Transcript Earth`s Crust - Southern Local Schools

How do Earth processes affect
our everyday lives?
Natural events such as
volcanic eruptions,
earthquakes, landslides, floods
and gigantic sea waves make
headlines and affect many
people in obvious ways.
Affects
Although we cannot prevent most of
these
natural
disasters
from
happening, the more knowledge we
have about what causes them, the
better we will be able to predict, and
possibly control the severity of
impact.
Geologists
Scottish geologist James Hutton, the
eighteenth-century geologist developed
the theory of uniformitarianism.
English geologist Sir Charles Lyell is given
the most credit for advancing the basic
principles of modern geology.
The Rock Cycle
Some people believe that "once a rock,
always that rock". But that is not always
true.
Rocks take different forms at
different times. A long time ago our earth
was very volcanic. As these volcanoes
cooled and vast oceans swept over the
earth, the cooled lava was broken or
crushed into small pieces. These small
pieces were cemented together to become
sedimentary rocks.
Rock Cycle contd.
These rocks were buried and the heat
and pressure changed them into
metamorphic rocks. They might even
have melted and become igneous
rocks once more. As you can tell, a
rock may change many times and the
rock you hold today may look entirely
different to someone a long time from
now.
The Rock Cycle
Convection
The mobile rock beneath the rigid
plates is believed to be moving in a
circular manner somewhat like a pot
of thick soup when heated to boiling.
The heated soup rises to the surface,
spreads and begins to cool, and then
sinks back to the bottom of the pot
where it is reheated and rises again.
This cycle is repeated over and over
to generate what scientists call a
convection cell or convective flow.
Convection contd.
While convective flow can be observed
easily in a pot of boiling soup, the idea of
such a process stirring up the Earth's
interior is much more difficult to grasp.
While we know that convective motion in
the Earth is much, much slower than that
of boiling soup, many unanswered
questions remain: How many convection
cells exist? Where and how do they
originate? What is their structure?
Convection
Ancestor Rock Types
Igneous
Metamorphic
Sedimentary
Igneous
Igneous rocks, also called volcanic rocks, are
formed from melted rock that has cooled and
solidified. When rocks are buried deep within the
Earth, they melt because of the high pressure
and temperature; the molten rock (called
magma) can then flow upward or even be
erupted from a volcano onto the Earth's surface.
When magma cools slowly, usually at depths of
thousands of feet, crystals grow from the molten
liquid, and a coarse-grained rock forms.
Igneous contd.
When magma cools rapidly, usually at or
near the Earth's surface, the crystals are
extremely small, and a fine-grained rock
results. A wide variety of rocks are formed
by different cooling rates and different
chemical compositions of the original
magma. Obsidian (volcanic glass), granite,
basalt, and andesite are four of the many
types of igneous rock.
Sedimentary
Sedimentary rocks are formed at the
surface of the Earth, either in water or on
land. They are layered accumulations of
sediments: fragments of rocks, minerals,
or animal or plant material. Temperatures
and pressures are low at the Earth's
surface, and sedimentary rocks show this
fact by their appearance and the minerals
they contain. Most sedimentary rocks
become cemented together by minerals
and chemicals or are held together by
electrical attraction; some, however,
remain loose and unconsolidated.
Sedimentary contd.
The layers are normally parallel or nearly parallel
to the Earth's surface; if they are at high angles
to the surface or are twisted or broken, some
kind of Earth movement has occurred since the
rock was formed. Sedimentary rocks are forming
around us all the time. Sand and gravel on
beaches or in river bars look like the sandstone
and conglomerate they will become. Compacted
and dried mud flats harden into shale. Scuba
divers who have seen mud and shells settling on
the floors of lagoons find it easy to understand
how sedimentary rocks form.
Metamorphic
Sometimes sedimentary and igneous
rocks are subjected to pressures so
intense or heat so high that they are
completely changed. They become
metamorphic rocks, which form while
deeply buried within the Earth's crust. The
process of metamorphism does not melt
the rocks, but instead transforms them into
denser, more compact rocks. New
minerals
are
created
either
by
rearrangement of mineral components or
by reactions with fluids that enter the
rocks.
Metamorphic contd.
Some kinds of metamorphic rocks--granite
gneiss and biotite schist are two
examples--are strongly banded or foliated.
(Foliated means the parallel arrangement
of certain mineral grains that gives the
rock a striped appearance.) Pressure or
temperature can even change previously
metamorphosed rocks into new types.
Earth Composition
The Earth is not just a ball of solid
rock. It is made of several layers
with different physical properties
and compositions. Scientists think
about the Earth’s layers in two
ways; by their composition and
physical properties.
Earth’s Composition
The Earth is divided into three layers

Crust- is the outermost layer ranging
from 5-100 km thick. Is the thinnest layer
and the most is known about this layer.
 Mantle- is the layer between the crust
and the core. It is thick and contains most
of the Earth’s mass.
 Core- believed to be made mostly of iron,
nickle, sulphur and oxygen.
Earth’s Crust
There are two types of crust:
 Continental crust- has a composition like
granite with an average thickness of 30
km.
 Oceanic crust- has a composition similar
to basalt with a thickness of 5 – 8 km thick.
Because basalt is denser than granite,
oceanic crust is denser than continental
crust.
PHYSICAL PROPERTIES

FIVE LAYERS
Five Layers
Lithosphere
 Asthenosphere
 Mesosphere
 Outer Core
 Inner Core

Mnemonics
meaning “rock”, the lithosphere is
the Earth’s solid, rocky crust
 Astheno- meaning ‘weak”, the
asthenosphere is a layer of slowly flowing
rock beneath the lithosphere
 Meso- meaning “middle’, the mesosphere
lies between the asthenosphere and outer
core.
 Litho-
Earth Structure
Earth Structure
Brain Food
The deepest hole ever drilled into the
continental crust was in the Kola
Peninsula, in Russia, in 1984. It was
12,226 m deep! It is very difficult to drill
much deeper because the deeper you go,
the hotter it gets. If you drill too deep, the
hot rock flows around the drill bit, filling the
hole faster than it can be drilled.
Earth Composition
Earth Composition
Inner Core – a solid iron-rich zone having a
radius of 756 miles.
Outer Core – a molten metallic layer 1410
miles thick.
Mantle – a solid rocky layer with a maximum
thickness of 1789 miles.
Crust – a relatively lighter outer skin that
ranges from 3 to 25 miles thickness.
Asthenosphere
A very important zone located
within the mantle. The
asthenosphere is located
between 60 and 150 miles
down and is a hot weak zone
that is capable of gradual flow.
Lithosphere
Situated
above
the
asthenosphere, this zone includes
the crust and uppermost mantle.
It can be considered cool and
rigid.
The
lithosphere
is
approximately 60 miles thick.
Mapping the Interior
How do we know these things about
the Earth’s interior?
Earthquakes produce seismic waves
that can be measured. These seismic
waves travel at different speeds through
materials because of the difference in
density of the material.
Speeds





Lithosphere- 7-8 km/sec
Asthenosphere- 7-11 km/sec
Mesosphere- 11-13 km/sec
Outer Core- 7-10 km/sec
Inner Core- 11-12 km/sec
Quiz
1. The crust is the Earth’s only solid layer.
2. The inner core of the Earth is solid and
made primarily of iron.
3. Temperature and pressure increase
toward the center of the Earth.
4. The asthenosphere is the thinnest.
Answers
False
2. True
3. True
4. False
1.
Plate Tectonic Theory
According to the plate tectonics
model, The Earth’s rigid outer shell,
the lithosphere, is broken into several
individual pieces called plates that
move over the asthenosphere.
Further, it is known that these rigid
plates are slowly, but nevertheless
continually moving.
Plate Tectonic Theory
This motion is driven by a thermal engine,
the result of an unequal distribution of heat
within the Earth. As hot material gradually
moves up from deep within the planet and
spreads laterally, the plates are set in
motion. Ultimately, this movement of
Earth’s plates generates earthquakes,
volcanic activity and mountain building.
Jigsaw Puzzle
Look at the world map on the following
slide and notice that each tectonic plate
fits the other tectonic plates that surround
it. The lithosphere is like a giant jigsaw
puzzle. Also, notice that not all tectonic
plates are the same. Compare the size
and composition, some have both types of
crust and some have only one type of
crust.
Plate Close-Up
From the previous slide, notice that
this tectonic plate consists of both
oceanic and continental crust. The
thickest part is under the Andes
mountains and the thinnest part is at
the Mid-Atlantic Ridge.
Continental Drift
 Geologist Alfred Wegener proposed the
theory of continental drift. Simply
stated, Earth’s land had once been joined
into a single super continent surrounded
by ocean.
 Pangea- super continent land mass
 Panthalassa- surrounding ocean
Continental Drift Theory
The continental drift theory explained
why the shorelines of different
continents fit together like a puzzle,
why the same fossil animals and
plants are found on different
continents
and
how
different
mountain ranges were once part of a
larger continuous mountain range.
Plate Movement
225 mya
135 mya
200 mya
35 mya
Present
Sea-Floor Spreading
As rising material from the mantle
spreads laterally, sea floor is carried
in a conveyor belt fashion away from
the ridge crest. Tears at the ridge
crest produced by the diverging plate
boundaries provide pathways for
magma to intrude and generate new
oceanic crust.
Sea-Floor Spreading
Mid- Ocean ridges are places where seafloor spreading takes place. Sea-floor
spreading is the process by which new
oceanic lithosphere is created as older
material is pulled away. As tectonic plates
pull away from each other, the sea floor
spreads apart and magma rises to fill the
gap.
Page 175, Figure 11
Notice that the crust increases with age
the farther away from the mid-ocean ridge.
This is because new crust continually
forms from molten material at the ridge.
The oldest crust in the Atlantic Ocean is
found along the edges of the continents. It
dates back to the time of the dinosaurs.
The newest crust is in the center of the
ocean.
Connection
In the 1950’s scientists broadcast
sonar waves toward the sea floor and
measured how long it took the waves
to return. The echoes revealed the
existence of oceanic valleys and
mountains. In short, the ocean floors
turned out to be as varied as the
continents.
Quiz
1. If the Earth’s crust is growing at mid-
ocean ridges, why doesn’t the Earth itself
grow?
2. What was pangea?
Answers
1. Because the Earth’s crust is part of the
rock cycle.
2. The large landmass that later broke up to
form two supercontinents and then
fragmented further to form the six
continents of today.
Magnetic Reversals
The molten rock at the mid-ocean ridges
contains tiny grains of magnetic minerals.
These mineral grains act like compasses and
align with the magnetic field of the Earth. Once
the molten rock cools, the record of these tiny
compasses is literally set in stone and is slowly
carried away from the spreading center as seafloor spreading occurs This new rock records
the direction of Earth’s magnetic field. When
Earth’s magnetic poles change place, it’s called
magnetic reversal. This record of magnetic
reversals was final proof that sea-floor
spreading does occur.
Paleomagnetism
A compass needle points toward the
magnetic north pole because it aligns
with the Earth’s magnetic field. Tiny
particles of iron-bearing magnetic
minerals are found in magma. These
minerals act like miniature compass
needles; as they cool to form new sea
floor, their magnetic fields align with
the Earth’s magnetic field.
Paleomagnetism
The orientation of the Earth’s
magnetic field at that particular time
becomes frozen in the rock as it
solidifies. The Earth’s magnetic field
reverses periodically but at irregular
intervals once every 300,000 to
500,000 years. A magnetometer
measures the amount and direction of
residual magnetism in a rock sample.
Plate Tectonics Theory
Plate tectonics theory is that the Earth’s
Lithosphere is divided into tectonic plates
that move around on top of the
asthenosphere. An incredible amount of
energy is needed to move something as
massive as a tectonic plate! We still don’t
know exactly why they move. But, recently
scientists have come up with some
possible answers.
Plate Tectonics
Plate Boundaries
 Divergent plate boundaries- zones where
plates move apart, as magma rises to the
surface, leaving a gap between them. (Sea
floor spreading)
 Convergent plate boundaries- zones
where plates move together, causing one of
the slabs of lithosphere to be consumed
(subduction zone) into the mantle as it
descends beneath an overriding plate.
Andes Mountains in South America.
Plate Boundaries
Transform plate boundaries- zones
where plates slide sideways past
each other, scraping and deforming
as they pass. (San Andreas Fault)
Continent - Continent
Convergence - Subduction
Practice
The distance between New York and
Paris increases every year. Currently, the
two cities are moving apart by about 2 cm
per year. This may not sound like much,
but calculate the increase in distance in 1
million years. How much will the distance
increase in 100 million years?
Practice Answers
 20 km
 2000 km
Quiz
1. Why are there several categories of
convergent plate boundaries?
2. Tell where you would expect to see the
following features:
a. tall, wrinkled mountains of a continent
b. a long parallel ridge on the ocean floor
surrounded by parallel zones of magnetic
reversal.
3. Explain the process of subduction
Answers
1. Plates that are pushed together behave
differently, depending on their
composition and density.
2. a. convergent continental/continental
boundary
b. divergent boundary
3. A denser oceanic crust is forced beneath
a less dense oceanic or continental plate
at a convergent boundary.
Tracking Plate Motion
Scientists use a network of satellites
called the Global Positioning System.
Radio signals are continuously beamed
from satellites to ground stations, which
record the exact distance between the
satellites and the ground station. By
measuring the time it takes for the GPS
ground stations to move a certain
distance, scientists can measure the rate
of each tectonic plate.
Deforming the Crust
When rock changes its shape due to
stress, this reaction is called deformation.
Stress is the amount of force per unit area
that is put on a given material. Much like
bending and then breaking uncooked
spaghetti noodles, rock layers can bend
when stress is placed on them. But, when
more stress is placed on them, they can
break.
Types of Stress
 Compression- this is a type of stress
when an object is squeezed, or when two
tectonic plates collide. The Rocky
Mountains and the Cascade Range are
two examples of compression at
convergent zones.
 Tension- this type of stress occurs when
forces act to stretch an object. Tension
occurs at divergent plate boundaries.
Folding
Folding occurs when rock layers bend
due to stress in the Earth’s crust. We
assume that all sedimentary rock
layers started out as horizontal layers.
So, when you see a fold, deformation
has occurred.
Types of Folding
 Anticline
Syncline
Monocline
When tectonic forces put stress on rock
layers, they can cause the layers to bend.
Anticlines and synclines form when
horizontal stress acts on rock. Monoclines
form when vertical stress acts on rock.
(See page 182, Figure 17.)
Faulting
The surface along which rocks break
and slide is called a fault. There are
three major types of faulting. A fault
will have two sides, the hanging wall
and footwall. Movement of the
hanging wall and the footwall with
relationship to each other, determines
which type of fault it is.
Normal Faults
The movement of a normal fault
causes the hanging wall to move
down with relation to the footwall.
Normal faults usually occur when
tectonic forces cause stress tension
that pulls rocks apart. (Page 183,
Figure 20).
Reverse Faults
The movement of a reverse fault
causes the hanging wall to move
up in relation to the footwall– the
“reverse” of a normal fault.
Reverse faults usually happen
when tectonic forces cause stress
compression that pushes rocks
together. Page 183, Figure 21.
Strike-Slip Faults
A third major type of fault is
called a strike-slip fault. They
occur when opposing forces
cause rock to break and move
horizontally. The San Andreas
Fault in California is a strikeslip fault.
Self-Check
How is folding different from faulting?
When folding occurs, sedimentary rock
strata bend but do not break. When faulting
occurs, sedimentary rock strata break
along a fault and the fault blocks on either
side move relative to each other.
Mountain Building
When
tectonic
plates
undergo
compression or tension, they can form
mountains in several different ways. Three
of the most common types of mountains
are:

Folded Mountains

Fault-block Mountains

Volcanic Mountains
Folded Mountains
Folded mountains form when rock layers
are squeezed together and pushed
upward. The Appalachian mountains were
once as great as the Himalayas. The
Appalachian mountains have been worn
down by hundreds of millions of years of
weathering and erosion. Both are
examples of folded mountains.
Fault-Block
Where tectonic forces put enough
tension on the Earth’s crust, a large
number of normal faults can result.
Fault-block mountains form when
this faulting causes large blocks to
drop down relative to other blocks.
(Page 186, Figure 25).
Volcanic Mountains
Most of the world’s major volcanic
mountains are located at convergent
boundaries. Volcanic mountains form
when molten rock erupts onto the Earth’s
surface. They form from new material
being added to the Earth’s surface. There
are so many volcanic mountains around
the rim of the Pacific Ocean that explorers
named it the Ring of Fire.
Quiz
1. What three features form when rock
layers bend?
2. Why are the Appalachian Mountains now
located in the middle of the North
American Continent?
Answers
1. Anticlines,
synclines
and
monoclines.
2. The Appalachian Mountains formed
when North America and Africa
collided. In time, the plates
separated and so much new crust
was created that the mountains
were no longer at the plate
boundaries.
VENT
Located at the summit of many
volcanoes is a steep-walled
depression, crater, which is
connected to a magma chamber via a
pipe like conduit, vent.
Density
mass
Density 
volume
g
g
Density  3 
cm
ml
= Specific Gravity
VOLCANOES
VOLCANOES
Most of the more than 600 active
volcanoes that have been identified are
located in the vicinity of convergent
plates. Further, extensive volcanic
activity occurs out of view along sea
floor spreading centers of the oceanic
ridge system.
VOLCANOES
Mt. Pinatubo
Earthquakes
An earthquake is the vibration of the
earth produced by the rapid release of
energy. The energy radiates in all
directions from its source, or focus. The
rapid release of energy or movement of
plates is associated with the large
fractures in the Earth called faults.
San Andreas Fault
Earthquake Waves
Seismographs reveal that two main groups
of seismic waves are generated by the
slippage of a rock mass.
 Surface Waves- travel along the surface
 Body Waves- travel through the Earth’s
interior
“L” - Waves
Surface waves consist of two types of wave
motion. One motion produces a complex upand-down motion similar to ocean swells,
while the other surface wave whips the ground
from side-to-side without any vertical motion.
Surface waves have a longer time interval
between crests and are referred to as L-Waves.
These waves cause the greatest destruction.
“P” - Waves
“P”
Waves(primary)
are
compressional waves and travel
through solids, liquids and gasses.
This wave action is like that
generated by human vocal chords as
they move air to create sound.
“S” - Waves
“S” Waves- (secondary) shake the
particles at right angles to the direction of
wave travel. This can be illustrated by
tying one end of a rope to a post and
shaking the other end. S waves do not
travel through liquids and gasses, only
through solids.
Earthquakes
The epicenter is the location on the surface of
the Earth directly above the focus (source) and
can be measured by a seismograph.
Seismograph- A device used to measure the low
frequency waves generated by an earthquake.
They are placed at over 100 locations throughout
the world and provided geologists with data
about the Earth’s composition, by understanding
the characteristics of the low frequency waves
generated by an earthquake.
Richter Scale
The Richter Scale is used worldwide to describe
earthquake magnitude. Using Richter’s scale, the
magnitude is determined by measuring the
amplitude of the largest wave recorded on the
seismograph. Large-magnitude earthquakes will
cause the seismograph pen to be displaced farther
than small-magnitude earthquakes.
Earthquakes
Seismograph of an Earthquake
Ohio Earthquake