Classifying Rocks

Figure 3 Texture helps geologists classify rocks. Forming Operational Definitions – Looking at the rocks below, describe the characteristics of a rock that help you define what a rock’s “grain” is.

Figure 3: Sample Answer

• Texture is described with the terms based on:

• Grain size– Fine-grained– Coarse-grained

• Grain shape– Smooth – Jagged

• Grain pattern– banded– nonbanded

What characteristics are used to identify rocks?

• When studying a rock sample, geologists observe the rock’s– Color– Texture– Determine its mineral composition

What are the three major groups of rocks?• Igneous rock

– Forms from the cooling of molten rock – either magma below the surface or lava at the surface

• Sedimentary rock– Forms when particles of other rocks or the

remains of plants and animals are pressed and cemented together

– Forms below the surface• Metamorphic rock

– Forms when an existing rock is changed by heat, pressure, or chemical reactions

– Most metamorphic rock forms deep underground

What tests do geologists use to identify minerals in rocks?

• Scratch Test (Mohs)• Use acid to determine whether the

rock contains the mineral calcite• Magnet test

Igneous Rock-0rigin

• Igneous rocks form when molten rock cools and solidifies. Molten rock is called lava when it is above the Earth’s surface and magma when it is below.

Intrusive Igneous Rocks• Igneous rocks that form

below the Earth’s surface are called intrusive igneous rocks .

• They form when magma enters a pocket or chamber underground that is relatively cool and solidifies into large crystals as it cools very slowly.

Intrusive Igneous Rock

EXAMPLES: 1. Granite 2. Gabbro 3. Diorite

Intrusive Igneous Rock:

• inside the volcano• cools slowly• large crystal• coarse grained

Extrusive Igneous Rocks• Extrusive igneous rocks, or

volcanic, form when magma makes its way to Earth's surface. The molten rock erupts or flows above the surface as lava, and then cools forming rock.

• Most extrusive (volcanic) rocks have small crystals. Examples include basalt, rhyolite, and andesite.

Glassy Igneous Rocks

• Pumice (left)• Scoria (bottom left)• Obsidian (bottom)• Note gasses in the lava can cause fine holes as

seen in the pumice and scoria.

Glassy Igneous Rocks cool so rapidly, that atoms don’t have enough time to get together, bond and form crystals. To cool this quickly the rocks MUST be extrusive.

Extrusive rocks:

•Outside the volcano•Cools quickly•Small crystal•Fine grained

Rock Cycle Questions

1. What are the 3 types of rocks?2. Why is the set of natural processes by which

rocks change into other types of rocks called a cycle?

3. If a rock began as a metamorphic rock, what could happen to it?

4. Draw a diagram showing how an igneous rock could change into a metamorphic rock and how the metamorphic rock could change into a sedimentary rock.

Sedimentary Rocks

Rocks made of bits & pieces of other rocks. Sedimentary rock forms

when sediments-rocks made of bits and pieces of other rocks are pressed

and cemented together, or when minerals form from solutions.

How Sedimentary Rocks Form:• When rocks, plants, and loose material get

pressed into layers over time.• This process includes:• Erosion-running water, wind, or ice loosen and

carry fragments of rock.• Deposition-is the process by which sediment

settles out of the water or wind carrying it.• Compaction-particles squeezed together under

great pressure.• Cementation- particles are glued together.

Types of sedimentary rocks?

• There are 3 main types of sedimentary rocks.

• 1.Rocks from sediments• 2.Rocks made from remains of plants and

animals. (contains fossils)• 3. Rocks made from minerals that are

dissolved in solutions

What is a metamorphic rock?• The term "metamorphic" means

"to change form." • Any rock (igneous, sedimentary,

or metamorphic) can become a metamorphic rock. If rocks are buried deep in the Earth at high temperatures and pressures, they form new minerals and textures all without melting. If melting occurs, magma is formed, starting the rock cycle all over again.

Metamorphic Rock Types

• There are two types of metamorphic rocks.

• Each is classified according to its composition and texture.– Foliated– Nonfoliated.

Foliated Metamorphic Rocks

• FOLIATED metamorphic rocks are those in which the minerals have been flattened and pushed down into parallel layers. The bands in foliated metamorphic rock look like pages in a book.

• Examples of foliated rocks are slate, shale, and gneiss.

Non-foliated• NON-FOLIATED metamorphic

rocks do not display layers. Rather, they are massive structures with no obvious banding. The mineral grains grow and rearrange, but they don’t form layers.

• A good example of non-foliated rock is quartzite, the smooth-textured, metamorphosed form of the mineral quartz.

• A coarse-textured non-foliated rock is marble.

• Anthracite, or hard coal, is a non-foliated rock that forms when intense pressure drives gases out of soft coal, causing it to harden.

The Rock Cycle

Sedimentary

Metamorphic

Igneous

The Rock Cycle

Rock – a naturally formed solid that is usually made up of one or more types of minerals

Rock Cycle – the set of natural processes that form, change, break down, and reform rocks.

3 Rock Types ReviewIgneous Rock – forms when molten rock cools

and becomes solid. This can happen within earth or on the surface

Sedimentary Rock – forms when pieces of older rocks, plants, and other materials get pressed and cemented together

Metamorphic Rocks – forms when heat and pressure causes older rocks to change into new types of rocks. These are formed deep under the earth.

Describing the Rock Cycle A rock’s cycle begins by becoming one of the

three types of rocks.

Describing the Rock Cycle

• If it begins as igneous, it can be changed with heat and pressure to become a metamorphic rock or it can reach the surface, be broken down and turn into a sedimentary rock.

• If the rock begins as sedimentary, then it can be changed by heat and pressure into a metamorphic rock or it can be broken down into sediment.

Describe the Rock Cycle

• If the rock begins as a metamorphic rock then it can be turned into magma which eventually becomes an igneous rock or it can be brought up to the surface of earth and broken down into sediment to become a sedimentary rock.

Rock Cycle Vocabulary• Erosion – the movement of rock pieces from

one place to another by wind, water, or ice• Weathering – the breaking down of rocks by

wind, water, ice, or plant roots• Heat – high temperatures• Pressure – caused by layering or weight• Melting – occurs due to magma• Deposition – when rocks are moved to a new

location due to erosion

Rock Cycle