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3.5.1 Evidence of Continental
Drift and Plate Tectonics
Topic Physical - Hazardous Earth
1.1 Variety of Evidence for the Theories of
Continental Drift and Plate Tectonics
Theories of continental drift and plate tectonics including:
the basic structure of the Earth including the lithosphere,
asthenosphere and the role of convection currents
evidence for sea-floor spreading; paleomagnetism; the age of sea floor
rocks
evidence from ancient glaciations
fossil records.
The Basic Structure of the Earth
Layers of the Earth
Inner Core - Solid iron-nickel, 6000 degrees, extremely dense.
Outer Core - Liquid iron-nickel; generates Earth’s magnetic field.
Mantle - Solid but capable of slow flow; composed of silicate minerals.
Crust - Thin outer layer; oceanic (basaltic) or continental (granitic).
Lithosphere & Asthenosphere
Lithosphere - Rigid layer comprising the crust and uppermost mantle;
broken into tectonic plates.
Asthenosphere - Partially molten, ductile upper mantle; plates move over it.
3.5.1 Evidence of Continental Drift and Plate Tectonics 1
, Plates “float” on the asthenosphere, driven by convection, slab pull and
ridge push.
Role of Convection Currents
Heat from radioactive decay drives mantle convection.
Rising hot material spreads beneath plates → divergence.
Sinking cold slabs at subduction zones → slab pull (dominant force).
Elevated ridges push plates apart → ridge push.
Evidence for Sea-Floor Spreading
Ocean Floor Mapping
SONAR mapping (Tharp & Heezen) revealed:
Mid-Ocean Ridges
Deep ocean trenches
Thin sediment cover (young crust)
Paleomagnetism
As basalt cools, iron minerals align with Earth’s magnetic field.
Magnetic reversals create symmetrical stripes on either side of ridges.
Vine-Matthews hypothesis proved new crust forms at ridges and moves
outward.
Age of Sea Floor Rocks
Radiometric dating shows:
Youngest rocks at ridges
Oldest (<200 Ma) near trenches
Confirms continuous creation and destruction of oceanic crust.
Evidence from Ancient Glaciations
Tillites and glacial striations found in South America, Africa, India, Australia,
Antarctica.
3.5.1 Evidence of Continental Drift and Plate Tectonics 2
, Impossible under modern positions → continents must have been joined.
Striations point to a common ice centre in southern Africa → supported
Gondwanaland.
Fossil Records
Identical fossils found on continents now separated by oceans:
Mesosaurus (freshwater reptile) - South America & Africa.
Glossopteris (fern) - Africa, India, Australia, Antarctica.
Lystrosaurus & Cynognathus - Africa, India, Antarctica.
Land animals could not cross oceans → continents were once connected.
1.2 There are Distinctive Features and
Processes at Plate Boundaries
Earth’s crustal features and processes, including:
the global pattern of plates and plate boundaries
the features and processes associated with divergent (constructive)
plate boundaries
the features and processes associated with convergent plate
boundaries including oceanic-continental, oceanic-oceanic
(destructive) and continental-continental (collision) boundaries
the features and processes associated with conservative plate
boundaries.
Global Patterns of Plates
Earth’s lithosphere divided into major and minor plates (e.g. Pacific,
Eurasian, African).
Boundaries are zones of intense tectonic activity: earthquakes, volcanoes,
mountain building.
Interior of plates = aseismic, tectonically stable.
Divergent (Constructive) Plate Boundaries
3.5.1 Evidence of Continental Drift and Plate Tectonics 3
, Processes
Plates move apart due to extensional forces.
Crust thins → decompression melting of mantle → basaltic magma.
Magma rises through feeder dykes → creates new oceanic crust.
Transform faults offset ridges; shallow earthquakes common.
Landforms
Mid-Ocean Ridges (e.g. Mid-Atlantic Ridge)
Axial Rift Valleys
Hydrothermal Vents (Black Smokers)
Horst and Graben structures on land (e.g. East African Rift)
Volcanicsm
Basaltic, low-viscosity magma → effusive eruptions.
Pillow basalts form underwater.
Convergent (Destructive) Plate Boundaries
Oceanic-Continental Convergence
Processes:
Dense oceanic plate subducts beneath continental plate.
Benioff Zone marks descending slab.
Water from slab causes flux melting → viscous, silicic magma.
Magma rises, melting continental crust → explosive volcanism.
Landforms:
Deep-Sea Trenches
Accretionary Wedges
Stratovolcanoes (e.g. Andes)
Fold Mountains
Granitic Batholiths
3.5.1 Evidence of Continental Drift and Plate Tectonics 4
3.5.1 Evidence of Continental
Drift and Plate Tectonics
Topic Physical - Hazardous Earth
1.1 Variety of Evidence for the Theories of
Continental Drift and Plate Tectonics
Theories of continental drift and plate tectonics including:
the basic structure of the Earth including the lithosphere,
asthenosphere and the role of convection currents
evidence for sea-floor spreading; paleomagnetism; the age of sea floor
rocks
evidence from ancient glaciations
fossil records.
The Basic Structure of the Earth
Layers of the Earth
Inner Core - Solid iron-nickel, 6000 degrees, extremely dense.
Outer Core - Liquid iron-nickel; generates Earth’s magnetic field.
Mantle - Solid but capable of slow flow; composed of silicate minerals.
Crust - Thin outer layer; oceanic (basaltic) or continental (granitic).
Lithosphere & Asthenosphere
Lithosphere - Rigid layer comprising the crust and uppermost mantle;
broken into tectonic plates.
Asthenosphere - Partially molten, ductile upper mantle; plates move over it.
3.5.1 Evidence of Continental Drift and Plate Tectonics 1
, Plates “float” on the asthenosphere, driven by convection, slab pull and
ridge push.
Role of Convection Currents
Heat from radioactive decay drives mantle convection.
Rising hot material spreads beneath plates → divergence.
Sinking cold slabs at subduction zones → slab pull (dominant force).
Elevated ridges push plates apart → ridge push.
Evidence for Sea-Floor Spreading
Ocean Floor Mapping
SONAR mapping (Tharp & Heezen) revealed:
Mid-Ocean Ridges
Deep ocean trenches
Thin sediment cover (young crust)
Paleomagnetism
As basalt cools, iron minerals align with Earth’s magnetic field.
Magnetic reversals create symmetrical stripes on either side of ridges.
Vine-Matthews hypothesis proved new crust forms at ridges and moves
outward.
Age of Sea Floor Rocks
Radiometric dating shows:
Youngest rocks at ridges
Oldest (<200 Ma) near trenches
Confirms continuous creation and destruction of oceanic crust.
Evidence from Ancient Glaciations
Tillites and glacial striations found in South America, Africa, India, Australia,
Antarctica.
3.5.1 Evidence of Continental Drift and Plate Tectonics 2
, Impossible under modern positions → continents must have been joined.
Striations point to a common ice centre in southern Africa → supported
Gondwanaland.
Fossil Records
Identical fossils found on continents now separated by oceans:
Mesosaurus (freshwater reptile) - South America & Africa.
Glossopteris (fern) - Africa, India, Australia, Antarctica.
Lystrosaurus & Cynognathus - Africa, India, Antarctica.
Land animals could not cross oceans → continents were once connected.
1.2 There are Distinctive Features and
Processes at Plate Boundaries
Earth’s crustal features and processes, including:
the global pattern of plates and plate boundaries
the features and processes associated with divergent (constructive)
plate boundaries
the features and processes associated with convergent plate
boundaries including oceanic-continental, oceanic-oceanic
(destructive) and continental-continental (collision) boundaries
the features and processes associated with conservative plate
boundaries.
Global Patterns of Plates
Earth’s lithosphere divided into major and minor plates (e.g. Pacific,
Eurasian, African).
Boundaries are zones of intense tectonic activity: earthquakes, volcanoes,
mountain building.
Interior of plates = aseismic, tectonically stable.
Divergent (Constructive) Plate Boundaries
3.5.1 Evidence of Continental Drift and Plate Tectonics 3
, Processes
Plates move apart due to extensional forces.
Crust thins → decompression melting of mantle → basaltic magma.
Magma rises through feeder dykes → creates new oceanic crust.
Transform faults offset ridges; shallow earthquakes common.
Landforms
Mid-Ocean Ridges (e.g. Mid-Atlantic Ridge)
Axial Rift Valleys
Hydrothermal Vents (Black Smokers)
Horst and Graben structures on land (e.g. East African Rift)
Volcanicsm
Basaltic, low-viscosity magma → effusive eruptions.
Pillow basalts form underwater.
Convergent (Destructive) Plate Boundaries
Oceanic-Continental Convergence
Processes:
Dense oceanic plate subducts beneath continental plate.
Benioff Zone marks descending slab.
Water from slab causes flux melting → viscous, silicic magma.
Magma rises, melting continental crust → explosive volcanism.
Landforms:
Deep-Sea Trenches
Accretionary Wedges
Stratovolcanoes (e.g. Andes)
Fold Mountains
Granitic Batholiths
3.5.1 Evidence of Continental Drift and Plate Tectonics 4
