









Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Community
Ask the community for help and clear up your study doubts
Discover the best universities in your country according to Docsity users
Free resources
Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors
Plate tectonics: unifying theory of geology in erly ideas about continental drift and paleomagnetism in earth magnetic field.
Typology: Slides
1 / 17
This page cannot be seen from the preview
Don't miss anything!
2.Plate Tectonics2.Plate Tectonics
Unifying theoryUnifying theory –– plate tectonics describeplate tectonics describe movement of continental and oceanicmovement of continental and oceanic plates and forces driving them. Itplates and forces driving them. It explains locations of mountain chains,explains locations of mountain chains, earthquakes, rock assemblages, andearthquakes, rock assemblages, and structures on sea floor.structures on sea floor.
For geology Plate Tectonics is similar inFor geology Plate Tectonics is similar in importance as the Discovery of DNA is toimportance as the Discovery of DNA is to biologybiology
Plate tectonics affects all of us, whether in relation to the destruction caused by volcanic eruptions and earthquakes, or politically and economically due to the formation and distribution of valuable natural resources. Plate tectonics tyies together many seemingly unrelated geologic phenomena and illustrating why Earth is a dynamic planet of interacting subsystems and cycles.
2.Plate Tectonics2.Plate Tectonics
Earlier related theories:Earlier related theories:
Over the past 200 years (up to late 1960s)Over the past 200 years (up to late 1960s) geologists had developed many theoriesgeologists had developed many theories about the characteristics & location ofabout the characteristics & location of EarthEarth’’s tectonic features.s tectonic features.
Mountains, volcanoes, earthquakes, etc:Mountains, volcanoes, earthquakes, etc:
UnderstoodUnderstood
Early Ideas About Continental Drift
The idea that continents have moved in the past is not new. The concept of continental movement was first suggested when it was noticed that Africa and South America had coastlines which appeared to be counterparts of one another This suggested they may once have been joined and drifted apart. Fig. 2.3, p. 32
Early Ideas About Continental Drift
Pangaea consisted of a northern landmass called Laurasia and a southern landmass called Gondwana.
As Pangaea broke up, the various continents moved to their present-day locations.
Fig. 2.1, p. 31
The Glossopteris fern, also known as the “Pangaea plant”
What is the Evidence for Continental Drift?
Wegener and others amassed a large amount of evidence in support of continental drift.
There is a close fit between the continents off the coast at a depth of about 2000 m.
Fig. 2.3, p. 32
Evidence for Continental Drift
Glacial tills and striations on the bedrock beneath the till provide evidence of glaciation at the same time on all the Gondwana continents, with South Africa located at the South Pole.
Evidence for Continental DriftEvidence for Continental Drift
Evidence for Continental Drift Evidence for Continental Drift
Evidence for Continental Drift
geologist, 1937)
**Evidence for Continental Drift
Wegener could not provide a convincing mechanism to demonstrate ‘how’ the continents could have moved. His ideas were largely ignored.
Fig. 2.2, p. 31
Evidence for Continental Drift Evidence for Continental Drift
Earth’Earth’s Magnetic Fields Magnetic Field • Strength and orientation of the magnetic field varies
Paleomagnetism and Seafloor Spreading
The existence of such magnetic reversals was discovered in continental lava flows by A) age dating B) determining the orientation of the remnant magnetism.
Fig. 2.10, p. 37 Fig. 2-10, p. 37
Stepped Art
North magnetic pole (reversed)
North magnetic pole (reversed)
South magnetic pole (normal position)
South magnetic pole (normal)
South magnetic pole (reversed)
South magnetic pole (reversed)
North magnetic pole (normal)
North magnetic pole (normal position)
Magnetic Reversals and Seafloor Spreading
He suggested that the seafloor separates at oceanic ridges, where new crust is formed by upwelling magma. As the magma cools, the newly formed oceanic crust moves laterally away from the ridge. Fig. 2.11, p. 38
Continental Drift, Seafloor Spreading, Continental Drift, Seafloor Spreading,
and Plate Tectonicsand Plate Tectonics
Mid-Atlantic Ridge
Seafloor SpreadingSeafloor Spreading
Seafloor SpreadingSeafloor Spreading
Seafloor Spreading orSeafloor Spreading or Plate Tectonics?Plate Tectonics?
Seafloor SpreadingSeafloor Spreading
new crust new crust formed dueformed due expandingexpanding
earth?earth?
Seafloor Spreading orSeafloor Spreading or Plate TectonicsPlate Tectonics
Plate Tectonics Theories (1960Plate Tectonics Theories (1960--68)68)
and symmetrical with the ridges
Magnetic Reversals and Seafloor Spreading
Sea floor spreading is confirmed by the ages of fossils in sediments overlying oceanic crust radiometric dating of rocks on oceanic islands. These indicate that oceanic crust is youngest at the spreading ridges and oldest at the farthest points from the ridges.
Fig. 2.13, p. 40
Magnetic Reversals and Seafloor Spreading
Further evidence confirming seafloor spreading came from the Deep Sea Drilling Project are the age and thickness of the sediments overlying the oceanic crust.
Fig. 2.14, p. 41
Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early 1970's.
The theory is widely accepted because it explains so many geologic phenomena, including volcanism, seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the distributions of natural resources.
Numbers represent average rates of relative movement, cm/yr
The lithosphere overlies the asthenosphere, and through some type of heat-transfer system within the asthenosphere, moves the plates.
As the plates move over the asthenosphere, they separate mostly at oceanic ridges and collide and are subducted into Earth’s interior at oceanic trenches.
Fig. 14 p. 41
Little Magadi soda lake
An Example of Ancient Rifting
Characteristic features of ancient continental rifting include faulting, dikes, sills, lava flows, and thick sedimentary sequences within rift valleys. Pillow lavas and associated deep-sea sediments are evidence of ancient spreading ridges.
Fig. 2.17, p.
Europe
Africa
North America
South America
Atlantic Ocean basin
Thousands
kilometersof
Palisades of Hudson River
There are three types of convergent boundaries.
An oceanic-oceanic boundary is where two oceanic plates collide, one ocean plate will subduct beneath the margin of the other plate.
An oceanic-continental boundary is where an oceanic plate and a continental plate collide, the oceanic plate will subduct.
A continental-continental boundary occurs when two continents collide
The Three Types of Plate Boundaries
Oceanic-Oceanic Boundaries
One oceanic plate is subducted beneath the other and a volcanic island arc forms on the non-subducted plate An oceanic trench forms parallel to the volcanic island arc where the subduction is taking place. The volcanoes result from rising magma produced by the partial melting of the subducting plate. (^) Fig. 2.18a, p. 47
The Three Types of Plate Boundaries
Oceanic-Continental Boundaries
An oceanic plate and a continental plate converge, with the denser oceanic plate being subducted under the continental plate. Just as with an oceanic-oceanic boundary, a chain of volcanoes forms on the nonsubducted plate. (^) Fig. 2.18b, p. 47
Hot spots enable geologists to determine absolute motion because they provide an apparently fixed reference point from which the rate and direction of plate movement can be measured.
Fig. 2.23 a-b, p. 51
Fig. 2.23a, p. 51
The average rate of plate movement is most commonly determined by dividing the distance from an oceanic ridge axis to any magnetic anomaly in the crust of the seafloor by the age of that anomaly. Satellite-laser ranging techniques are also used to determine the rate of movement and relative motion of one plate with respect to another.
The Driving Mechanism of Plate Tectonics
Most geologists agree that some type of convective heat system is the basic process responsible for plate motion.
Fig. 2.25, p. 52
The Driving Mechanism of Plate Tectonics
Two models involving thermal convection cells have been proposed to explain plate movement. 1. thermal cells are restricted to the asthenosphere 2. the entire mantle is involved.
Problems with both models involve the source of heat for the convection cells and how heat is transferred from the outer core to the mantle. (^) Fig. 2.25, p. 52
The Driving Mechanism of
Plate Tectonics
Some geologists think a gravity-driven mechanism also plays a major role.
“Slab-pull” involves pulling the plate behind a subducting cold slab of lithosphere “Ridge-push” involves gravity pushing the oceanic lithosphere away from the higher spreading ridges and toward the subduction trenches (^) Fig. 2.26, p. 53
The Driving Mechanism of Plate Tectonics
In the early 1970s J. Tuzo Wilson put forth the hypothesis of a large-scale global cycle of supercontinents. Supercontinents like Pangea form, break up, and re-form in a cycle spanning approximately 500 million years. The breakup forms rift valleys within the supercontinent that eventually becomes a long, linear ocean basins as the crust is depressed below sea level. As the width of the narrow sea continues to expand an open ocean develops. As the ocean basins close, another supercontinent forms.
Plate Tectonics and the Distribution of
Natural Resources
Many metallic mineral deposits are related to igneous and associated hydrothermal activity, so it is not surprising that a close relationship exists between plate boundaries and the occurrence of these valuable deposits. Many of the world’s major metallic ore deposits are associated with convergent and divergent plate boundaries. Copper, iron, lead , zinc, gold and silver ore deposits are associated with plate boundaries. Fig. 2.27, p. 54
Plate Tectonics and the Distribution of Life
Fossil evidence provided one of the first proofs for plate tectonics. Together, plate tectonics and evolution have changed the way we view our planet.
The world’s plants and animals occupy biotic provinces controlled mostly by: Climate Geographic barriers
The location of these provinces is mostly controlled by plate movement.
Fig. 2.28, p. 56