W2-1 Tectonic

• Earth's surface is divided into large lithospheric plates that float on the mantle
• Tectonics involves the study of the movement and deformation of these plates
• Major topographic features on Earth are a direct result of the motion and interaction of lithospheric plates

Contributions to Plate Tectonics Theory

• The plate tectonics theory emerged through the work of scientists like DaVinci, Hutton, Darwin, and Wegener
• Today, it serves as a unifying theory in geology
• Leonardo DaVinci discovered fossil seashells in the Italian mountains in 1508
• DaVinci concluded that part of the seafloor had been uplifted
• James Hutton realized in the late 1700s that uplift exposes rocks to weathering and erosion
• Charles Darwin experienced an earthquake in Chile in 1835
• This earthquake brought mussel shells 10 feet above the high water mark

Continental Drift Hypothesis

• Alfred Wegener published the Continental Drift hypothesis in 1914
• The hypothesis tried to explain the parallel shapes of Atlantic coastlines, matching glacial landscapes, mountain ranges, and plant and animal fossils
• These were on continents separated by ocean basins
• Wegener suggested that landmasses were once joined in a supercontinent called "Pangaea"
• Wegener's presented observations to support that continents slowly drifted apart
• He was unable to provide a mechanism for how the continents moved
• There were few supporters of his hypothesis by the time he died in 1930

Evidence for Plate Tectonics

• In the 1950s, volcanic rocks were used to measure the past polarity of Earth's magnetic field
• Geophysicists found evidence that Earth's poles appeared to have wandered across the globe
• The apparent wandering paths for Europe and North America differed
• Scientists determined that continents had moved, carrying magnetized rocks, rather than the magnetic poles

Seafloor Spreading

• Continental drift lacked a mechanism explaining the movement
• In the 1950s, geologist Harry Hess hypothesized seafloor spreading while examining oceanic ridges
• The topography could be explained by the ocean crust moving laterally and splitting along the oceanic ridges

Paleomagnetism and Magnetic Reversals

• Paleomagnetism revealed records of magnetic reversals
• Older crust is farther from the ridge due to the splitting and spreading of the oceanic crust
• New lava fills the gap along the ridge and becomes magnetized with the Earth's magnetic field polarity

Seafloor Spreading Confirmed

• Seafloor spreading explained how continents could move
• Continents are passively carried atop lithosphere fragments that move away from oceanic ridges

Plate Motion and the Driving Force

• Observational evidence to support Wegener's supercontinent was assembled by the 1960s
• A driving force behind seafloor spreading had yet to be identified by the 1960s
• In 1919, Sir Arthur Holmes proposed continents were carried along by convective movements within the mantle in response to Weneger's continental drift hypothesis
• The solid rock of Earth’s interior is hot, weak, ductile, and undergoes slow convection
• Convection brings masses of hot rock upwards from the Earth’s interior
• Near the surface, it spreads sideways and cools, becoming denser, and eventually sinks back down in the mantle
• The precise way heat energy, brought up by convection, causes the plates to move is uncertain
• The lithosphere has the energy of motion, and the source of this energy is Earth’s internal heat
• Rising magma at oceanic ridges may push plates away from each other
• Lithosphere breaks, sinks into the asthenosphere, dragging the plate away
• The whole plate may be sliding downhill away from the spreading ridge
• There is evidence suggesting that each mechanism operates to some extent
• Old, cold lithosphere breaks, begins to sink, and pulls downward on the plate
• Downhill slides and ridge push combine to keep the process going
• The exact configuration of convection cells in the mantle is a cause for scientific debate
• Satellites measure the distance between two points on Earth with an accuracy of 1 cm
• Measuring distances several times a year provides present-day plate velocities directly and accurately
• Plates that carry a lot of continental crust move relatively slowly
  • Example: the African plate
• Plates without any continental crust move relatively rapidly
  • Example: the Pacific plate
• Plates are changing in size
• As new oceanic crust is formed at ridges, an equal amount of old crust must be getting destroyed somewhere else
• Constructive plate boundaries are balanced by destructive plate boundaries, recycling the tectonic plates

Plate Interactions and Earth's Landscapes

• The lithosphere is broken into nine major and several smaller plates
• Interactions between plates occur along their edges, which are also known as plate boundaries or margins
• There are three basic types of plate boundaries: divergent, convergent, and transform

Divergent Plate Margins

• This is where two plates move apart
• Divergent plate margins are known as spreading centers or constructive margins
• Divergent plate margins occur in plates capped by either continental or oceanic crust
• The process starts on a continent and evolves into an ocean
• Huge continental masses act as thermal blankets which slowly heat up and expand
• Eventually, they split, forming a rift, and starting the cycle of spreading

Convergent Plate Margins

• This is where two plates move toward each other
• Also known as destructive margins
• Convergent plate margins occur between plates carrying either oceanic crust, continental crust, or both
• Plates carrying different types of crust behave differently when they converge due to the different physical characteristics of oceanic and continental crusts

Ocean-Ocean Convergent Margin

• One plate will undergo subduction into the mantle beneath the other plate
• This zone is called a subduction zone
• Which of the two converging oceanic plates will be subducted depends upon the velocity and angle of approach

Ocean-Continent Convergent Margin

• Continental crust is much less dense than oceanic crust
• Continental crust rides up and over while the oceanic crust gets subducted beneath it
• This zone is called a subduction zone

Continent-Continent Convergent Margin

• Both plates are low density and cannot undergo subduction, so they collide
• This collision forms a continental collision zone
• Marks the final disappearance of an ocean basin and forms mountain ranges
• This is also known as a continental suture zone

Transform Fault Plate Margins

• Two plates slide past each other in a horizontal or strike-slip motion
• Can occur in oceanic or continental crust
• Conservative margins are where crust is neither created nor destroyed
• The San Andreas Fault is the most famous example of this type of boundary

Wilson Cycle

• Rifting, producing new crust, opening an ocean basin, convergence, the destruction of old crust, and closing ocean basins is the Wilson Cycle

• Plate tectonics explains the major topographic and geologic features on Earth's surface

• Plate tectonics applies to both ocean basins and continents

• Major topographic features of ocean basins include mid-ocean ridges, continental shelves, continental slopes, continental rises, and abyssal plains

• Subduction zones occur in closing oceanic basins and include oceanic trenches and arc-shaped chains of volcanic islands

• Arc-shaped chains of volcanic islands are called island arcs or magmatic arcs

• Continents and collision zones, composed of heterogeneous crust, do not have simple features

• Active continental margins, formed by ocean-continent subduction, have a continental volcanic arc

• Continent collision zones form massive mountain chains like the Himalaya

• Transform faults form topography like a giant knife cut through the crust

Building the Continents

• On the scale of a continent, two structural units are distinguished: cratons and orogens
• Cratons are the stable core of very ancient rock assemblages that formed, evolved, and stabilized over long time periods
• Orogens are elongate regions of crust that have been intensely bent and fractured during continental collisions
• Together, cratons and orogens form a continental shield
• Some continental mass was present by the end of the Hadean Eon
• Continental mass continued to grow during the Archean Eon, reaching 80-90% of today's mass by the beginning of the Proterozoic Eon
• There is strong evidence for continental collisions by the Proterozoic Eon
• Continental collisions lead to the assemblage of cratons into large continental complexes called supercontinents
• The breakup of Pangaea, the most recent supercontinent, eventually formed modern-day continents