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