AGE DATING THE EARTH

1
AGE DATING THE EARTH

• Geologic Techniques and
• The Geologic Time Scale

2
INTRODUCTION

• Speculations about the nature of the Earth as
  well as the Age of the Earth inspired much of the
  lore and legend of early civilizations.
• In the 3rd century B.C., Eratosthenes depicted a
  spherical Earth and even calculated its diameter
  and circumference
• The concept of a spherical Earth was beyond the
  imagination of most men at that time.
• Only 500 years ago, sailors aboard the Santa
  Maria begged Columbus to turn back lest they sail
  off the Earth's "edge."
• Herodotus, ancient historian, made one of the
  earliest recorded geological observations in the
  5th century B.C
• Fossil shells found inland in Egypt and Libya
• He suggested Mediterranean Sea had once extended
  farther south
• Few believed him, however, and this idea did not
  catch on.
• Similar opinions and prejudices about the nature
  and age of the Earth have waxed and waned through
  the centuries
• Even today - traditional beliefs among certain
  religious groups suggest the Earth is quite
  young--that its age might be measured in terms of
  thousands of years, but certainly not in
  millions.

3
AGE OF THE EARTH

• Scientists have established the age of the Earth
  as 4.54 billion years old.

4
HOW DO GEOLOGISTS KNOW THE AGE OF THE EARTH?

• The evidence to age-date the Earth is concealed
  in the rocks that form the Earth's crust and
  surface.
• The rocks are not all the same age -- or even
  nearly so -- but, like the pages in a long and
  complicated history book, rocks record the
  Earth-shaping events and life of the past.

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WHAT DO GEOLOGISTS USE?

• Two time-measuring systems are used to date past
  Earth-shaping events and to measure the age of
  the Earth
• (1) RELATIVE DATING A relative time measuring
  system
• based on the sequence of layering of the rocks
  and the evolution of life as recorded in the
  rocks
• (2) ABSOLUTE DATING A radiometric time
  measuring system
• based on the natural radioactivity of chemical
  elements in some of the rocks

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RELATIVE DATING NICOLA STENOS LAWS and
STRATIGRAPHY (BEDROCK LAYERS)

• Principle of Original Horizontality
• - Bedrock layers formed from sedimentary
  material are deposited in a horizontal position
  under gravity
• - Any deviations from this position are due to
  outside forces disturbing the rocks later.
• Law of Superposition
• Wherever uncontorted (no faults, no folds) layers
  are exposed at the surface, the bottom layer was
  deposited first and is the oldest layer exposed
• Each succeeding layer, up to the topmost one, is
  progressively younger.
• In any rock layer, each layer represents a
  specific interval of geologic time.

Nicola Steno 1638 - 1686
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THE BEGINNING JAMES HUTTON FATHER OF MODERN
GEOLOGY

• In the late 18th century, James Hutton, a
  Scottish geologist, proposed a fundamental
  principle in Geology
• Uniformitarianism
• A theory that natural agents (wind, rain, etc.)
  at work on and within the Earth have operated
  with general uniformity through immensely long
  periods of time
• Uniformitarianism went directly against
  Catastrophism
• Prevailing thought that all Earths features were
  formed by catastrophic events

Picture at right may have been Huttons first
clue as to age of material with later disruption,
assuming that materials on bottom were originally
deposited horizontally and then uplifted much
later
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Charles Lyell Principles of Geology

• Charles Lyell, in the 1830s, followed up on James
  Huttons work.
• Lyell viewed the history of the Earth as vast and
  directionless.
• Lyell worked from the theory of
  Uniformitarianism not Catastrophism
• Lyell found evidence that valleys were formed
  through the slow process of erosion, not by
  catastrophic floods.
• Changes to the Earths surface were gradual and
  over time, great changes could be effected the
  Earth was vastly old.
• The present is the key to the past
• Darwin used Lyells Principles of Geology to
  decipher the volcanic rocks on the Canary Islands

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WILLIAM STRATA SMITH Fossil Record and
Relative Time Scale

• William "Strata" Smith, a civil engineer and
  surveyor in the early 19th century, collected and
  cataloged fossil shells from rocks
• Discovered that certain layers contained fossils
  unlike those in other layers Index Fossils
• Index Fossils existed in limited periods of
  geologic time, but were widespread geographically
• They can be used as guides to age of rocks
• Age-dating the fossils also provides an age-date
  for the rock layer in which they were found.

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PUTTING IT ALL TOGETHER

• DEVELOPING A RELATIVE TIME SCALE
• Studying origin of rocks (petrology), combined
  with studies of rock layers (stratigraphy) and
  studies of the evolution of life (paleontology),
  allow geologists to reconstruct the Earth using
  four basic principles.
• Original Horizontality
• Superposition
• Lateral continuity
• Cross-cutting relationships

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GEOLOGIC TIME AND DATING 4 BASIC PRINCIPLES OF
RELATIVE DATING
(1) Principle of Original Horizontality Beds of
deposited sediment form as horizontal or nearly
horizontal layers. (2) Principle of
Superposition Within a sequence of undisturbed
sedimentary or volcanic rocks, the layers become
younger going from bottom to top. (3) Lateral
Continuity An original sedimentary layer extends
laterally until it tapers or thins at its
edges (4) Cross-cutting Relationships A
disrupted pattern is older than the cause of the
disruption.
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PRINCIPLE OF ORIGINAL HORIZONTALITY and
SUPERPOSITION
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Principles of Dating
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RELATIVE DATING - How it works

• Correlation by Physical Continuity
• Physically tracing the course of a rock unit to
  correlate rocks between two different places
• Correlation by Similarity of Rock Types
• Correlation of two regions by assumption that
  similar rock types in two regions formed at same
  time, under same circumstances
• Correlation by Fossils
• Plants and animals that lived at the time rock
  formed were buried by sediment
• If there are fossil remains preserved in the
  layers of sedimentary rock fossils nearer the
  bottom (in older rock) are more unlike those
  near the top (in younger rock)
• Observations formalized into Principle of Faunal
  Succession fossil species succeed one another
  in a definite and recognizable order.
• Index Fossil a fossil from a short-lived,
  geographically widespread species known to exist
  during a specific period of geologic time.

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CORRELATION OF ROCK UNITS

• Each column represents the sequence of
  sedimentary beds at a specific locality
• The same beds are bracketed within the lines
  connecting the three columns.
• Adjoining beds that possess similar or related
  features (including fossils) are grouped into a
  single, more conspicuous unit called a formation

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FAUNAL SUCCESSION
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Faunal Succession
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INDEX FOSSIL CHART
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Relative Dating Reading the Sedimentary Layers

• When reading the sedimentary layers, geologists
  not only look at the layers and the fossils
  within them (whats there) - geologists also
  look for an Unconformity (whats missing)
• An Unconformity is a surface representing a gap
  in the geologic record
• Types of Unconformities
• Disconformity parallel strata missing a layer
• Angular Unconformity younger horizontal layer
  overlying a folded or tilted layer
• Nonconformity a plutonic/metamorphic rock layer
  covered by younger sedimentary or volcanic rock

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Formation of An Unconformity
Disconformity
Discnformity
Angular unconformity
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Angular Unconformity
Examples of angular unconformities
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DISCONFORMITY
Disconformity, Death Valley, California.
Disconformities are unconformities in which the
younger material is roughly parallel to the
contact. This photograph shows the rocks being
parallel on the left side. However, on the right
it shows the gravel cutting down intothe marble
to indicate erosion. Photo is approximately 1
meter across.
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DISCONFORMITY

• The upper 2/3 of the cliff is Redwall Limestone,
  whereas the lower part is Cambrian carbonate rock
  
• The age difference between these units is roughly
  150 Ma.
• The contact between the two rock units represents
  a significant span of geologic time and is termed
  a disconformity

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NON CONFORMITY
Stratified rocks upon unstratified rocks
(sedimentary rocks overlying metamorphic or
plutonic rocks).
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ABSOLUTE DATING RADIOMETRIC

• Radiometric dating Absolute Dating - based on
  radioactive decay of isotopes
• An isotope is a form of an element containing
  different atomic mass (Carbon-12 vs Carbon-14,
  for example)
• Same number of protons, but different number of
  neutrons in nucleus
• Most isotopes are radio-active and unstable
• Radioactive decay
• any number of processes by which unstable
  isotopes emit radioactive particles and
  eventually become stable elements
• Radioactive decay rate can be quantified because
  it occurs at a constant rate for each known
  isotope and is measured in half-life
• Half-life is the time required for a quantity
  of radio-active material to decay to half of its
  initial value
• Unstable radioactive (Parent) isotope ? stable,
  non-radioactive
• (daughter ) isotope
• The half-lives of isotopes have all been measured
  directly
• Using a radiation detector to count the number of
  atoms decaying in a given amount of time from a
  known amount of the parent material
• Measuring the ratio of parent-to-daughter atoms
  in a sample that originally consisted completely
  of parent atoms
• Measuring ratio of parent-to-daughter isotopes
  determines absolute ages of some rocks.

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RADIOMETRIC DATING

• The decay of Radio-active atoms compares to sand
  grains falling in an hourglass.
• You cannot predict when the individual sand grain
  will fall, but you can predict from one time to
  the next how long the whole pile of sand takes to
  fall to the bottom.
• Similarly, you can predict how long it takes for
  all the radio-active atoms in a given amount of
  rock to decay to a non-radioactive form.

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RADIOMETRIC DATING
In exponential decay the amount of Parent
material decreases by half during each half-life
rapidly at first, then slowly with each
succeeding half-life.
The daughter  element or isotope amount increases
rapidly at first and more slowly with
each succeeding half life
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ABSOLUTE DATING ISOTOPES

• URANIUMLEAD (U238?Pb206)
• Half-life 4.5 billion years
• Dating range 10 million 4.6 billion years
• URANIUMLEAD (U235 ?Pb207)
• Half-life 713 million years
• Dating Range 10 million 4.6 billion years
• POTASSIUM-ARGON (K40?Ar40)
• Half-life 1.3 billion years
• Dating Range 100,000 4.6 billion years
• CARBON-14 (C14?N14)
• Half-life 5730 years
• Dating Range 100 40,000 years

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Absolute Dating Half-life
Uranium half-life
Radio-carbon half-life
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Radio Carbon Carbon 14

• All living organisms absorb radiocarbon (C14), an
  unstable form of carbon.
• After death and fossilization, C14 continues to
  decay without being replaced (half-life of about
  5,730 years).
• Radiation counters are used to detect the
  electrons given off by decaying C14 as it turns
  into nitrogen (N14).
• Remaining amount of C14 is compared to the
  remaining amount of C12, the stable form of
  carbon, to determine how much radiocarbon has
  decayed to date the fossil.

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Radiocarbon Dating
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Relative and Radiometric Dating
Using Relative and Radiometric Dating together
gives the most accurate time-scale for geologic
time
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Absolute Dating non-Radiometric -
Dendrochronology

• Annual growth of tree rings
• Dating back 11,500 years Holocene Epoch

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Principles of Dendrochronology

• The dating of past events (climatic changes)
  through study of tree ring growth
• A chronology (arrangement of events in time) can
  be made by comparing different samples

35
Cross-dating in Dendrochronology

• Process of matching rings of trees in an area
  based on patterns of ring widths produced by
  regional climate.
• More accurate age than ring counting

36
Cross-dating techniques
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Absolute Dating -Varve Chronology

• Parallel strata deposited in deep oceans or lakes
• Varves are a pair of sedimentary layers deposited
  on seasonal cycles
• Winter/summer
• Date back to over 200 million years

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Varves
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Geologic Time Scale

• Fossils in rock used to age date rocks
• Time scale consists of periods of time broken
  into smaller and smaller units eons (100s of
  millions of years), eras, periods, epochs
  (millions to thousands of years)
• Eons, eras, periods and epochs are listed with
  oldest at the bottom of the scale and youngest at
  the top
• Names of eras, periods and epochs based on global
  location
• PreCambrian from rocks near Wales Cambria
• Jurassic from limestone found in Jura Mountains,
  France

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THREE MAJOR ERAS IN GEOLOGIC TIME SCALE

• (1) Paleozoic Era appearance of complex life
• Approximately 600 million years ago to 250
  million years ago)
• (2) Mesozoic Era Age of Dinosaurs
• Approximately 250 million years ago to 65 million
  years ago)
• (3) Cenozoic Era Age of Mammals
• Approximately 65 million years ago to present

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TERTIARY
Red Arrows point to mass extinction dates
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The Great Permian Extinction

• At the end of many large time units on the
  Geologic Time Scale, mass extinctions took place.
• Index Fossils used for dating, remember
• The end of the Permian, approximately 250 million
  years ago (also the end of the Paleozoic era),
  was marked by the greatest extinction of the
  Phanerozoic eon.
• During the Permian extinction event, whose
  cause(s) remain controversial, over 95 of marine
  species became extinct, while 70 of terrestrial
  taxonomic families suffered the same fate of
  extinction!

43
PERMIAN EXTINCTION CAUSES?

• (1) Climate change, possibly caused by glaciation
  and/or volcanic activity, has been associated
  with many mass extinctions. It seems likely that
  climate change is a consequence of the cause of
  extinction rather than the root cause itself.
• The Siberian Traps triggered a massive, sudden
  glaciation as well as other environmental
  consequences of volcanic eruptions.
• The opening of the Atlantic Ocean basin as the
  result of sustained volcanic eruptions (the
  Central Atlantic Magmatic Province) led to the
  release of toxic fumes, greenhouse gases, and
  ultimately, global climate change perhaps
  triggering an ice age
• Formation of Pangaea has been invoked as a cause
  for the extinction.
• Pangaea's presence may have led to extreme
  environments with hotter interior areas of the
  continent and colder polar areas, possibly
  producing glaciation.
• (2) Poisoning of the ocean has been suggested due
  to an apparent drop in carbon isotope data
  obtained from marine sediments formed at the time
  of the extinction.
• The cause of this apparent drop off in the
  photosynthetic rate in the seas has not yet been
  determined
• (3) Extraterrestrial Objects
• Evidence of a large impact at the close of the
  Permian is not strongly supported, although some
  indirect evidence suggests an impact did occur
  during the Permian, although possibly not at the
  time of the extinction crisis.

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CRETACEOUS-TERTIARY EXTINCTION

• Impact Theory
• 1980 L.W. Alvarez and colleagues published a
  paper proposing that, approximately 65 million
  years ago, the earth was struck by an
  asteroid-sized object on Yucatan Peninsula
  Chicxulub, Mexico
• Evidence
• Boundary Clay with high levels of iridium
• A very rare mineral in terrestrial rocks
• More common in extraterrestrial rock samples
• Microtektites hollow, microscopic, glass-like
  spheres that form when a violent explosive event
  occurs in association with molten rock
• "Shocked" quartz grains, where the regular,
  crystalline structure has been distorted by the
  application of large forces

The disappearance of dinosaurs from the fossil
record 65 million years ago
ma
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Shocked Quartz

• To visualize this type of deformation, imagine a
  perfectly vertically stacked deck of playing
  cards.
• Now slant the stack by pushing the upper part of
  the deck a little to the side. This is a rough
  analogy of what happens when quartz goes through
  a lattice offset.
• As the compression wave from the blast passes
  through the sand grains, planes of atoms in the
  quartz get "shifted" slightly to the side
  relative to adjacent planes of atoms.
• These latice offsets create zones of optical
  interference in the sand grain which, under a
  microscope, show up as two or more groups of dark
  lines that intersect each other

46
Microtektites
How do they form? 1) A comet or asteroid impacts
the Earth, probably at an oblique angle. 2)
Terrestrial (Earth) rock is melted and ejected
into the upper atmosphere at hyper-velocities. 3)
Tektites rain down.
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Iridium Boundary Layer Clay

• The asteroid hit a geologically unique,
  sulfur-rich region of the Yucatan Peninsula and
  kicked up billions of tons of sulfur and other
  materials into the atmosphere.
• Darkness prevailed for about half a year after
  the collision.
• This caused global temperatures to plunge near
  freezing

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Chicxulub Crater, Yucatan Peninsula
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ALTERNATE THEORY FOR CRETACEOUS-TERTIARY
EXTINCTION

• VOLCANISM
• CAMP Central Atlantic Magmatic Province
• Massive basaltic eruptions that broke up the
  super-continent Pangaea and opened up the
  Atlantic Ocean Basin.
• The eruption of the Deccan Traps approximately
  65-64 million years ago is the largest volcanic
  event since the Permian-Triassic event at 245 Ma
• The impact at Chicxulub, Mexico predates Dinosaur
  extinction by 300,000 years.
• Selective extinction only dinosaurs

50
Central Atlantic Magmatic Province

• Volcanic events flooded the center of the former
  supercontinent of Pangaea with molten rock
• The areawhich today stretches around the
  Atlantic, across parts of Canada, the eastern US,
  Europe, South America and Africais referred to
  as the CAMP, or Central Atlantic Magmatic
  Province.

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CENTRAL ATLANTIC MAGMATIC PROVINCE
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Deccan Volcanic Province in India

• The Deccan volcanic province in India today
  covers an area the size of France or Texas.
• The original size is estimated twice this size,
  but was reduced by erosion.
• Arrows show the direction of the largest lava
  flows 1500 km across India and into the Gulf of
  Bengal.

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EARTH STRUCTURES

• The Earth is composed of four major layers
• Inner Core
• Outer Core
• Mantle
• Crust

54
EVIDENCE OF EARTHS LAYERS

• DIRECT EVIDENCE OF LAYERING
• Mantle rock brought up to surface through
  volcanism
• Intrusion and erosion of diamond-bearing
  kimberlite pipes
• Lower layers of oceanic lithosphere brought to
  surface at subduction zones
• Ophiolite Suite a sequence of rocks that appears
  to represent a section through oceanic crust
• INDIRECT EVIDENCE OF LAYERING
• Seismic reflection return of energy from seismic
  waves bouncing off rock boundaries.
• Similar to light off a mirror, rock boundaries of
  differing densities set up a reflection of
  seismic waves
• Seismic refraction bending of seismic waves as
  they pass through rock layers of differing
  densities
• Seismic waves change speed or direction passing
  through different rock boundaries

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DIRECT EVIDENCEVOLCANOES
Magma ejected by a volcano may have its origin in
the Mantle layer of the Earth Mantle minerals
include pyroxenes, amphiboles, biotites and
plagioclase
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KIMBERLITE PIPE DIAGRAM
The complex volcanic magmas that solidify into
kimberlite and lamproite are not the source of
diamonds, only the elevators that bring them
with other minerals and mantle rocks to Earth's
surface
57
DIRECT EVIDENCE OPHIOLITE SUITE (SEA FLOOR)
The sequence of layers of rock found on all
ocean floors. An idealized ophiolite sequence
shows an upper layer consisting of deep sea
sediments (limestones, cherts, and shales),
overlying a layer of pillow basalts.  Pillow
basalts overly the sheeted dikes of basalt
material. Beneath the sheeted dike complex are
gabbros that likely represent the magma chambers
for the basalts.  The marine sediments are
typically from animal and terrestrial material
settling at the bottom of the ocean The Pillow
lavas, dykes, gabbros and peridotite are typical
mantle materials
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INDIRECT EVIDENCE SEISMIC WAVES
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CORE INNER AND OUTER

• INNER AND OUTER CORE represent approximately
  31 of Earths mass
• The intense heat of the core is derived from
  decay of radio-active isotopes
• Inner core
• The inner core is under such extreme pressure
  that it remains solid
• Composed mostly of iron (Fe) and some Nickel (Ni)
• Temperatures over 7,0000 F (4,3000 C)
• 2200 Km across
• Outer core
• The outer core is under less pressure and is
  molten
• Composed mostly of iron (Fe), Sulphur (S), and
  Nickel (Ni)
• Temperatures of 6,700-7,7000F (3,700 4,3000C)
• 3000 Km across

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MANTLE AND CRUST

• MANTLE represents 68 of Earths mass -2900 km
  thick
• At over 1000 degrees C, the mantle is solid but
  can deform slowly in a plastic manner
• Composed of iron (Fe), magnesium (Mg), aluminum
  (Al), silicon (Si), and oxygen (O) and a number
  of other minerals
• LITHOSPHERE
• Upper mantle more rigid, bonded to Crust
• 100 km thick
• ASTHENOSPHERE
• Mantle below Lithosphere more plastic, weaker and
  more molten
• 100-200 km thick
• CRUST represents 1 of Earths mass
• The crust thinnest of the layers 5-70 km thick
• Continental crust Granitic
• Oceanic crust Basaltic

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EARTHS CRUST

• The Earths crust is composed of almost all of
  the basic elements.
• Listed below (in order of abundance) are the
  eight (8) basic elements that compose
  approximately 99 of the crust
• Oxygen (O) Silicon (Si) Aluminum (Al) Iron
  (Fe)
• Calcium (Ca) Potassium (K) Sodium (Na)
  Magnesium (Mg)
• Continental Crust is composed mainly of a
  granitic rock type
• High silica content (a combination of Oxygen and
  Silicon)
• Lower density (2.7 grams per cubic centimeter)
• Thicker (20-70 km thick)
• Underlies most continents
• Oceanic Crust is composed mainly of a basaltic
  rock type
• Low silica content
• Higher density (3.0 grams per cubic centimeter)
• Thinner (5-10 km thick)
• Underlies most ocean basins

62
TECTONIC PLATES

• Earths Lithosphere (crust and upper mantle) is
  broken into large moving slabs Tectonic (or
  Lithospheric) Plates
• Interaction between plates drives mountain
  building
• Volcanic mountains
• Folded mountains
• Faulted mountains