Transcript 2. Minerals

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
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
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
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)
How are rocks classified?
• By how they form
– Rocks change, typically over thousands to
millions of years – they break down and reform
• Rock types:
– Igneous Rock
– Sedimentary Rock
– Metamorphic Rock
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
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
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
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.
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
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
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
Mono Lake
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
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
Metamorphic Rock
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
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%
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
Minerals in a Granite Sample
Mineral
Fraction of
granite sample
quartz
1/4
feldspar
13/20
mica
3/50
Dark minerals
1/25
Percentage of
granite