Early Observations
• Late 1500’s Abraham Ortelius observed the fit of continents on both sides of the Atlantic Ocean
Early Observations
• Eduard Suess suggested that the
Southern Continents were once joined together in a landmass known as
Continental Drift
• Alfred Wegener proposed the hypothesis of continental drift
• He claimed that all the land on the Earth was a supercontinent called Pangaea
Alfred Wegener (1903) suggested and presented evidence that the continents
were once a single
supercontinent, called Pangaea, which divided ~ 200 Million years ago into Laurasia and Gondwanaland and later into
Continental Drift
• Evidence:
– Similar rock types found on different sides of the Atlantic Ocean
– Similar fossils found all over the world – Coal beds found in Antarctica
Evidence
• Wegener gathered most of his evidence from fossils
• Remains of past organisms were found all over the world
Evidence
• Other fossils that Wegener studied were:
– Mesosaurus – an aquatic reptile – Glossopteris – a seed fern
Fossils of the land-bound lizard Lystrosaurus were found in Africa, Antarctica and India, suggesting that these continents were one
Ancient Climates
• Wegener also found evidence of past climates all over the world
Ancient Climates
• Glacial deposits were found in Africa, India, Australia and South America
• Glaciers do not exist on these continents
Geologic evidence for “continental drift”
Distribution of warm-water fossils
Continental Drift
• This hypothesis was initially rejected
• No evidence to explain what was making the continents move
Rejection
• Wegner first suggested that the rotation of the Earth caused this
– Physicists disproved this
• Wegner also said that the continents were plowing through ocean floor
Seafloor Spreading
Early Beliefs
• Up until the mid-1900’s everyone believed the ocean floor to be flat
• It was believed that:
– Oceanic crust didn’t change
– Ocean crust was much older than continental crust
Technology
• Two types of technology that helped to study the ocean floor
– Sonar
Sonar
• Sonar-Uses sound waves to measure water depth
Sonar
Magnetometer
• A magnetometer is a device that can detect small changes in magnetic fields • Magnetometers record the magnetic field
More evidence to support sea floor spreading comes from the study of
paleomagnetism, the magnetic properties of ancient rocks.
When a magnetometer (a
device that detects the magnetic properties of rocks) is towed across the ocean it indicates that there are alternating zones of rock with either normal or reversed polarity.
In the green zones bits of magnetic iron in the rocks are pointing to where north is today. They act like countless little magnets trapped in the rock when it solidified from magma.
Scientists discovered that the ocean floor contained:
Mountain chains Ocean ridges
Deep Sea Trenches
• Deep sea trenches are elongated
depressions in the seafloor and form as a result of these mountain chains
• Scientists as well made two very important discoveries:
– Ages of rocks near ocean ridges were
younger than rocks near deep sea trenches – Thickness of sediments increased with
Magnetism
• Paleomagnetism-The study of the magnetic record
Magnetism
• Studies of Basalt flows have shown a pattern of magnetic reversals
Magnetism
Isochron
• Scientists were able to construct an isochron map
• An isochron is a line on a map that
Seafloor Spreading
• All of this data was compiled and analyzed and an American scientist by the name of Harry Hess proposed the theory of
seafloor spreading
• Seafloor spreading states that new ocean crust is created at ocean ridges and
Seafloor Spreading
• Magma is forced towards the crust
• Cools once it comes in contact with the water and fills in the missing gaps
Continental Drift
• Seafloor spreading completes Wegener’s model of continental drift
• Wegener could not explain what caused landmasses to move or how they move
Theory of Plate Tectonics
• The theory of plate tectonics states that Earth’s crust and rigid upper mantle are broken into enormous slabs called plates • These plates interact with each other at
Types of Plate Boundaries
• Divergent-Places where two tectonic plates are moving apart
• Most are found near ocean ridges on the seafloor
• When they occur on the Earth they form Rift Valleys
Divergent Plate Boundaries
• Formation of new ocean crust occurs at Divergent Boundaries
Divergent Plate Boundaries
• May cause an ocean basin to grow wider • The Atlantic Ocean is spreading at about
Divergent Plate Boundaries
• Divergent Boundary-Area on the Earth where two of Earth’s tectonic plates are moving apart.
Divergent Boundary
• The place where two plates move apart. • Most occur at the
mid-ocean ridge and on land. • The boundary forms rift
valleys.
Rio Grande Rift
• Extends
Convergent Plate Boundaries
• Convergent Plate Boundary-Area on the Earth where two of Earth’s tectonic plates are moving toward each other
Types of Plate Boundaries
Convergent Plate Boundaries
• Oceanic-Oceanic-Causes subduction zone and a deep sea trench
• Oceanic-Continental-Causes a series of volcanoes along the continent
Oceanic-Oceanic Boundary
• The denser plate is subducted beneath the other
• This forms a deep sea trench
Oceanic-Oceanic Boundary
• Some of the magma is forced back to the surface and forms an arc of volcanoes
• Examples:
Oceanic-Continental
• Subduction occurs as well
• This forms a series of mountains along the edge of a continental plate
• Example:
Continental-Continental
• The ocean basin between the two continents is subducted
• The colliding edges of the continents are uplifted and form a mountain range
• Example:
– Himalayas
Transform Boundaries
• Place where two plates slide horizontally past each other
• Crust is not created or destroyed it is only deformed or fractured
Convection Current
• The heated material of the mantle rises because it decreases in density while the cooler part sinks
• This is called a convection current
