Download mid-ocean ridge

EARTH’S STRUCTURE
Drifting Continents
Piecing It All Together
German scientist Alfred Wegener noticed that the coastlines of some
continents seem to fit together like a jigsaw puzzle. Look at the world map
below and see if you can match the continents that seem to line up. What
might this mean?
Continental Drift
• German scientist Alfred Wegener
developed the idea that the continents were
once _joined together_ in a single landmass
and have since _drifted apart_.
• He called this single landmass Pangaea .
• His idea became known as Continental Drift
.
The Evidence
Wegener used three types of evidence to support
his idea:
Evidence from landforms included mountain ranges
on the continents of South America and Africa
that line up when maps of the continents are put
together. Also, coal fields in Europe and North
America match up.
Evidence from fossils
included Glossopteris, a
fernlike plant that lived 250
million year ago, and the
freshwater reptiles
Mesosaurus and Lystrosaurus.
Glossopteris fossils have
been found in Africa,
South America, Australia,
India, and Antarctica.
Mesosaurus
and
Lystrosaurus
were both
freshwater
reptiles.
These fossils
have been
found in
places that
are now
separated by
oceans.
Lystrosaurus
Evidence of climate
change included fossils of
tropical plants on the island
of Spitsbergen in the Artic
Ocean. Why is this
evidence that the
continents might have
moved?
The fossil
Spitsbergen
The live plant
Drifting Continents
Pangaea and
Continental Drift
Many types of evidence
suggest that Earth’s
landmasses were once
joined together.
Scientists Reject Wegener’s Theory
• At the time, most scientists rejected
Wegener’s hypothesis because he could not
identify the force that pushes or pulls the
continents.
• For nearly half a century, from the 1920s to
the 1960s, most scientists paid little attention
to the idea of continental drift. Then new
evidence surfaced…
Continental Drift Foldable
Grading Rubric
• Provide a complete description of Wegener’s theory (30 pts)
• Provide a complete description of the 3 types of he evidence used to support
his theory (30 pts)
• Neat and legible (10 pts)
• Colorful illustrations (10 pts)
• Turned in today (20 pts)
Mid-Ocean Ridge
Scientists using sonar to map the ocean floor
in the 1940s discovered long chains of
mountains that rise up from the ocean floor.
These are called mid-ocean ridges , and they
extend into all of Earth’s oceans.
Sea-Floor Spreading
Ocean Floors
Mid-ocean ridges rise
from the sea floor like
stitches on the seams of
a baseball.
At the mid-ocean ridge, molten material from inside
Earth rises, erupts, cools, and hardens to form a solid
strip of rock. This process is called sea-floor spreading
and it adds more crust to the ocean floor. At the same
time, older strips of rock move outward from either side
of the ridge.
mid-ocean ridge
Mid-ocean ridge erupting
Pillow lava
Sea-Floor Spreading
In the central valley of mid-ocean ridges, geologists
discovered pillow lava. These rocks form only when
molten material hardens quickly after erupting
under water.
The pattern of magnetic stripes on one side of a mid-ocean ridge
is usually a mirror image of the pattern on the other side of the
ridge.
Scientists tested the age of rock samples taken from
the ocean floor. The youngest rocks were always
found at the center of the ridges. The farther away
the rock sample was taken, the older the rock was.
Sea-Floor Spreading
Some mid-ocean ridges have a valley that runs
along their center. Evidence shows that molten
material erupts through this valley and then
hardens to form the ocean floor.
In a process taking tens of millions of years, the ocean
floor moves away from the mid-ocean ridge. As it
moves, it cools and becomes more dense .
Eventually, the crust might collide with the edge of a
continent where gravity pulls the older, denser crust
down into a deep underwater canyon called a deep
ocean trench and back into the mantle . This
process is called subduction .
Sea-Floor Spreading
Deep-Ocean
Trenches
The deepest part of
the ocean is along the
Mariana Trench.
Several trenches in the
Pacific Ocean are
shown in yellow.
Sea-Floor Spreading
Subduction
Oceanic crust created
along a mid-ocean ridge is
destroyed at a deep-ocean trench.
During the process of subduction, oceanic crust sinks down beneath
the trench into the mantle. Sea-floor spreading and subduction work
together. These processes support the idea of continental drift.
Earth’s Interior
The three main layers of Earth
are the crust , the mantle ,
and the core . The crust and
the upper mantle are grouped
together into a single layer
called the lithosphere . The
lithosphere is broken into
large pieces called plates
that are separated by cracks.
Below the lithosphere, the
material is hotter and under
increasing pressure. As a
result, the part of the mantle
just beneath the lithosphere is
less rigid than the rock above,
although it is still solid. This
softer layer is called the
asthenosphere .
Earth’s
lithosphere, its
solid outer
shell, is like
an eggshell
broken into
pieces
separated by
cracks.
These pieces
are called
plates. Earths
plates meet at
boundaries.
Convection and the Mantle
Heat from the Earth’s core and the mantle itself causes convection in the mantle.
Convection occurs when the heat from Earth’s core causes the mantle material to
become hotter. The hotter rock is less dense and rises toward the crust. As it
moves further from the core, it cools off, becomes more dense , and sinks back
through the mantle. Over and over, the cycle of rising and sinking takes place, and is
known as convection currents .
Convection and the Mantle
Earth’s plates move because they are the top part of
the large convection currents in Earth’s mantle.
The Theory of Plate Tectonics
Plates move apart from
each other, or diverge,
at a divergent
boundary. Most
divergent boundaries
occur in the oceans at
the mid-ocean ridge .
Where plates diverge on
land, a deep valley
called a rift valley
forms.
Divergent
Boundaries
The Theory of Plate Tectonics
Plates come together, or converge, at
a convergent boundary. What
happens when two plates collide
depends on the density of the plates.
Volcanoes often form where two
plates collide.
Convergent
Boundaries
The Theory of Plate Tectonics
Plates slip past each other, moving in opposite
directions, at a transform boundary. Earthquakes
often occur when the plates suddenly slip along the
boundary that they form.
Transform
Boundaries
The Theory of Plate Tectonics
The Theory of Plate Tectonics
• Scientists combined what they knew about sea-floor
spreading, Earth’s plates, and plate motions into a theory
called plate tectonics . The theory of plate tectonics
states that Earth’s plates are in slow, constant motion,
driven by convection currents in the mantle. Plate
tectonics explains the formation, movement, and
subduction of Earth’s plates.
• Scientists use satellites to measure plate motion precisely.
The plates move very slowly – from about 1 to 12
centimeters per year. Over millions of years the
movement of Earth’s plates has greatly changed the
location of the continents and the size and shape of the
oceans .
The Theory of Plate Tectonics
Earth's Changing Crust
As plates move, they produce mountains,
volcanoes, and valleys as well as midocean ridges and deep-ocean trenches.
Use the terms from the list to label the
diagram.
Monitoring Earthquakes
Earthquakes
Around the
World
Earthquakes are
closely linked to
plate tectonics.
The map shows
where past
earthquakes have
occurred in
relation to plate
boundaries.
Monitoring Earthquakes
Earthquake Risk
The map shows
areas where
serious
earthquakes are
likely to occur,
based on the
location of past
earthquakes across
the United States.
Volcanoes and Plate Tectonics
The Ring of Fire
The Ring of Fire is a
belt of volcanoes that
circles the Pacific
Ocean. As with most
of Earth’s volcanoes,
these volcanoes form
along boundaries
of tectonic plates.