Continental Drift Theory

Theories of Mountain Building

• Several hypotheses and theories aim to explain the origin of mountains
• The theories include the Contraction Hypothesis, Expansion Hypothesis, Continental Drift Hypothesis, Oscillation and Undation Theory, Convection Current Hypothesis, and the Theory of Plate Tectonics

Contraction Hypothesis

• Warision introduced the contraction hypothesis in the 19th century.
• Jeffreys later developed it in 1929, based on Earth's thermal history
• The inner mantle below 700 km was considered stable in temperature and volume
• The outermost layer, about 100 km, had reached equilibrium and wasn't changing in volume
• Cooling and contraction concentrated between 100 km and 700 km depth, creating tension
• Contraction and thinning in this zone resulted in compression in the layer above 100 km

Expansion Hypothesis

• Carey suggested the oceanic area and the globe were expanding, according to paleoclimatic and palaeomagnetic studies in 1958
• Carey believed that the Earth's original diameter was less than half its current size, with the surface area being less than a quarter of the present area
• Worldwide expansion causes the Earth's crust to break into blocks
• The blocks rotate anticlockwise (sinistrally) in the southern hemisphere and clockwise (dextrally) in the northern hemisphere.
• The rotation occurs due to shearing forces caused by the adjustment of angular momentum between polar and equatorial regions

Oscillation and Undation Theory

• Haarman proposed in 1930 that vertical movements caused by an undefined cosmic factor created disequilibrium in the Earth's crust
• This led to the formation of 'geotumours' divided by 'geodepressions’
• The Geodepressions were due to sub-crustal sialic matter from depressions to tumours
• Tumours moved when the cosmic influence shifted, resulting in oscillation of emergence and submergence
• Sediments in depressions were uplifted and glided down the slopes of new tumours in the second phase
• Folded and structured rocks were uplifted, forming mountain chains in the third phase
• A major criticism is the unknown basis of cosmic influence, and gliding tectonics cannot explain all structural features

Theory of Continental Drift

• Scientists realized that the coastlines on both sides of the Atlantic Ocean fit together like a jigsaw puzzle
• Continents are not stationary and have moved in the geological past
• F.B. Taylor, an American physicist, initiated the concept of continental mobility in 1910
• Alfred Lothar Wegener is recognized as the originator and pioneer of the theory, who first suggested this theory in 1912
• Wegener was a German meteorologist and geophysicist
• Wegener termed the supercontinent 'Pangaea' and the oceanic part 'Panthalassa' (meaning "all oceans")
• Pangaea fragmented into smaller continents, which drifted to their present locations
• Evidences include geological data, continuity of older structures, stratigraphic formations, and fossil fauna and flora across the continental shorelines
• Carboniferous times (~225 million years ago) affected most of the southern continents while the northern continents had tropical conditions
• Continental drift occurred in response to the centripetal force caused by the Earth's rotation
• Alexander Du Toit (1937) proposed that Pangaea initially split into Laurasia and Gondwanaland
• Laurasia consisted of North America, Greenland, Europe, and Asia
• Gondwana included South America, Antarctica, Africa, Madagascar, India, and Australia
• Laurasia was derived from Laurentia and Asia
• Gondwanaland was named after an ancient tribe in Madhya Pradesh, India
• Laurasia and Gondwanaland were separated by the 'Palaeo-Tethys' Ocean

Hypothesis of Sea Floor Spreading

• Harry Hess proposed the hypothesis of sea-floor spreading in the early 1960s
• Basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges
• Sea floor moves from the ridge towards deep-sea trenches, where it is subducted and recycled back into the mantle
• Hess proposed that the drift between North America and Europe occurred through the Atlantic Ocean's growth.
• South America and Africa moved 2500 km apart over 250 million years from the mid-Atlantic ridge at ~10 mm/year
• Oceanic ridges are located on rising thermal convection currents from the mantle
• Oceanic crust moves in opposite directions from mid-oceanic ridges
• Molten lavas cool and solidify, forming new crust along divergent plates
• New crust is continuously created along mid-oceanic ridges, while the expanding crusts are destroyed along oceanic trenches

Faults

• Normal Fault: The hanging wall moves downward relative to the footwall, these are common at divergent plate boundaries where Earth's crust is stretching
• Reverse Fault (Thrust Fault): The hanging wall moves upward relative to the footwall, typically occurring at convergent plate boundaries where tectonic plates are colliding and compressing
• Strike-Slip Fault: Movement is mainly horizontal with minimal vertical displacement, rocks slide past each other horizontally
• Transform Fault: A type of strike-slip fault that forms the boundary between two tectonic plates

Plate Tectonic Theory

• Theory supported considers Earth's crust and upper mantle composed of large, thin, rigid plates that move relative to each other
• Slip on faults that define plate boundaries commonly results in earthquakes
• Several styles of faults bound the plates
• Styles of faults include thrust faults (subduction), oceanic spreading ridges (new crustal material production), and transform faults

Basic Principles of Plate Tectonics

• Plates move apart at divergent boundaries and converge at convergent boundaries
• New oceanic crust is continuously generated along mid-oceanic ridges
• Divergent plate margins (constructive plate margin)
• Divergent zones occur where molten rocks from the mantle upwell and create new oceanic crust
• Convergent boundaries (destructive or consuming plate boundaries) occur where the denser plate subducts below the lighter one
• Subduction is the downward motion of one plate under another, occurring at the Benioff zone and is characterized by deep focus earthquakes
• Concept of Plate Tectonics: Came into light in the 1970s and explains continental drift, volcanism, folding, faulting and orogeny

Concept of Plate Tectonics

• Plate tectonics theory has revolutionize Earth sciences making it possible to explain diverse phenomena like earthquakes and sea floor spreading
• The globe is made up of rigid masses called 'plates' consisting of lithosphere.
• These plates float and move along the convection current over the asthenosphere
• H. Hess postulated the concept of 'plate tectonics' in 1960 to support continental drift.
• The continents and oceans move due to the movement of the plates
• The continents and ocean basins attained their current arrangement because of the relative movement of plates since the second Pangaea in the Carboniferous period

Plate Movement Types

• Constructive Plate Margins (Divergent/Accreting)
• Zones of divergence with continuous upwelling of molten material
• New oceanic crust is continuously formed
• Destructive Plate Margins (Convergent)
• Plates move towards each other or converge
• One plate overrides the other, leading to subduction, and part of the crust is lost in the mantle
• Conservative Plate Margins (Shear/Transform)
• Plates slide past one another along transform faults
• Crust is neither created nor destroyed

Earth's Layers

• The Earth has three layers: the crust, the mantle, and the core
• The crust is made of solid rocks and minerals
• The mantle, beneath the crust, consists of rocks, minerals and semi-solid magma
• Earth's center is a hot, dense metal core

Oceanic Crust

• Averages 5-10 kilometers or 3-6 kilometres beneath the ocean floor, composed of basalts
• Geologists refer to oceanic crust rocks as "sima" for silicate and magnesium, the most abundant minerals in oceanic crust (Basalts)
• Oceanic crust is dense, nearly 3 grams per cubic centimeter
• Oceanic crust is consistently formed at mid-ocean ridges and as magma cools, it becomes young oceanic crust.
• The age and density of oceanic crust increases its distance from mid-ocean ridges and is destroyed in subduction zones
• Dense oceanic lithosphere subducts under less dense lithosphere at convergent plate boundaries
• At boundaries between continental and oceanic lithosphere, the dense oceanic lithosphere subducts beneath the continental one
• In the Japan Trench, the dense Pacific plate subducts beneath the less-dense Okhotsk plate
• Lithosphere sinking into the mantle becomes more plastic/ductile
• The lithosphere subducts, it sinks into the mantle, becoming more plastic and ductile which surfaces as crust-making lava at mid-ocean ridges and volcanoes thru convection
• Samples are gathered while drilling the ocean floor using submersible and analyzing ophiolites
• Ophiolites, sections of oceanic crust formed above sea level because of tectonic plate movement are often more accessible to scientists than oceanic crust at the bottom of the ocean

Continental Crust

• Composed of granites and referred to as "sial" for silicate and aluminum, the most abundant minerals
• Sial is thicker than sima (70 kilometers or 44 Miles thick), but with a density of around 2.7 grams per cubic centimeter
• Continental crust is created by plate tectonics
• At convergent plate boundaries, continental crust is thrust up in the process of orogeny - the thickest parts are at mountain ranges
• Cratons are the oldest and most stable parts of the continental lithosphere, located in the interior of most continents with shields being cratons that expose ancient basement rock out into the atmosphere
• These features are divided into sheilds, meaning cratons where rocks crop ot into the atmosphere and platforms, undergrounds below sediment
• Continental crust is older than oceanic crust, and sections are nearly the same age as Earth

Explore the continental drift theory, including Wegener's contributions, the supercontinent Pangaea, Laurasia, and Gondwanaland. Understand the evidence supporting continental drift and the reasons for initial skepticism toward Wegener's theory. Also, consider alternative theories like the contraction and expansion hypotheses.