Chap06 Sedimentary and Metamorphic Rocks

6 What You’ll Learn • How sedimentary rocks are formed. • How metamorphic rocks are formed. • How rocks continuously  change from one type to another in the rock  cycle.

Sedimentary  and Metamorphic Rocks

Why It’s Important Sedimentary rocks provide information about surface conditions and organisms that existed in Earth’s Earth ’s past. In addition, addition, mineral resources are found in sedimentary and metamorph meta morphic ic rocks. The rock cycle further provides evidence that Earth is a dynamic planet, planet, con con-stantly evolving and changing.

To find out more about sedimentary and metamorphic rocks, visit the Earth Science Web Site at earthgeu.com

Mount Kidd, Alberta, Canada 120

ab  Discovery  L   L ab  Sedimentary rocks are usually  found in layers. layers. How do these these layers form? In this activity, activity, you will investiinvestigate how layers form from particles that settle in water. . Obtain 100 100 mL of soil from from a location specified by your teacher. Placee the soil in a tall, Plac tall, narr narrow ow,, jar jar.. . Add water to the jar until it is three-fourths three-fou rths full. Put the lid on the jar so that it is tightly sealed. . Pick up the jar with both hands and turn it upside down several times to mix the water and soil.

6.1 6.1 OBJECTIVES

• Sequence the formation  of sediment sedimentary ary rock rocks. s. • Explain the formation  and classification of  clastic sediments. • Describe features of  sedimentary rocks. VOCABULARY

sediment bedding   clastic  graded bedding  deposition  lithification  cross-bedding  cementation 

. Quickly turn the jar upright and set it on a flat surface.

In your science journal, draw dra w a diagram of what you you observe. observe. What Wh at type of of part partiicles settled out first? What Wh at type of of part partiicles form the topmost layers? How is this activity related to the layering that occurs in sedimentary rocks?

Formation of  Sedimentary Rocks  You learned in Chapter 5 that igneous rocks are the most common rocks in Earth’s Earth’s crust, yet when you look at the ground, you may not see igneous rocks. In fact, you usually don don’t ’t see any solid solid rock at all. all. Whyy is this? Much Wh Much of Earth’ Earth’ss surface is cover covered ed with sediments. Sediments are pieces pieces of solid material that that have been been deposited deposited on Earth’’s surface by wind, wate Earth waterr, ice ice,, gravity gravity,, or chemical chemical precipitati precipitation. on. When sediments sediments become cemented cemented together, together, they form sedimentary  rocks. The formation of sedimentary rocks begins when weathering and erosion produce sediments.

WEATHERING Wherever Earth’s Wherever Earth’s crust crust is exposed at the surface, it is continuously  beingg worn bein worn away away by weathe weathering, ring, a set set of ph physic ysical al and and chemic chemical al processes proces ses that break rock rock into smaller pieces. pieces. Chemical weathering weathering occurs when the minerals in a rock are dissolved or otherwise 6.1 Formation of Sedimentary Rocks  121

A

Chemical weathering attacks less resistant minerals

Figure 6-1 The process of chemical weathering is illustrated in (A). Granite breaks apart easily under the effects of chemical weathering (B). weathering  (B).

Resistant grains such as quartz

Resistant grains are released as weaker grains break down

B chemica chem ically lly changed changed.. Stu Study dy Figure 6-1. What happens to moreresistant minerals during weathering? While the less-stable minerals are chemically chemically broken down, the more-resistant more-resistant grains are broken broken off of the ro rock ck as as small smaller er grain grains. s. Dur During ing ph physi ysical cal we weath atheri ering, ng, on the other hand, minerals remain remain chemically chemically unchanged. unchanged. Rock fragmentss simply ment simply break break off of the solid solid rock rock along along fracture fracturess or grain boundaries. Weathering produces rock and mineral fragments known as clastic sedime sed iments nts.. The wor word d clastic  comes from the Greek word klastos, meaning “broken.” Clastic sediments range in size from huge boulders to microscopic particles. Table 6-1 summarizes the classification of clas clastic tic sediment sedimentss based on size. size. Clast Clastic ic sediment sediment particles particles usually  usually  have worn surfaces surfaces and rounded rounded corners corners caused by physical abrasion abra sion during erosion and transport.

EROSION AND TRANSPORT After rock fragment fragmentss have have been been weathe weathered red out of outcr outcrops, ops, they are transported transported to new locations. locations. The removal removal and movement movement of  surface materials from one location to another is called erosion.

Table 6-1  Classification of Clastic Sediments Particle Size

Sediment

> 256 mm 256–64 mm 64–2 mm

Gravel

2–0.062 mm

Sand

}

Boulder Cobble Pebble

Rock Conglomerate Sandstone

0.062–0.0039 mm

Silt

Siltstone