Plate Tectonics Quiz

Endogenic Processes of the Earth

• Endogenic processes are internal Earth processes.
• Course outcome 5, GEO01 - Earth Science.
• Materials prepared by Ryo Jerome C. Tuzon, LPT.
• Video lecture by Perseval S. Pineda, LPT.
• Website: www.mapua.edu.ph

Plate Tectonics: The Unifying Theory

• Earth's surface consists of a few large, thick plates that move slowly and change size.
• Plate boundaries are where plates move away, towards, or past each other causing intense geologic activity.
• The theory of plate tectonics developed from continental drift and seafloor spreading hypotheses.

Plate Tectonics

• Earth's surface is composed of large, thick plates
• Plates move and interact at their boundaries
• Plate movement is driven by intense geologic activity
• Based on continental drift and seafloor spreading

The Major Plates of the World

• A diagram of the major plates of the Earth is presented.
• The image shows various plates including: North America, South America, Eurasian, African, Indo-Australian, Pacific, Antarctic, and Scotia plates.
• Plate boundaries are also labeled (divergent, convergent, transform).

The Early Case for Continental Drift

• The fit of continents (Africa and South America) has long been recognized.
• Alfred Wegener noted similar late Paleozoic rocks and fossils in South America, Africa, India, Antarctica, and Australia.
• Fossils of Glossopteris (plant), Lystrosaurus, and Cynognathus (animals) were found on multiple continents.
• Mesosaurus (reptile) fossils were found only in Brazil and South Africa.

Pangaea

• Pangaea was a proposed supercontinent.
• Laurasia was the northern supercontinent (North America, Asia, excluding India).
• Gondwanaland was the southern supercontinent (South America, Africa, India, Antarctica, and Australia).
• Evidence for Pangaea includes late Paleozoic glaciation patterns and coal beds in different continents.

Paleoclimate Belts and Polar Wandering

• Paleoclimate belts suggest polar wandering (potential evidence for continental drift).
• Polar wandering is the apparent movement of the poles.
• Possible explanations include continents remaining motionless and poles moving, poles staying put and continents moving, or both.

Skepticism About Continental Drift

• Land bridges could explain the distribution of land-dwelling reptiles in scattered continents
• Wind or ocean currents could explain the distribution of fossil plants in separate continents
• Polar wandering could be explained by moving poles rather than continents moving
• Wegener's mechanism was not accepted as centrifugal and gravitational forces were proven insufficient.

The Revival of Continental Drift

• Paleomagnetism is the study of ancient magnetic fields
• Magnetite, a mineral, becomes magnetized as lava cools below the Curie Point
• Rocks that formed closer to the magnetic poles have steeper dip angles
• Rock age indicates pole positions migrating from the present ones

Evidence from Paleomagnetism

• Apparent polar wander curves for various continents suggests real movement.
• Permian rocks on different continents show different pole positions, unlikely if continents were stationary.
• Reconstructing continents to form Pangaea shows similar polar wandering paths, indicating previous joint positioning.

Geologic Evidence for Continental Drift

• Matching continental edges after redefinition of edges is better fit.
• Matching isotopic ages, glacial striations, and rock types supports continental movement.
• Paleomagnetic data indicates the direction and rate of continental movement.
• Pangaea split 200 million years ago, but movement has spanned 2-4 billion years.

Seafloor Spreading

• Seafloor spreading is the concept that the seafloor moves like a conveyor belt.
• It occurs away from the mid-oceanic ridge.
• The rising hot mantle material creates new seafloor, while cold oceanic crust sinks at the ocean trenches. The driving force is deep mantle convection.

Explanations

• Hot mantle rock rises beneath mid-oceanic ridges.
• Decompression melting occurs creating new rock.
• Circulation pattern diverges, moving rock away from the ridge.
• A rift valley forms from tensional forces at the ridge crest.
• Cooled rock becomes denser and sinks back into the mantle at oceanic trenches.
• Overall young age of seafloor rocks (<200 million years) is explained.

Plates and Plate Motion

• Tectonic plates are composed of relatively rigid lithosphere.
• Lithospheric thickness and age increase with distance.
• Plates rest on ductile asthenosphere and interact at boundaries.
• Divergent, convergent, and transform boundaries are forms of plate interactions.

How Do We Know That Plates Move?

• Marine magnetic anomalies, alternating positive and negative magnetic anomalies, form a stripe-like pattern parallel to mid-oceanic ridges.
• The Vine-Matthews Hypothesis states that new basaltic magma continually extrudes, cools, and records Earth's magnetism, including reversals.
• Matching patterns of reversals in continental rocks allows determining movement rates and seafloor age.

Rates of Motion and Seafloor Age

• Comparing marine magnetic reversals to land-based reversals.
• Rate of plate motion equals distance from the ridge divided by the age of rocks.
• Seafloor age increases with distance from the mid-oceanic ridge, indicating plate motion away.

Fracture Zones and Transform Faults

• Mid-oceanic ridges are offset along fracture zones.
• Transform faults are segments of fracture zones between offset ridge crests.
• Relative motion along faults results from seafloor spreading.

Measuring Plate Motion Directly

• Plate motion is measured using satellites, radar, lasers, and global positioning systems.

Divergent Plate Boundaries

• Plates move apart, often in the middle of oceans or continents.
• Rifting, volcanic activity, and eventual ridge uplift are associated.
• Formation of new ocean basins.

Transform Plate Boundaries

• Plates slide horizontally past each other.
• Transform offsets of mid-oceanic ridges result in approximate curved boundaries.

Convergent Plate Boundaries

• Plates move toward each other.
• Ocean-ocean convergence forms ocean trenches, volcanic island arcs, and Benioff zones.
• Ocean-continent convergence creates ocean trenches, volcanic arcs, and mountain belts.
• Continent-continent convergence forms mountain ranges and thrust faults.

Do Plate Boundaries Move?

• Plate boundaries can move over time and migrate, driven by forces associated with different plate motions like subduction angles and trenchward motions.
• Transform boundaries can shift.

Can Plates Change Size?

• Plate sizes can change over time.
• The North American Plate is increasing in size.
• The Nazca Plate is getting smaller as the leading edge is subducted under South America.

What Causes Plate Motions?

• Any proposed mechanism must explain why mid-oceanic ridges are hot and elevated while trenches are cold and deep, why ridge crests have tensional cracks, and why some plates subduct while others don't.
• Mantle convection is a possible driving force.

Ridge Push, Slab Pull, and Trench Suction

• Ridge push – new plate cools and thickens, leading to subsidence and resulting in a driving force.
• Slab pull – cold lithosphere sinks pulling the surface plate.
• Trench suction – forces subducting plates pulling overlying plates seaward.

Mantle Plumes and Hot Spots

• Mantle plumes are columns of hot mantle rock rising through the mantle, stationary with respect to moving plates.
• Mantle plumes may spread and tear apart the overlying plate creating hot spots (like Hawaii, Yellowstone).
• These can lead to flood basalt eruptions and rifting of continental land masses forming divergent boundaries.

Hot Spot Volcanism

• Mantle plumes produce volcanic chains as plates move over a stationary plume.
• Volcanic rock age can be used to determine plate movement rates.
• The Hawaiian Islands is an example.

Emperor Seamounts of Hawaii

• A map shows the Emperor Seamount chain and the Hawaiian Ridge, indicating the direction of plate movement over time.
• The ages of volcanoes on the chain indicate the relative movement of the Pacific Plate.

Earth’s Interior & Geophysical Properties

• The Earth's interior is complex and studied indirectly through geophysical methods.

How Is Earth's Interior Studied?

• Indirect study methods are used to obtain information on the Earth's interior including observing data from volcanic eruptions and drill holes.

Evidence from Seismic Waves

• Seismic waves from large earthquakes travel through Earth.
• Seismic reflection and refraction patterns are used to map the internal layering and structure.

Earth's Internal Structure

• Main zones include the crust, mantle, and core.
• The crust is the outermost solid rock layer.
• The mantle is the thick shell of dense rock underneath the crust.
• The core is the central metallic zone of the Earth.

Earth's Internal Structure: The Oceanic Crust

• Average thickness of 7km (4.3 miles)
• Seismic waves travel around 7km/sec.
• Composed of mafic rocks, basalt

Earth's Internal Structure: The Continental Crust

• Average thickness between 30-50m (18.6-31miles).
• Seismic waves travel approximately 6km/sec.
• Composed of felsic rocks, granite.

Earth's Internal Structure: The Mantle

• P-waves and S-waves pass through the mantle.
• Higher seismic wave velocities signify higher density, ultramafic composition.
• Crust + upper mantle = lithosphere (70 km).
• The low-velocity zone, asthenosphere, is a plastic layer below the lithosphere.

Seismic Shadow Zones

• Seismic waves passing through Earth create shadow zones on the opposite side from the epicenter.
• P-wave shadow zone (103-142 degrees) is due to refraction at the core-mantle boundary.
• S-wave shadow zone (>103 degrees) suggests the outer core is liquid.
• Inner core is solid based on P-wave refraction.

Earth's Internal Structure: The Core

• Iron-nickel alloy mixed with lighter elements.
• Density is approximately 10-13 grams/cubic centimeter.
• Estimated based on astronomical and seismic wave data.

The Core-Mantle Boundary

• A D'' layer marks the boundary between the mantle and the outer core.
• There are significant changes in seismic velocity, density, and temperature at this boundary.
• The region containing the low-velocity zone (ULVZ) in the layer D".

Isostasy

• Isostasy is the equilibrium of adjacent blocks of brittle crust floating on the upper mantle.
• Isostatic adjustment results from rising or sinking of crustal blocks to achieve balance.
• Large mass removal results in crust rise, leading to crustal rebound after ice sheet removal.

Gravity Measurements

• Gravity force is determined by mass and distance.
• Gravity meters detect changes in gravity, related to mass beneath a point.
• Gravity is higher over dense materials and lower over less dense materials.

Earth's Magnetic Field

• A magnetic field region of magnetic force has north and south magnetic poles.
• These poles are recorded by magnetic minerals in igneous rocks cooled below their Curie Point.
• Magnetic reversals occur when poles switch, timing is chaotic.
• Paleomagnetism is the study of ancient magnetic fields in rocks used to reconstruct plate motions over time.

Magnetic Anomalies

• Local increases or decreases in the Earth's magnetic field strength.
• Positive and negative magnetic anomalies represent larger and smaller deviations from the average magnetic field strengths respectively.

Earth's Magnetometers

• Used to measure local magnetic field strength.
• Can detect metallic ore deposits, igneous rocks, and thick layers of nonmagnetic sediments.

Earth's Temperature: Geothermal Gradient

• Temperature increases with depth.
• Gradient sharply decreases below the lithosphere.
• Increased temperatures cause little melting, except in the outer core.

Heat Flow

• Gradual loss of heat through Earth's surface.
• Major heat sources include original heat and radioactive decay.
• Areas with magma near the surface have higher heat flow.
• Similar values for oceanic and continental crust, but from different sources.

Geologic Structures

• Dynamically produced patterns or arrangements in rock or sediment resulting from forces in the Earth
• Produced by change in rock shape (folded and altered orientation).
• Structural geology is the study of shapes, arrangement, and interrelationships of rock units, as well as the forces that cause features.

Tectonic Forces At Work

• Stress is a force per unit area.
• Basic types of stress are compressive, tensional, and shear.
• Strain is a change in size or shape in response to stress.
• Geologic structures can be used to understand stress type and physical rock properties.

How Do Rocks Behave When Stressed?

• Rocks can be elastic, ductile, or brittle depending on stress type, and stress application rate, rock type, and temperature and pressure.
• Elastic deformation is reversible, ductile deformation is non-reversible.
• Brittle deformation is characterized by fracturing.

How Do We Record and Measure Geologic Structures?

• Rock structures are determined on the ground by observing rock outcrops (places where bedrock is exposed).
• Geologists use geologic maps, standardized symbols, and patterns for rock types and geologic structures.

Geologic Maps and Field Methods

• Planar features like tilted beds, joints, faults, which have orientation.
• Strike is the compass direction from intersections of an inclined plane with a horizontal plane.
• Dip is the orientation from horizontal in which a plane is oriented.
• Geological cross sections represent a vertical slice to show Earth's portion.

Folds

• Wavelike bends in layered rocks, usually under compression.
• Anticlines – upward, synclines – downward.
• Axial plane – divides a fold into two limbs.
• Hinge line – is the surface trace of the axial plane on a fold.

Geometry of Folds: Domes & Basins

• Plunging fold – a fold where the hinge line is not horizontal.
• Domes – structures with beds dipping away from a central point (sometimes referred to as doubly plunging anticlines).
• Basins – structures with beds dipping towards a central point (sometimes called doubly plunging synclines).

Interpreting Folds

• Open folds – gently dipping limbs
• Isoclinal folds – parallel limbs
• Overturned folds – dipping limbs are in similar directions
• Recumbent folds – limbs are overturned until horizontal

Joints

• Fractures in bedrock where there is no movement.
• Columnar jointing – contraction of cooling lava flows.
• Sheet jointing – expansion due to pressure release.
• Joint Sets – multiple parallel joints.

Faults

• Fractures where movement has occurred.
• Dip-slip faults – movement parallel to the dip of the fault plane.
• Strike-slip faults – movement parallel to the strike of the fault plane.
• Oblique-slip faults – movement with both vertical and horizontal components.

Normal Faults

• Dip-slip where the hanging wall block has moved down relative to the footwall block due to tensional stress.

Features of Normal Faults

• Grabens – fault blocks bounded by normal faults that drop down.
• Rifts – associated with divergent plate boundaries.
• Horsts – fault blocks bounded by normal faults that are uplifted.

Reverse Faults

• Dip-slip fault where the hanging wall block has moved up relative to the footwall block due to compressional stress.
• Thrust faults are reverse faults with dip angles less than 30 ° from horizontal.

Strike-Slip Faults

• Predominantly horizontal movement parallel to the strike of the fault plane.
• Right-lateral strike-slip faults the offset is toward the right.
• Left-lateral strike-slip faults the offset is toward the left.

The Sea Floor

• Covers approximately 70% of the Earth's surface.

Origin of the Ocean

• Oceans originated primarily from volcanic degassing of water vapor from Earth's interior.
• Earth cooling led to water vapor condensation and rainfall.

Methods of Studying the Sea Floor

• Seafloor rocks are studied using rock dredges, seafloor drilling, or submersibles, along with sonar and seismic reflection profiling.

Features of the Sea Floor

• Passive continental margins comprise continental shelf, continental slope, continental rise, and abyssal plain.
• Active continental margins include continental shelves, slopes, and oceanic trenches with associated volcanic arcs and Benioff zones.
• Mid-oceanic ridge systems encircle the globe.
• Numerous conical seamounts rise from the deep ocean floor.

Continental Shelves and Slopes

• Continental shelves are gently sloping shallow platforms along continents (range 0-500 km).
• Covered with young sediments.
• Continental slopes extend steeply downward from the shelf edge to the abyssal plain.

Submarine Canyons

• V-shaped valleys that cross continental shelves and slopes.
• Deliver continental sediments to abyssal fans.
• Turbidity currents (sediment-laden water) move down continental slopes.

Passive Continental Margins: The Continental Rise

• Gently sloping wedges of sediment along the continental slope extending to the abyssal plain.
• Sediment deposited by turbidity and contour currents.
• Ends at the abyssal plain, approximately 5 km deep.
• Lie on oceanic crust.
• Extremely flat regions beyond the continental rise, with slopes <0.01°.
• Sufficient turbidity currents bury rugged topography.

Active Continental Margins

• Consists of a shelf, slope, oceanic trench, and an island arc or volcanic arc.
• Trenches are the deepest parts of the oceans.
• Earthquake foci (Benioff zone) begin at trenches.
• Volcanoes are found above the Benioff zone, aligned along ridges paralleled to trenches.
• Very low heat flow and high negative gravity anomalies associated.

Mid-Oceanic Ridges

• Giant undersea mountain ranges.
• Rift valley runs down the crest.
• Primarily composed of young basalt flows, has shallow focus earthquakes and extremely high heat flows.
• Offset along transform fracture zones
• Unique biological communities, such as mussels, crabs, tube worms, and thermophilic bacteria, are supported by hot springs.

Other Features

• Fracture zones are lines of weakness in Earth's crust crossing mid-oceanic ridges at right angles.
• Earthquakes are associated with transform faults.
• Seamounts are conical undersea mountains.
• Guyots are flat-topped seamounts.
• Aseismic ridges are chains of seamounts without earthquakes.

Reefs

• Wave-resistant ridges formed by coral, algae, and other calcareous organisms.
• Found in warm, shallow, sunlit, low-sediment waters.
• Types include fringing reefs, barrier reefs, and atolls.

Sediments of the Sea Floor

• Terrigenous sediments are land-derived sediments.
• Pelagic sediments settle slowly through ocean water.
• Clay, and shells of microscopic organisms primarily contribute.
• Pelagic sediments are rare on mid-oceanic ridge crests.

Ocean Crust and Ophiolites

• Oceanic crust is about 7 km thick.
• Layers include marine sediment, pillow basalts, overlying basaltic dikes, and sill-like gabbro intrusions.
• Ophiolites are sequences found in mountain ranges.
• Suggests ancient seafloor crust and upper mantle.

The Age of the Sea Floor

• All deep-sea floor rocks and sediments are younger than 200 million years.
• Continents preserve rocks up to 4 billion years old.
• Understanding seafloor age helps in studying plate tectonics.

Reference

• Lecture Outlines Physical Geology, 17th Edition, Plummer, Carlson & Hammersley.
• Physical Geology, 17th Edition, Charles C. Plummer, Diane H. Carlson, and Lisa Hammersley.

Test your knowledge on Plate Tectonics and continental drift! This quiz covers the fundamental concepts of the theory, major pieces of fossil evidence, and the supercontinent proposed by Alfred Wegener. Challenge yourself to see how well you understand these geological processes.