Tab: Exercise 2.2


Lesson 2.2: Three Classes of Rocks; Igneous, Sedimentary, and Metamorphic


INTRODUCTION: Three Classes of Rock
Up to this point we have looked at the building blocks of rock. Now it is time to see how rocks are formed and reformed over time. There are three classes in which rocks can be categorized as, based on how they were formed. They are igneous, sedimentary, and metamorphic rock.

Some of these rock forming processes may take millions of years and some can occur very rapidly. Due to the way each is formed, it can usually be identified by its appearance. Let's a closer look at the three classes...



ACTIVITY A: Igneous Rock


Required Readings

Science in Action 7
pages 377 to 378
or
Science Focus 7
page 361

Rock is always being cycled in the core of the Earth. When this liquid rock, Magma, rises to Earth's crust and is cooled and solidified Igneous Rock is formed.

Any rock or mineral that is heated at great depths can melt into magma. Sometimes the magma melts away the mantle and crust of the Earth to create a entry point on the surface.

Igneous rock is classified into two groups based on where it is formed. If it forms below the surface it takes longer to cool and harden. This type of igneous rock is called intrusive rock. This type of rock is only found on the surface, when the surface rocks are eroded away. The picture below shows intrusive rock after the outer cone has eroded away. This is Devil's Tower a US national monument in Wyoming.

Extrusive rock forms when magma breaks through the Earth's surface in volcanic eruptions. Rock forms when lava cools.

The size of the crystals in igneous rock depends on how fast or slow the rock cools. Molten rock which cools slowly underground has longer to create crystals, therefore, the crystals or grains in intrusively cooled igneous rock are larger than those cooled extrusively on the surface.



ACTIVITY B: Sedimentary Rock


Required Readings

Science in Action 7
pages 379 to 380
or
Science Focus 7
pages 364 to 365

Over 70 percent of the Earth's surface is made up of sedimentary rock. With the help of wind and water, sediment is deposited as layers which are slowly compacted under pressure to form rock. This process is called compaction. Another similar process is called cementation. In this process minerals dissolve as water soaks into the rocks which forms a natural cement and bonds the layers together.

Sedimentary Rock becomes closely packed in layers and these layers can often be seen along canyons where erosion has taken place. Stratification is the term used to describe the visible layers.

Some of the more common forms of sedimentary rocks consist of Shale, Limestone, Sandstone, and conglomerate. Shale is formed from mud under pressure. Sandstone is formed from sand which is usually quartz. Pebbles and stones can become cemented together to form conglomerate. Limestone, which is mainly composed of calcite and the organic material deposited at the bottom of bodies of water from animals which have died, is the best material to find fossils in.

 


ACTIVITY C: Metamorphic Rocks


Required Readings

Science in Action 7
pages 380 to 381
or
Science Focus 7
pages 366 to 367
Yosemite National Park in California is home to some of the most majestic granite peaks such as the one to the left, El Capitan and the famous Half Dome. Granite is a metamorphic rock. The word metamorphic means to change form. These rocks start out as another class of rock such as sedimentary or igneous and they are changed by intense heat and pressure deep within the earths crust.

Often metamorphic rock is difficult to find on the Earth's surface, unless top layers have eroded away. Nonetheless, metamorphic rock can be strong and beautiful building materials when found. Marble, granite, and slate are all metamorphic rocks. All of which are used in many construction projects.

Read the section on Metamorphic rock in your textbook and answer the following questions.

Exercise 2.2: Rock Types