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6.2 Rocks form in different ways

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1 6.2 Rocks form in different ways
6. Minerals and Rocks 6.1 Minerals are all around us 6.2 Rocks form in different ways 6.3 Natural processes break down rocks 6.4 Geologic maps show Earth’s surface features

2 6.2 Rocks form in different ways
Before, you learned: Minerals and rocks are basic components of Earth Minerals have four characteristics Most rocks are made of minerals Now, you will learn: About the three types of rocks How one type of rock can change into another How common each rock type is in Earth’s crust

3 Warm-up Questions 1. What are the four characteristics of a mineral?
Forms in nature, is a solid, has a definite chemical makeup, has a crystal structure 2. How are minerals classified? Groups based on chemical makeups 3. What is the most common group of minerals? What percentage of the crust do they make up? Silicate group is the most common; 90% 4. Can oil and natural gas be classified as minerals? Why or why not? No because they are not solids, do not form crystals, and do not have a definite chemical makeup

4 Our world is built of rocks
Earth’s surface is only a very thin covering of the planet Surface features: rocks, soils, plants, rivers, oceans… Below think layer, and above the Earth’s metallic core: solid and molten rock Rock’s uses: Building materials, sources of metals, art work and sculptures, pavement for roads Rocks are: Long lasting, beautiful, historic (monuments and sculptures, Great Wall, Great Pyramids, Mount Rushmore)

5 How are rocks classified?
By how they form Rocks change, typically over thousands to millions of years – they break down and re-form Rock types: Igneous Rock Sedimentary Rock Metamorphic Rock

6 Igneous Rock Form when molten rock cools and becomes solid
Deep in the early – temperatures hot enough – 750C to 1250C – to melt rock! This molten rock is called magma magma is less dense than the surrounding solid rocks  it rises toward the surface It may settle within the crust or erupt at the surface from a volcano as a lava flow Magma - below the Earth's surface Lava - erupts onto the Earth's surface through a volcano or crack

7 Igneous Rock Depending on where they form, igneous rocks are either:
Intrusive: forms when magma cools within the crust Example: granite Extrusive: forms when lava cools above the surface Example: rhyolite Intrusive or Extrusive can have the same mineral composition But the rocks will have different names, because the size of their minerals crystals will be very different

8 Igneous Rock Crystal Size
Granite and rhyolite are the same mineral composition, but their crystals sizes are different Large crystals form in intrusive rocks because: slow cooled (very hot) Small crystals form in extrusive rock because: faster cooling Granite Rhyolite

9 Intrusive Rocks at the Surface?
Can reach the surface by forces of nature When mountains form (tectonic plates shifting) Water and wind break apart and carry away surface rocks, exposing deeper rocks Devils Tower National Monument, Wyo. An igneous intrusive body exposed by erosion. Photograph by F. W. Osterwald, U.S. Geological Survey.

10 Sedimentary Rock Forms when pieces of minerals and rocks, plants, and other loose material get pressed or cemented together Loose material carried by wind or water which then settle on the surface are called sediments Build up in layers – younger on top of older layers Also formed as water evaporates, it leaves behind materials dissolved in it Minerals form from the materials

11 Sedimentary Rock Animation
The distance sediments are carried depends on the size of sediments and speed of water/wind Large heavy sediments settle quickly as speed decreases Small light sediments can be carried a longer distance Animation Lower layers of sediments can get pressed into rock by the weight of layers above them New minerals can grow in the spaces between the sediments, cementing them together

12 Sedimentary Rock and Fossils
Fossils are formed by a similar process Fossils are the remains or traces of organisms from long ago Limestone: usually made up of fossils of ocean organisms Shells and skeletons settle to ocean floor Coal: remains of ancient plants pressed into rock

13 Mono Lake

14 Metamorphic Rock Forms when heat or pressure causes older rocks to change into new types of rocks “Metamorphism”: the process Can begin as either igneous or sedimentary, or another metamorphic! (called the parent rock) Ex: limestone (sedimentary) is the parent rock of marble (metamorphic) Usually occur over large areas with both high temperatures AND pressures If just one is high – over smaller areas

15 Metamorphic Rock Rocks remain SOLID!
Pressure can cause a rocks minerals to flatten out in one directions Rocks remain SOLID! They do NOT melt when they undergo metamorphism If they melted – result would be igneous Heat and pressure can break the bonds that join atoms in minerals – new bonds form: recrystalization Individual mineral crystals can grow larger Atoms can combine in different ways, and new minerals form in place of older ones

16 Metamorphic Rock

17 Rocks can change into other types of rocks
Over time… Sediments on the surface may become sedimentary rock Rocks at or near the surface may become metamorphic rocks Or they may melt and cool, forming igneous rocks The Rock Cycle – set of natural processes by which rocks form, change, bread down, and re-form



20 Math in Science Each mineral makes up a certain proportion, or fraction, of a granite sample You can compare mineral amounts by expressing each mineral’s fraction as a percentage To change a fraction to a percentage, you must find an equivalent fraction with 100 as the denominator 1/5 to percent? First, divide 100 by the denominator 5 = 20 Then multiply the numerator and denominator by 20 = 20/100 = 20%

21 Rocks in the Crust Igneous, sedimentary and metamorphic rocks are all found in Earth’s crust 95% is igneous and metamorphic rock 5% is sedimentary, a thin covering on Earth’s surface Surface of Crust: 75% sedimentary, 25% Ign & Met. Entire Crust: 5% sedimentary, 95% Ign & Met. Sedimentary is most common at surface because formed by processes that occur at the surface Igneous and metamorphic are formed by process that occur deeper within Earth

22 Minerals in a Granite Sample
Fraction of granite sample Percentage of granite quartz 1/4 feldspar 13/20 mica 3/50 Dark minerals 1/25 25, 65, 6, 4

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