Transcript sea-floor spreading

The Theory of Plate Tectonics and
Continental Drift
• Write down the underlined items for your
notes. Feel free to put the material in your
own words.
Tectonic Plates and
Continental Drift
Tectonic plates are pieces of the lithosphere
that move around on the asthenosphere.
Continental drift is the hypothesis which states
the continents once formed a single landmass
called Pangaea and have drifted into their
current positions when Pangaea broke apart.
Tectonic plates: Pieces of lithosphere that move around on
top of the asthenosphere.
Take a look. Are the plates just continental or oceanic or are they a
combination of both?
A Giant Jigsaw Puzzle
• All of the tectonic plates fit together, like a
puzzle, but are not the same size.
• Along the plates’ edges (boundaries) movement
occurs.
• Plates move away from each other, toward each
other, and slide past each other.
• Earthquakes, volcanoes, & mountain ranges
occur along these boundaries.
Continental Drift
• Alfred Wegener; 19121915
• 1 land mass in world
– called Pangaea
– Drifted apart over 250
million years
Wegener’s Theory
• Wegener’s theory explained and was
supported by several things:
– Puzzle Theory: the continents look like they fit
together like a puzzle & they do.
– Fossil Pattern: Fossils of the same plant &
animal species are found on continents
oceans apart.
– Rocks: Similar types of rocks are found on
several different continents.
Oceanic Crust and Sea-Floor Spreading
•
Problem for Wegener:
– How could items as big
as continents move?
He couldn’t prove how
the continents had
separated.
– Solution: sea-floor
spreading
• The adding of
ocean crust as
magma bubbles up
through a midocean ridge and
hardens on the
ocean floor.
• New crust is closer
to the ridge.
• Old crust farther
away, indicating
plates are moving
apart.
Further evidence for sea-floor spreading: magnetic reversal (polarity changes).
Rocks record the magnetism of the earth. This changes periodically, so the
changes are seen in the rock record.
The pattern of "stripes" or anomalies is symmetrical around the oceanic ridge.
The youngest oceanic rocks are near the ridges with the oceanic rocks
becoming older as they move away from the ridge. The black stripes
represent rocks that cooled under "normal" conditions and are normally polarized,
while the white stripes represent rocks that cooled under "reversed" conditions
and are reversely polarized.
Mid-Ocean Ridges & Sea Floor Spreading
• A chain of
submerged
mountain ranges
that runs through
the world’s oceans
= mid-ocean
ridge.
.
• Most common one
= Mid-Atlantic
Ridge.
Sea- Floor Spreading
Sea-floor spreading is the
process by which new
oceanic lithosphere (ocean
crust) forms as magma rises
to the surface & hardens.
As tectonic plates move
away from one another, the
sea floor spreads apart and
magma fills in the gaps,
creating new ocean floor—
creating new crust, new
Earth.
Sea-Floor Spreading
•Sea-floor spreading takes place along the mid-ocean
ridges all around the world.
•Mid-ocean ridges are also the site of other geological events:
•Mountain building:
•Where plates collide, great mountain ranges may be pushed up, such as the
Himalayas or Andes.
•If one plate sinks below another (subduction zone), deep oceanic trenches and
chains of volcanoes are formed.
•Earthquakes are by far most common along plate boundaries and rift zones, and
they can trigger tsunamis.