Rocks and The Rock

Cycle

A. Rocks- any solid mass of mineral or mineral-like matter that occurs naturally as part of our planet

B. There are 3 major types or families of rocks:

1. Igneous rocks

2. Sedimentary rocks

3. Metamorphic rocks

II. Igneous Rocks A. How Igneous Rocks are formed

1. Magma- molten material underground

2. Lava- magma that reaches the surface

3. Igneous rocks are formed from magma that has cooled and hardened either beneath the surface or from a

volcanic eruption

B. Formation processes influence Igneous Rock types

- Magma contains some gases, including water vapor- this make it less dense, so it rises

- As magma rises, it cools and forms crystals

- The longer the cooling time the larger the crystals

1. Intrusive Igneous Rocks

- Form when magma hardens beneath Earth’s

surface

- Magma intrudes into existing rocks

- Typically see crystals

2. Extrusive Igneous Rocks

- form when lava hardens on the surface of the

Earth

- Extruded onto the surface

- Typically don’t see crystals

Granite

Rhyolite

C. Classification- Igneous rocks are classified by texture and composition

1. Texture refers to the appearance of an igneous rock crystals

a. Coarse- grained Texture- form as a result of slow cooling, ions can move = large crystal size; example: granite

b. Fine-grained Texture- form as a result of quick cooling time, ions lose motion= small crystal size; example: basalt

c. Glassy Texture- form from

lava that cools rapidly,

ions don’t have time to

arrange = glassy texture;

example: Obsidian

d. Porphyritic Texture- rocks with different size minerals that form from varying cooling rates; example: Andesite

2. Composition refers to the proportions of light and dark minerals in the rock

a. Granitic Composition- contain mainly quartz and feldspar. Make up major rocks of continental crust

b. Basaltic Composition- contain mainly dark colored minerals.

-Darker and denser than granitic

composition

- Make up the ocean floor

Box of Rocks- Igneous Rocks

1. Radioactivity- in atoms that are unstable, the nuclei spontaneously break apart or decay.

- The radioactive isotope is known as the parent, product is known as the daughter isotope

D. Dating With Radioactivity

2. Radioactive Dating- rates of decay for certain isotopes have been measured and DO

not vary in Earth’s outer layers - Parent isotopes decay at a constant rate since the formation of the rock they are in

- The decay product has also been accumulating at a constant rate

3. Half-life- the time necessary for one half of the unstable isotope to decay to a stable isotope

- To find the age of a sample you need to know the parent to daughter ratio, the measured graph of half-life decay, and the number of half- lives.

4. Dating with Carbon-14- used to date recent events

• All living things contain carbon-14

• When an organism dies the amount of carbon 14 gradually decreases to carbon 12

• Knowing the ratio of carbon 14 to carbon 12, radiocarbon dates can be determined.

Radiometric Dating Activity

III. Sedimentary Rocks- formed from compacted and cemented sediments

A. Layers of sedimentary rock oftencontain fossils and hold the clue to Earth’s geologic and biologic history

B. Source of Sediments

1. Weathering breaks rocks down

2. Erosion involves weather and the removal of rock

3. Deposition is the dropping of sediments

4. Sediments are deposited according to size

C. How are Sedimentary Rocks formed

1. Sediment- small/fine pieces of rock

2.Sediments are moved by wind, water, ice, and gravity

3. Sediments eventually are dropped/deposited and form layers that get cemented together

4. Final formation includes compaction and cementation

a. Compaction- the squeezing out of water within and between layers due to the weight of material.

b. Cementation occurs when the dissolved minerals seep into the spaces between sediments and

binds them together

D. Classification of Sedimentary Rocks- based on formation

1. Clastic Sedimentary Rocks

- formed from weathered bits of rocks and minerals

- Grouped according to size of the sediments in the rock- example: conglomerate,

sandstone, shale

2. Chemical Sedimentary Rocks are formed when dissolved minerals precipitate from water solution- Examples include limestone, coquina,

Box of Rocks- sedimentary rocks

Samples 6-10

E. A Brief History of Geology1. Uniformitarianism - the same forces

and processes we observe on the Earth now have been at work for a very long time.

2. Relative dating – gives us a sequence of events; allows us to see what layer is older or younger; no numbers

F. Relative Dating Key Principles1. Law of superposition – in an

unchanged layer of rocks, each layer is older than the one above it

2. The principle of original horizontality- Layers of sedimentary rock begin in a horizontal position but get changed over time

3. The principle of cross-cutting relationships

a. faults or breaks in the layers are

younger than the last layer it occurs in

b. Intrusions- places where rock

intrudes into another- are younger

than the rock layer they are in

4. Inclusions are rocks contained within other rocks

a. Rocks containing inclusions are

younger than the inclusions

they contain.

5. Rock is deposited, tilted, eroded, and then new layers are deposited on top of it

a. The boundary

between the layers

is known as an

unconformity.

G. Reading Rock Layers

See if you can do it!

IV. Metamorphic Rocks- form from existing rocks that undergo a change

A. Formation of Metamorphic Rock

1. Contact metamorphism- metamorphic rocks that form from magma intruding into existing rock; the area of contact changes the existing rock

- Low grade change; minor changes

- Ex- marble; often forms when magma

intrudes into limestone

2. Regional Metamorphism – large scale change

- Occurs over large areas during the mountain building process when rocks are exposed to extreme pressures and temperatures.

- Large scale deformation, high grade metamorphism

B. Agents of Metamorphism- what causes metamorphic rocks to change1. Heat- provides energy for

chemical reactions→ reactions cause minerals to recrystallize or form new minerals

- heat comes from magma and increase temperature deep within the earth

2. Pressure (Stress)

- increase in depth of the earth causes an increase in pressure

- increase pressure causes rock to flow not fracture- result is flat and elongated mineral grains

3. Reactions to Solution

- Hydrothermal solutions from magma cause recrystallization when original minerals are dissolved

C. Classification of Metamorphic Rocks- based on texture & composition

1. Foliated Metamorphic Rocks

- Contact metamorphism compacts the mineral grains, increases the pressure and temperature, and causes the minerals to form in the same direction

- Banded appearance from aligning of the mineral directions is known as foliation

2. Nonfoliated Metamorphic Rocks

- Metamorphic rock that does not have a banded appearance.

Box of rocks Metamorphic rocks

Samples 11-15

V. Rock Cycle- rocks move in and out of the three rock families, they can spend any amount of time frame in one rock family and can take alternate paths to each family