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Earth’s Crust Overhead Notes
Most of the land features that you see every day – rocks, rivers, lakes,
hills – have changed over time and are still changing.
Some changes happen rapidly, while others occur over long periods of
time.
 ROCKS are all around you. Food, building material, soil,
toothpaste, glass, pencils, and much more comes from rocks.
 Rocks that have different properties, depending on how they are
formed, and what different materials they are made from.
MINERALS
 pure, naturally occurring substances that are found in Earth’s
crust
 all rocks are made of minerals – “building blocks” of rocks
GEOLOGY – study of rocks (classification and identification)
CLASSIFICATION OF ROCKS
Colour
 Could be an important clue, but does not stand on its own as an
identifier
Streak
 The colour of the powdery mark that some minerals make when
they are scratched across a hard surface
 Use a streak plate (unpolished piece of porcelain tile)
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 Streak may be the same colour as the mineral, or a different
colour – some minerals can be identified by their streak
 Example: fool’s gold (pyrite) and gold are both coloured yellow,
but gold has a yellow streak, and pyrite has a brown streak
Lustre
 The degree of shininess
 Includes, glassy, dull, and metallic
Hardness
 Related to the hardness of other minerals
 Mineral is harder if it can make a scratch on the other mineral
 Mohs hardness scale uses ten standard minerals, ranging from
very soft to very hard, to compare hardness
Mohs Hardness Scale
1. talc
2. gypsum
3. calcite
4. flourite
5. apatite
6. feldspar
7. quartz
8. topaz
9. corundum
10. diamond
SOFTEST
Hardness Scale of
materials
you can easily find
1. soft pencil point
2. - 3. fingernail
3. - 4. copper penny
5. - 6. nail or glass bottle
6. - 7. steel file
7. - 8. sandpaper
9. emery paper
HARDEST
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Crystal Structure
 All minerals are crystals
 Crystals have regular shapes because they are made up of tiny
particles that are connected in a repeating pattern
 Size of crystals tells how quickly a mineral cooled from a liquid to
a solid - large crystals = mineral cooled slowly (granite); small
crystals = mineral cooled rapidly (basalt).
 Most crystals too small to be seen without magnification
Cleavage
 Describes the way minerals break or fracture into rough, uneven
surfaces, or split or crack along parallel or flat surfaces
 Can be tested by breaking with a hammer or splitting off into
sheets with a table knife
 E.g. mica splits into thin sheets, halite splits into cubes
Magnetism
 The ability of a mineral to attract a magnet
 Only minerals that contain iron are magnetic
Reaction with Certain Chemicals
 Some minerals can be identified by their reaction with certain
chemicals
 E.g. the carbonate material in calcite, marble, and limestone react
with acidic solutions (e.g. vinegar), creating fizzing or bubbling of
carbon dioxide (CO2) on the surface.
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FAMILIES OF ROCKS
There are many different minerals, but they are usually found mixed
together in rocks. E.g. granite contains mica, quartz, and feldspar.
Three families based on how they are formed:
IGNEOUS ROCK
 Hot molten rock beneath Earth’s surface is called MAGMA.
 Rock that forms from the hardening of liquid magma is called
igneous rock, e.g. granite
 Most of Earth’s surface is composed of igneous rock.
If the magma cools underground, the rock that is formed is called
INTRUSIVE igneous rock. It will appear only after the layers of rock
over it have eroded.
 If the magma if forced out onto Earth’s surface, it is called LAVA.
 Rock that forms from the hardening of cooled lava is called
EXTRUSIVE igneous rock. E.g. basalt, obsidian
Basalt
Obsidian
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 The rate at which the molten rock cools determines the size of the
crystals in the rock.
 Cool very slowly: crystals can be seen with the unaided eye
 Cools very quickly: tiny crystals that cannot be
seen without magnification.
 Some igneous rocks (pumice) float due to
gases trapped in frothy lava, which cools
quickly.
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SEDIMENTARY ROCK
 Smaller pieces of rock, sand, clay, mud, gravel, and boulders
carried downstream is called SEDIMENT.
 Water approaches a wider body of water (lake, ocean), the
sediment slows down and sinks to the bottom.
 Sediment gradually piles up in layers.
Millions of years >>> weight of upper layers presses down on the lower
layers >>> compacted and pushed together (higher density) >>>
dissolved minerals fill in the gaps between the pieces and act as a
natural cement that hardens the lower layers into rock.
EROSION >>> DEPOSITION >>> COMPACTION >>> LITHIFICATION =
SEDIMENTARY ROCK
 May also contain plant and animal remains that were deposited
along with the sediment.
 Layers are compressed >>> form different kinds of rock,
depending on the nature of the particles in the sediment.
 Fast-moving streams or rivers on a mountain slope can move large
rocks, while a slow-moving wide river can only carry fine clay
particles.
 Study of the sedimentary layers can give clues to the land
formations of the past.
 Most visible rock on Earth’s surface is sedimentary >>> being
added to all the time by continuous weathering and erosion by
wind and water.
 example: sandstone (formed from layers of compressed sand);
conglomerate (contains rounded pebbles and small stones); shale
(tiny particles of clay or silt)
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METAMORPHIC ROCK
 Igneous or sedimentary rock becomes buried at a great depth,
subjected to increased temperature and pressure.
 Magma moving through Earth heats and squeezes these rocks
 Rock may change in appearance and/or in the minerals it contains
– some change so much
Parent Rock
Metamorphic Rock
that they no longer
shale (sedimentary)
slate
resemble the parent rock
granite (igneous)
gneiss
limestone
(sedimentary)
sandstone
(sedimentary)
Granite
Shale
marble
quartzite
Gneiss
Slate
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Sandstone
Quartzite
Limestone
Marble
More heat and pressure on metamorphic rocks can change them into
other types of metamorphic rocks.
More heat – slate turns to phyllite, which can turn into schist, one of
the strongest rocks in the world.
Phyllite
Schist
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FOSSILS
 Impressions or remains of organisms that were covered by
sediment before they could decompose
 Covered quickly by falling into mud or quicksand, or suddenly
buried by a landslide of sediment or blowing volcanic ash
 Covered by further layers of sediment that eventually becomes
sedimentary rock
 Minerals in the body parts are replaced by minerals that are in the
wet sediment.
 Final result = a fossil that looks
exactly like the original
organism but in a rock-like
form
Fossil found near Cache Creek, BC
FOSSIL RECORD
 Shows what kinds of plants and animals lived millions of years ago
 Fossils buried in sediment in the order of their time on Earth, from
bottom to top
 time line of the changes of life on Earth: geologic time scale.
WEATHERING BREAKS DOWN ROCKS
 The process that slowly breaks down natural materials, e.g. rocks
and boulders, into smaller pieces
 Also affects human-made structures (roads and buildings)
 Caused by physical forces or chemical reactions
 Includes changing temperature, wind, rainfall, and snowfall
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THREE KINDS OF WEATHERING
MECHANICAL WEATHERING: caused by a
physical force
Ice wedging:
 Rainwater trapped in the cracks of rocks
freezes in the winter, expands, forces the
cracks to widen, causing pieces of the
rocks to break off
Wind
 Sand particles and small rocks carried by the wind wear down
exposed surfaces into small pieces or particles of rock
Water
 Fast-flowing water works in the same way as wind, wears away
and smoothes the outer surfaces of the rocks
 Pounding waves on a sea shore can break large rocks into smaller
fragments.
Glaciers
 Rocks trapped under glaciers scrape the ground below and
remove large chunks of
rock
 Long scratches in rock are
called striations.
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CHEMICAL WEATHERING
 Occurs when there is a chemical reaction between air, water, or
another substance and the materials in the
rocks.
 Water can dissolve some rock
materials, especially when it contains
natural or man-made acids
 Rainwater collects carbon dioxide gas
to form a mild acid, CARBONIC ACID
(H2CO3)
 Carbonic acid dissolves the mineral
calcite in limestone on bedrock and on
structures (remember the experiment with the eggshell and the
vinegar? It’s similar to the effects of carbonic acid on limestone,
because the shells have calcite as well)
 Oxygen in the air can rust any iron in minerals found in rocks
BIOLOGICAL WEATHERING
 Some living things cause mechanical or chemical
weathering.
 E.g. LICHEN grows on rocks
and uses some of the materials in
the rock as a source of nutrients.
It produces an acid that dissolves
and wears down the rocks. New
lichens can grow in the
weathered material that is left
from the previous lichen.
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 E.g. the roots of plants that grow in the cracks in rocks can split
the cracks even more. The plants grow in the weathered material
blown into the cracks by wind and water
EROSION: The movement of weathered rock materials from place to
place.




Pieces range from grains of sand to giant boulders
Rapid erosion – landslide down a mountain
Slower – can also take hundreds of thousands of years
Distances range from a few centimetres to hundreds of kilometres
away from the source (PARENT ROCK)
 Gravity, wind, water, and ice all help to move weathered rock
materials
DEPOSITION – when the eroded rock materials stop moving and settle
on Earth’s surface
GRAVITY
 Causes rock falls
and avalanches
 Can be triggered by
earthquakes
WIND
 Particles carried by

 the wind are
deposited when the wind stops
 On beaches and deserts, the wind picks up loose sand and
deposits it in regular piles called DUNES.
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 During times of drought, the wind may pick up and carry away
rich layers of topsoil and deposit it several kilometres away
WATER
 Rivers can move billions of tonnes of rock from the land it crosses
 Water cuts into the land and makes deeper and deeper valleys
(any low region of land between hills and mountains) – tend to be
V-shaped
 Rivers cross flat areas called plains near the coasts of oceans – the
water slows down and deposits the heavier sediment on the
riverbed of riverbanks
 Rivers also slow down when they reach a lake or ocean – much of
the sediment is deposited on the bottom of the lake or ocean,
builds up over time and eventually causes the river to fan out over
a large area, often the shape of a triangle – called a DELTA –
breaks into a number of smaller channels, separated by islands of
sediment
E.g. the Fraser River Delta was built in this manner – the cities of
Richmond and Delta were formed from the deposition of sediment
over thousands of years. The pictures below show Metro Vancouver 10
thousand years ago, and today (deposition in yellow). You can see that
the mouth of the Fraser River used to be at New Westminster, but the
river has deposited an additional 14 km of sediment at the mouth and
some places inland. It continues to grow, in some spots, at a rate of 5m
per year.
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THE ROCK CYCLE
The ways that igneous, sedimentary, and metamorphic rocks relate to
each other – each family of rock is linked to the others in this cycle
 New rocks of each kind are constantly
being formed and may eventually become
exposed on Earth’s surface
 Weathering wears them down >>>
sediment forms layers >>> compression >>>
forms sedimentary rock
 Some rocks get pushed far into Earth >>>
high pressures and temperatures >>> rocks melts and turns into
magma
 Magma can arise and erupt out of a volcano or cool gradually near
the surface >>> igneous rock
EARTH’S CRUST IS MADE UP OF MOVING PLATES
CRUST (0 – 50 km):
 Thin layer of solid rock that makes up Earth’s outermost layer
 Materials in the crust tend to be
lighter than the materials below
 Earth’s crust floats on inner layers
MANTLE (2900 km)
 Hot, thick layer of solid and partially
melted rock
 Increased pressure because of upper
layers pushing down on you
 Moves sluggishly, like syrup
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OUTER CORE (2200 km):
 Dense, hot region made up mostly of liquid iron and some nickel
 Pressure is very high
 Material flows more sluggishly
INNER CORE (1250 km):
 Large ball of iron and nickel
 Pressure from other layers compresses the inner core and keeps it
solid
 Temperature is nearly as hot as Sun’s surface
Sources of heat inside Earth – theories:
 Decay of radioactive material within the mantle;
 Heavy metals descending to Earth’s core
 Heat may be generated by tidal force on the Earth as it rotates;
since land cannot flow like water it compresses and distorts,
generating heat.
 Heat remaining from the impact of falling meteorites from Earth’s
formation
Geothermal Gradient: Heat increases as you move towards the center
of Earth: 25-30°C per km of depth in most of the world.
EARTH’S CRUST IS MADE UP OF MOVING PLATES
Alfred Wegener (1880-1930) first came up with idea that Earth’s
continents were once joined as one supercontinent called Pangaea;
continents separated and moved to their current locations in a process
called continental drift.
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Wegener observed that South America
and Africa fit together like pieces of a
jigsaw puzzle.
SUPPORT FOR CONTINENTAL DRIFT
 Fossil Record: Scientists have found
fossils of identical plants and animals on
different sides of the ocean; they could not have travelled across
the ocean, therefore they must have lived on the same continent
at some time in the past.
 Landforms: landforms on the continents matched, e.g. mountain
ranges and unusual rock formations between South Africa and
Brazil, Eastern USA and Scotland.
 Ancient Ice Age: Similar striation patterns left by glaciers along
the coasts of South America and Africa, indicating that they were
once situated around the South Pole
CRITICISM OF CONTINENTAL DRIFT
 No explanation of how the continents drifted
OCEAN FLOOR
Not flat, as originally thought: many long, deep trenches running
parallel and near the edges of oceans
Mid-ocean ridge: a 50 000 km long
mountain range that almost circles the
Earth and runs through the middle of
the oceans
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AGE OF ROCKS ON THE OCEAN FLOOR
 The layer of sediment on the ocean floor is quite thin: the ocean
floor is not as old as they thought – if it was unchanged for
millions of years, then it would have been much thicker
 Young rocks found at the top of mid-ocean ridges – the further
away from the ridge, the older the rocks
Conclusion:
 the ridge is where the crust is splitting apart
 Magma is rising at the ridge to form new crust
 The sea floor at the mid-ocean ridge is increasing in size as new
crust if formed
 Process is called SEA-FLOOR SPREADING
MAGNETIC REVERSALS
 Magnetic Field has reversed several times over millions of years
 Grains of magnetite displaying the magnetic field at different
times was locked into the
rock at the time it was
formed
 Stripes of rock parallel to
the mid-ocean ridge
alternate normal and
reversed magnetic fields,
indicating that new rock
was formed at the ridge.
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THEORY OF PLATE TECTONICS
 The surface of the Earth consists of about a dozen large plates
that are continually moving
 The parts of Earth’s crust that have continents on them are called
continental crust
 Plates that have ocean floor on them are called oceanic crust
 Plates can be made up of either crust, or both
Plates move at different rates: fastest is 15 cm per year, slowest is 2.5.
DIVERGENT BOUNDARIES
 Boundaries between plates that are moving apart
 Plates separate >> hot magma rises to Earth’s surface to form new
crust
 Separation and production of new crust is called SEA-FLOOR
SPREADING
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 Magma cools and hardens, forming
ridges of new rock up to a kilometre from
the ocean floor
 Entire length of the Atlantic Ocean has
the MID-ATLANTIC RIDGE spreading at a rate
of 2.5 cm per year
Divergent plate boundaries on land are called RIFTS – this is a picture of
a rift between the North
American Plate and the
Eurasian Plate visible on land in
Iceland.
CONVERGENT BOUNDARIES
 Located where plates are
moving against each other
 Recycling of old crust while new crust is formed at divergent
boundaries
 Collisions that occur can
last millions of years
 One plate sinks below
the other in the
SUBDUCTION ZONE
 The new landform
depends on whether the
converging plates are
ocean-ocean, oceancontinental, or continental-continental.
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OCEANIC PLATE CONVERGING WITH CONTINENTAL PLATE
When an oceanic plate collides with a continental plate, the oceanic
plate is subducted under the continental plate because its rock is
denser. This creates deep ocean trenches along the edge of a
continent. In BC, this is seen where the Juan de Fuca Plate is being
subducted under the North American Plate. In the cutaway diagram,
you will see the trench
located at the subduction
zone (note the location of
Mt. St. Helens in relation
to the rising magma). This
type of convergence can
also cause landmasses to
rise and create mountains,
either by the crumpling of
the plates, or by the scraping of the subducting plate: this is called
continental accretion.
OCEANIC PLATE CONVERGING WITH OCEANIC PLATE
When two oceanic plates converge, one plate subducts below the
other. The deepest trench in the
world is the Mariana Trench,
located in the Pacific Ocean on the
east side of the Philippine Plate,
just east of The Philippines. It has
been estimated to be nearly 11 km
deep (deeper than Mount Everest
is tall).
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CONTINENTAL PLATE CONVERGING WITH CONTINENTAL PLATE
When two continental plates meet, neither one is subducted because
the rock in each plate is the same density. The crust buckles and
crumbles, pushing up mountains or
areas of high level ground called
PLATEAUS. The Himalayan Mountains
are the result of the India Plate
converging with the Eurasian Plate 50
million years ago. It also pushed up the
Tibetan Plateau, which is higher than
the Alps mountain range in Europe.
TRANSFORM FAULT BOUNDARIES
A FAULT is an area where rocks are broken by
a movement in the crust. Divergent and
Convergent plate boundaries can end
abruptly and “transform” into faults. The zone between plates that are
slipping past each other are called TRANSFORM FAULT
BOUNDARIES. Most are found on the ocean floor. One
of the most famous found on land is the San Andreas
Fault, where the Pacific Plate is moving past the North
American Plate in California. Shallow earthquakes are
common along transform fault boundaries. At left is a
picture of the San Andreas Fault.
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EARTHQUAKES
Plates in Earth’s crust move, and the rough edges lock together. Over
time, one or both of the plates move, releasing the elastic energy
stored in the rocks. The sudden release of energy causes vibrations in
the Earth’s crust called EARTHQUAKES.
Earthquakes can occur in all three types of plate boundaries:
convergent, divergent, and transform fault.
EARTHQUAKES AT DIVERGENT BOUNDARIES
 Two plates are being pushed apart >>> hot magma rushes
towards the opening in the crust >>> pressure builds up >>>
pushes the plates apart >>> crust shakes, causing an earthquake.
EARTHQUAKES AT CONVERGENT BOUNDARIES
 Oceanic plate is subducted under the continental plate >>> gets
stuck against the top plate >>> force builds up >>> top plate
suddenly moves >>> causes a large earthquake.
 Longer the plate is stuck >>> stronger the earthquake when it
breaks free.
 Coast of BC has over 200 earthquakes a year as the Juan de Fuca
Plate is subducted under the NA Plate.
EARTHQUAKES AT TRANSFORM FAULT BOUNDARIES
 Plates moving past each other >>> one gets stuck against the
other >>> pressure builds up until one plate suddenly moves >>>
causes an earthquake
 Longer the plate is stuck >>> stronger the earthquake when it
breaks free.
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EFFECTS OF EARTHQUAKES
Recent earthquakes in Japan
and China are examples of
the devastation they cause.
At right is a picture from the
Alaskan earthquake in 1964.
 Earthquakes start at
the FOCUS, or where
the plates are moving
 Surface of the earth
directly above the focus is called the EPICENTRE
 Shock waves sent out by an earthquake are called SEISMIC
WAVES.
 Smaller tremors that can occur for months afterward are called
AFTERSHOCKS
Energy from an earthquake travels in all directions.
Strength of the energy of an earthquake depends on the amount of
energy that was released from the plate movement.
Two kinds of waves:
PRIMARY WAVES




Travels through liquids and solids
Pushes and pulls materials as they move through the Earth
Travels about 8km per second
First movement felt in an earthquake
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SECONDARY WAVES




Travels through solids only
Makes rocks vibrate up, down, and sideways
Travels at about 4.5 km per second
Usually causes the most building damage
Earthquake begins >>> ground starts to shake >>> buildings sway back
and forth >>> tunnels and overpasses may collapse >>> gas lines may
rupture, causing fires >>> water and sewer lines are damaged or broken
>>> most devastating in urban areas
TSUNAMI
 Ocean waves caused by moving sections of sea floor or
underwater
volcano
 Can travel at a
speed of 800
km per hour
 Approach
shallow water
>>> energy is
shifted to form
giant waves to
30 metres or
more in
narrow inlets
 Can cause massive destruction and flooding in coastal areas.
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VOLCANOES
 Any opening in the Earth’s crust through which molten rock and
other materials erupt is called a volcano
 Most are located along the edges of Earth’s plates
 Some flow frequently and relatively quietly, while others erupt
every few hundred years
 Many volcanoes erupt out of sight at the bottom of the ocean
VOLCANOES AT DIVERGENT BOUNDARIES
 Three-quarters of all the lava
produced on Earth comes
from eruptions at divergent
plate boundaries on the ocean
floor
 Magma pushes to the surface
where plates are moving apart
>>> lava erupts and cools to form a ridge on each side of the crack
 When the lava rises high enough to reach the surface, an island
may form, e.g. Iceland
 Divergent boundary volcanoes in BC are located in the northwest
corner of BC, near Alaska – 150 years since last eruption
VOLCANOES AT CONVERGENT BOUNDARIES
 Most of the volcanoes on land are created near convergent plate
boundaries where one plate is being subducted beneath another
plate
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 Most powerful
eruptions take place here
because the magma tends
to be thick and sticky >>>
pressure builds up and it
erupts explosively as the
pressure is released
 High-pressure
eruption carries lava and
ash
 Last volcanic eruption in BC was 2 000 years ago (Mt. Meager)
 Other volcanoes on the convergent boundary between the Juan
de Fuca Plate and the NA Plate are Mount Silverthrone, Mount
Cayley, and Mount Garibaldi, located near Whistler.
VOLCANOES THAT FORM AT HOT SPOTS
 Some volcanoes can occur away from plate boundaries, e.g.
Hawaiian Islands are 3 000 km away from the nearest boundary
 Volcanoes form over HOT SPOTS, or parts of the mantle where
the temperature is higher than normal >>> enormous pool of
magma melts a hole in the rock above it >>>
pours out of the hole onto Earth’s surface as
lava
 Lava is typically runny and fluid, therefore does
not erupt explosively
 If it forms on the ocean floor, it would harden
more quickly than on land, and form a coneshaped mountain that may become a land mass
>>> Hawaiian Islands were formed this way.
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 BC has a hot spot area called
the Anahim Volcanic Belt from
the coast to Quesnel
THE EFFECTS OF VOLCANOES
Destructive properties of volcanoes:
 death by clouds of hot ash and
poisonous gases
 being buried by volcanic
mudslides.
 Death by starvation after crops
and livestock have been
destroyed
 Whole villages and cities have
been destroyed by volcanoes
 250 years ago, a volcano in northern BC destroyed two villages
and killed about 2500 people with CO2 gas
Positive effects of volcanoes:
 Volcanic ash can improve the soil for farmland
 Volcanic rocks contain many useful minerals and gems
 Igneous rock OBSIDIAN can be used to make sharp tools
To view a collection of extrusive rocks and learn more about them,
check out this web page:
http://geology.about.com/od/more_igrocks/ig/extrusives
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VOCABULARY
Pluton
lichen
Rock
volcano
parent rock
Mineral
basalt
delta
Geology
granite
crust
Colour
mechanical
weathering
mantle
Streak
Lustre
Hardness
chemical
weathering
Cleavage
biological
weathering
Magnetism
erosion
Clarity
deposition
Igneous
compaction
Sedimentary
lithification
Metamorphic
glacier
Crystal
striations
Intrusive
carbonic acid
Extrusive
dunes
Lava
talus slope
Magma
mass wasting
inner core
outer core
rock cycle
geothermal
gradient
continental drift
mid-ocean ridge
sea-floor spreading
continental crust
oceanic crust
Pangaea
Plate tectonics
Divergent
boundaries
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Convergent
boundaries
Subduction zone
Plateau
Earthquake
Hot spot
Transform fault
boundaries
Seismic waves
Primary waves
Secondary waves
Aftershocks
Focus
Tsunami
Volcanic ash
Obsidian