive igneous rock because it comes out of ( extrudes ) onto the surface. Other magma never reaches the surface and solidifies underground to form intrusive igneous rock – so called because it squeezes into ( intrudes ) the surrounding rocks. Intrusions come in many shapes. Massive blobs are called plutons and the largest of these are called batholiths. Others that form thin sheets cutting across layering in the wall rock (the rock around the intrusion) are called dikes , and those forming thin sheets parallel to the layers of wall rock are called sills (Figure 5.1) Figure 5.1 Intrusive and extrusive igneous rock bodies Volcano Lava flow Batholith Sill Dike Ash fall 1
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
When geologists look at an igneous rock, we want to know: Where did it cool— was it intrusive or extrusive? Where in the Earth did the rock's parent magma form? In what tectonic setting—ocean ridge, mid-continent, subduction zone, hotspot—did it form? In this chapter, you will learn how to answer these questions and how to identify common types of igneous rock.Few new skills are needed, just good observation and some geologic reasoning. Exercise 5.1 shows how easy the process is. EXERCISE 5.1: A FIRST LOOK AT IGNEOUS ROCKS There are many ways to classify igneous rocks, but for now let’s use three easily observable criteria: grain size, color, and specific gravity. Using the set of igneous rocks provided by your instructor, first group the specimens by grain size and record the specimen numbers in the appropriate column of the following table. Then group the specimens by color and specific gravity. Grain size Color Specific gravity (heft) Coarse Fine Light colored Dark colored Relatively high Relatively low a. Which two groupings are similar to one another? Which is different? b.
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/03/2009 for the course GEOL 101 taught by Professor Jackel during the Spring '09 term at CUNY Queens.

Page1 / 25

Chapter 5 Igneous Rocks - 2009 Allan Ludman and Stephen...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online