6. Minerals and Rocks

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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 Earths surface features

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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 Earths crust

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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

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Our world is built of rocks

• Earths surface is only a very thin covering of
  the planet
• Surface features rocks, soils, plants, rivers,
  oceans
• Below think layer, and above the Earths metallic
  core solid and molten rock
• Rocks 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)

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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

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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

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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

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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
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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.
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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

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Sedimentary Rock

• 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

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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

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Mono Lake
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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

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Metamorphic Rock

• 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

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Metamorphic Rock
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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

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Math in Science

• Each mineral makes up a certain proportion, or
  fraction, of a granite sample
• You can compare mineral amounts by expressing
  each minerals 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

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Rocks in the Crust

• Igneous, sedimentary and metamorphic rocks are
  all found in Earths crust
• 95 is igneous and metamorphic rock
• 5 is sedimentary, a thin covering on Earths
  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

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