Transcript The entire earth is still changing, due to the slow convection of soft

Plate Tectonics
Chapter 17
Great Idea:
The entire earth is still changing, due to the slow
convection of soft, hot rocks deep within the planet.
Chapter Outline
• The Dynamic Earth
• Plate Tectonics: A Unifying View of Earth
• Another Look at Volcanoes and
Earthquakes
The Dynamic Earth
• Small-scale changes
– Construction site
• Erosion by rain
• Large-scale changes
– Volcanoes
– Earthquakes
– Erosion
The Case of the
Disappearing Mountains
• Erosion
– Few hundred million years
– Mountains continually forming
• Earth’s surface is not static
Map of continents
A map of the world’s continents reveals the similar
shapes of coastlines on the two sides of the Atlantic
Ocean.
The Movement of the Continents
• F. Bacon
– Continents like a puzzle
• Wegener
– Continental Drift
• Continents in
motion
• Current Evidence
– Ocean Floors
– Magnetic Reversals
– Rock Ages
The German
scientist A.
Wegener (1880 1930) postulated
that a
supercontinent
once existed —
and later broke
apart — called
Pangea.
Plate Reconstructions
Once the motion of
continents was
determined, scientists
could predict how the
Earth’s surface might
have looked in the past.
More than 200 million
years ago, the presentday continents were
joined together as the
ancient continent
Pangaea.
Ocean Floors
• Mapping
– Ocean floor dynamic
• Canyons, mountains
• Mid-Atlantic Ridge
– Earthquakes,
volcanoes,
lava flows
Magnetic Reversals
• Earth’s magnetic field
– Changes periodically
• Magnetite
– Crystals in lava align to magnetic field
• Paleomagnetism
• Seafloor Spreading
– New rock comes to surface
Measures of tectonic rates: magnetic:
Spreading Ridge magmatism (Iceland)
Magnetic stripes that
parallel ocean ridges
must form as new
magma wells up from
the fissure and pushes
out to the sides.
In this cross-sectional
view, older rocks lie
farther from the ridge.
(The “lithosphere”
includes the uppermost
mantle and all of the
crust.)
Magnetism of the sea floor
Measurements in the
late 1950s and early
1960s revealed
magnetic stripes
running nearly parallel
to the Vancouver
province and
Washington state
coastlines.
Rock Ages
• Radioactive Isotopes
– Rocks near Mid-Atlantic Ridge younger
– Rocks farther away older
New Support for the Theory
• Measuring motion of continents
• Radio Astronomy
– Measured arrival of radio waves
– Repeated over several years
• North America and Europe
– Separating at 5 cm per year
Plate Tectonics: A Unifying
View of Earth
Plate Tectonics
• Plate Tectonics
– Large-scale surface features
– Related phenomena
• Tectonic plates
– Rigid, moving sheet of rock
– Crust and upper mantle
– Continental
• 100 km thick
• Lower density (granite)
– Oceanic
• 8-10 km thick
• Dense rock (basalt)
• Earth’s surface
– ¼ continent, ¾ water
Plate motions
The major plates of the Earth with their directions
of motion shown by arrows.
The Convecting Mantle
• Mantle convection
– Motion driven by Earth’s interior heat energy
• Sources of energy
– Gravitational potential energy
– Decay of radioactive elements
• Movement
– Heat moves to cooler regions
– Convection cells in mantle
– Very slow
• 200 million years for one cycle
Seafloor Spreading:
In the 1960’s,
Harry Hess, Robert
Deitz and other
geologists and
oceanographers, put
forward the
hypothesis of
‘seafloor spreading.’
The key data for this
hypothesis were the
topographic maps of
the ocean floors….
The ‘Ring of Fire’
Earth’s lithosphere is a mosaic of 13 major plates:
Plate boundaries may be convergent, divergent, and transform….
Earth’s
lithosphere is a
mosaic of 13
major plates:
Plate boundaries may be convergent, divergent, and transform….
Divergent, oceanic boundaries:
Mid-Atlantic
Ridge, Iceland
Divergent, continental boundaries:
The ‘Afar triangle’
A satellite photograph
of a portion of Africa’s
Great Rift Valley. The
narrow body of water
defines a divergent plate
boundary where new
plate material is being
created and plates are
moving out to either
side.
Gulfs formed between rifted
continental fragments:
Convergent Oceanic boundary:
Convergent Ocean-Continent boundary:
Continent-Continent collision boundary:
Transform Plate Boundary
• Two plates move past each other
– NOT smooth
– Earthquakes as a result of movement
Transform, or ‘Strike Slip’ boundaries:
The layers of Earth
The principal layers,
which differ in chemical
composition and physical
properties, are the core,
the mantle, the crust, and
the atmosphere (not
shown). When looked at
in detail, each of these
layers is itself composed
of smaller layers.
Volcanoes and EarthquakesEvidence of Earth’s Inner Forces
• Volcano
– Magma breaks
through surface
• Earthquake
– Rocks breaks along
fault
– Energy transmitted as
wave
– Richter scale
Volcanic settings
A cross-section of a volcano
reveals a magma chamber,
which stores molten rock,
and a system of pipes,
cracks, and vents that lead to
the surface. The terms in the
orange area refer to the kinds
of rock formations resulting
from cooled magma.
Xenoliths are the original
rocks encased in this cooled
magma.
Divergent Plate Boundaries
A divergent plate
boundary defines a
line along which new
plate material is
formed from volcanic
rock.
Subduction magmatism
Hawaiian example of plate motion
Kauai, currently the oldest, is between 3 and 5.5 million
years old, while Hawaii, the youngest, is less than 0.8
million years old.
Another Look at Volcanoes
and Earthquakes
• Plates and Volcanism
– Divergent Plate
Boundaries
– Convergent Plate
Boundaries
• Subduction zones
– Hotspots
• Source stationary, plates
move
• Chain of volcanoes
• Earthquakes
– At plate boundaries or
elsewhere
Seismology: Exploring Earth’s
Interior with Earthquakes
• Seismology
– Study of sound vibrations within earth
– Used to determine earth’s inner structure
• Seismic waves
– Compressional or longitudinal
– Transverse or shear waves
Seismology: Exploring Earth’s
Interior with Earthquakes
A Clicker Question.
How do we know that the Earth’s
tectonic plates move (continental drift)?
a)
Observation of the Earth’s rivers
b)
Direct measurements, observation of the rifts in the ocean
floors, magnetic reversals
c)
Calculations by Einstein
d)
Erosion of mountains
The Geological History of
North America
• Northeastern Canada and Greenland
– Several billion years old
• Western US
– Terranes
• Added to continent over time
• Appalachian Mountains
– Formed 450-300 million years ago
– Continental-continental convergence zone
• Rocky Mountains
– 60 million years ago
– Warping, folding and fracturing of continent
• The Colorado Plateau
– Gentle uplift
• The Sierra Nevada
– Molten rock pushed up sediments