Download amazonbasingeologicaldevelopment

Stages in the
evolution / development of the
Natural Landscape
Stage One: “PRE-ANDEAN STAGE” – approx 200 million years ago
(Jurassic)
Water flows west into the
Pacific Ocean, depositing
sedimentary material.
Western Gondwanaland
– continental shield
“South America”
“Africa”
Pacific Ocean
Nazca Plate (Oceanic Plate)
American Plate (Continental Plate)
This was before the Andes Mountains were
formed by subduction, folding and volcanism.
There was no Amazon Basin at this stage.
Stage One
• “South America” and “Africa” were joined
together as part of the Gondwanaland “supercontinent”. There was no “Atlantic Ocean”.
How do we know this?
• Matching rocks and fossils in Eastern South
America and Western Africa;
• Matching coastline shapes;
• Magnetic “stripes” on Atlantic sea-floor rocks on
either side of the Mid-Atlantic Ridge
Water ran off the continent to the west,
depositing sediments into the Pacific Ocean
Bands of fossils in their original locations
Tectonic plates and their direction of movement
Nazca Plate
American Plate
“Glossopteris”
These fossils can now be seen in new locations
Sea-floor spreading from a Mid-Ocean Ridge
(eg the Mid-Atlantic Ridge)
New crust is constantly being created,
pushing the older rocks and land masses
further away. The ocean becomes wider.
Mountain building
Subduction Zone
Sea-floor spreading from a Mid-Ocean Ridge (eg Mid-Atlantic Ridge)
This diagram shows the relationship between the Mid-Ocean Ridge and a Subduction
Zone to the east. New crust is created at the M.O.R. then “swallowed up” at the
subduction zone. On the western side of the ocean a continent is being pushed away.
This reconstruction shows the breakup of the Gondwana and Laurasia
super-continents, as the continents move to their present positions
Laurasia
Gondwana
Stage Two: “ANDES AND AMAZON LAKE STAGE” – approx 180 million years ago
(Jurassic)
Deformation of the crust
forms Fold Mountains
Volcanic activity creates
mountains (Andesitic)
Pacific Ocean
American Plate (Continental)
Nazca Plate (Oceanic)
This plate is being
subducted beneath
the American Plate.
This plate is being pushed westwards,
away from the Mid – Atlantic Ridge. It is
rising above the Nazca Plate.
Subduction Zone
This stage occurred due to the breakup of
Gondwanaland.
Stage Two
• The Atlantic Ocean began to open due to the
process of sea-floor spreading.
• This caused the Gondwanaland “supercontinent” to break up. South America was
pushed away from Africa towards the Pacific
Ocean
• The Nazca Plate was subducted beneath
the American Plate.
• The Andes mountain range emerged due to
folding and volcanism (andesite rock)
Stage Two: “ANDES AND AMAZON LAKE STAGE” – approx 180 million years ago
(continued)
(Jurassic)
“Lake Amazon”:
As water ran off from the growing Andes Mts
it ponded to the west of the Shield
Pacific Ocean
Nazca Plate (Oceanic)
American Plate (Continental)
The result was a build-up of thick
sedimentary layers in the lake bed.
As water flowed eastwards from the Andes it
ponded to the west of the Shield. This led to the
formation of the Amazon “Lake”, trapped
between the Andes and the Shield.
Stage Three: “AMAZON RIVER STAGE” – approx 24 million years ago
Rapid erosion of the Andes supplies
huge amounts of sediment.
(Oligocene)
Pacific Ocean
American Plate (Continental)
The Subduction Zone continues to operate
The Andes continue to grow
through folding and volcanism.
Vast amounts of sediment were deposited in
the lake bed.
Stage Three: “AMAZON RIVER STAGE” – approx 24 million years ago
(continued)
(Oligocene)
The Amazon lakewaters found an
escape point and flowed eastwards
to the Atlantic Ocean, draining the
lake and forming the river channel.
Pacific Ocean
American Plate (Continental)
These sediments have become the infertile
yellow latosols found on the “Terra Firme” of
the Amazon Basin.
Ibidos: the point where the
Amazon broke through a
weakness in the Shield to
flow eastwards. Today it is
the narrowest and deepest
part of the Amazon channel.