Plate Margins and Geological Features

• Hydrogeology studies surface water, while hydrology is the study of underwater
• Tectonic plates are created from rigid crust and the uppermost mantle
• Continental position is fixed by land bridges that sink to create oceans, this is called Permanentism
• Seismology is the study of the interior structure of the earth, including the crust, mantle, and core
• Guyots are flat-topped mounds that were once volcanoes
• Fracture zones are large cracks running perpendicular to MOR's (mid-ocean ridges)
• Mobilism: the theory that continents move as sheets due to forces
• Mountains are made by shifting continents
• Frank Bursley Taylor, in 1910, suggested in "Earth's Plan" continents move as sheets by centripetal or tidal forces
• Alfred Wegener proposed the concept of Continental Drift in 1915
• Evidence for continental drift was found in matching rock types and fossil evidence from multiple continents
• Plate tectonic theory was revived due to Marie Tharp's work
• Mary Tharp provided the map of the ocean floor, revealing mid-ocean ridges and trenches which provided evidence for seafloor spreading and supported the theory of continental drift
• Mary Tharp used geophysical data and located mid-ocean ridges

Key steps in Developing Plate Tectonic Theory:

• 1600: Bacon suggested continents fit together
• 1858: Snider Pellegrini proposed an ancestral continent
• 1915: Wegener introduced continental drift and Pangea
• 1962: Hess presented seafloor spreading
• 1968: Wilson formalized the plate tectonics concept
• Identical animal fossils found on the coasts of North America and Europe suggest a continental drift

Finding Identical Fossils on Different Continents Suggests:

• Continental drift means the continents were once connected
• Similar environments mean a shared ecosystem
• Geological history supports the theory of Pangea
• Erosion and sedimentation allowed fossils to survive across distances
• Tuzo Wilson's work in 1963 led to the development of the plate tectonic theory in 1968 with the United Plates
• Arthur Holmes proposed convection currents in 1928 moved the continents
• Magma is molten fluid rock, a mix of fluid and crystals
• Paleolatitude measures the inclination of magnetite
• Edward Bullard's, in 1965, computer reconstruction confirmed the ‘fit' of modern day continents
• Harold Hess introduced the idea of seafloor spreading in 1961
• Seafloor is created at ridges and recycled into the mantle at trenches
• Trenches are formed by plate subduction, where one plate is forced under another, destroying the plate
• Ridges are created at mid-ocean ridges where plates pull apart, allowing magma to rise and form new oceanic rust
• The hypothesis of old seafloor spreading occurs as a result of old seafloor being pushed away and off of the ridge by new seafloor
• Subduction: plates are destroyed, creating earthquakes that dip into the interior
• Recognition of subduction was made by Benioff Zones
• Mid-ocean ridges create plates
• Magnetometers can be used to detect submarines and find the magnetic signature of rocks
• Transform faults: allow spreading either side of sinuous mid-ocean ridges by diving a ridge into segments
• Pangaea split to form new oceans between continents
• Earth’s magnetic field records magnetic reversals within rocks
• Magnetic reversals occur every 500,000 years on average
• Vine-Matthews-Morely hypothesis explains magnetic stripes found on the ocean floor

Main Features of the Mid-Atlantic Ridge:

• A divergent boundary where new oceanic crust is formed.
• Plates are created at ridges
• Seafloor spreading
• Magnetic stripes indicate seafloor age.
• Segmented and sinuous structure
• Transform faults allow spreading on either side
• Seafloor is recycled into the mantle at trenches
• Mapping (geophysics), drilling and analyzing paleoclimate and paleomagnetic data are all methods to study the ocean floor
• Ice-rafted boulders impact geological formations by eroding landscapes as they move
• Ice-rafted boulders create raised beaches, altering coastlines, indicating past glacial activity, and climate changes
• Ice-rafted boulders can contribute to uneven land rebounding
• Evidence for plate tectonics comes from fitting continents and from seafloor spreading
• Mantle plumes are upwellings of hot rock from the mantle that create hotspots

Challenges When Mapping the Ocean Floor:

• Depth and pressure
• Limited access to remote area
• Complete terrain and features
• Significant costs
• The supercontinent cycle highlights the cyclical formation and breakup of supercontinents over approximately 500 million years and was proposed by John Dewey in 1974
• Mohorovicic discontinuity marks the boundary between the crust and mantle
• Ted Irving provided paleomagnetic data and apparent polar wander curves to confirm continental drift
• Presence of similar rock formations across distances indicates continental drift and historical landmass connection
• Ice sheets influence erosion and sediment transport
• If two sites share similar and overlapping polar wander curses, this indicates both sites have shifted around the globe, while remaining in close contact for the past 100 million years
• Earthquakes happen in the descending slab until it gets too warm to rupture in subduction zones
• Divergent plate boundaries are where plates move AWAY (ex. Mid-ocean ridges, continental rifts), which produces oceanic crust
• Convergent plate boundaries move TOWARDS; tectonics are destroyed at this margin
• Oceanic crust converging with oceanic or continental crust creates subduction zones
• Continental crust converging with continental creates obduction zones
• Transform plate boundaries: plates slide past each other
• A passive continental margin is not a plate boundary
• Approximate velocity of Pacific Plate → Nazca plate : 40 mm per year
• Crust: the outer layer of relatively silica-rich rocks
• Isostasy: the vertical buoyancy of Earth materials based on their density
• Icebergs floating on water is an example of isostasy
• The Continental crust doesn't end at today's coastline, but acts like a blanket preventing heat from the mantle getting to surface
• Has lots of radiogenic elements that generate heat especially where there's old and thick crusts
• Continents vs plates: Both oceanic and continental crust can coexist within the same plate

Continental Lithosphere vs Oceanic Lithosphere:

• Continental crust: 8km thick of low density, mostly felsic rocks
• Oceanic crust: 10km thick of high density mostly mafic rocks

The Upper Mantle:

• Asthenospheric mantle: plastic or ‘partially melted'; 200km thick
• Lithospheric mantle: rigid 'plates'; 100 km thick
• The measuring of the size of earthquakes: the Richter Scale is a logarithmic scale based on amplitude of deflection on a seismogram
• Moment magnitude: based on actual energy released, where Energy released is 32 times greater with each increase in magnitude

Body Seismic Waves:

• Both wave types refract when traveling across an interface, reflect off surfaces, and change velocity depending on the density of the material

P-Waves:

• Primary or compressional
• Faster
• Vibrations are parallel to the axis of motion
• P-Waves can travel thru liquid

S-Waves:

• Shear or secondary
• Slower
• Vibration is perpendicular to the axis of motion
• S-waves cannot travel thru liquid
• Lithoprobe: creating synthetic earthquakes
• Vibroseis trucks: is used to create dancing elephants (seismic waves)
• Sampling mantles: Mantle xenoliths are very rich in olivine and peridotite, which gives it its green colour
• Mantle is made of silicate minerals like the crust, more riched in iron and magnesium (ultramafic)
• Sampling core of Earth: Meteorites provide clues to planetary cores and mantles
• The Moho signifies the crust-mantle boundary, being a seismic discontinuity caused by a change in composition and density
• In deep earth, the velocity of seismic waves reflects density and state (solid vs liquid)
• Seismic tomography shows that the Earth's deep interior is more complex than the "onion skin" model
• Seismic tomography shows how plates form to drive mantle convection
• Plates show mantle plumes, ultra-low velocity zones, slab avalanching, and decompression melting
• Humans are likely to notice surface seismic waves, only
• Most of the earth is mantle

• Earthquakes are waves
• Wave refraction causes seismic waves to bend as they travel thru media of changing density
• Density and composition also affects the velocity of the seismic wave
• Continental Shelves tend to be widest on passive margins adjacent divergent plate boundaries
• Continental Shelves form on the transition from Continental to Oceanic Crust
• The Wilson Cycle describes how oceans open and close

Igneous Rocks and Silica:

• Magma is generated from melting of rock
• Convergent margins are felsic (silica rich)
• Andesite is associated with stratovolcanoes
• Divergent margins are mafic (iron rich and silica poor)
• Basalt is typical of oceanic crust
• Continental rifting and the creation of passive margins

Stages of Continental Rifting and Creation of Passive Margins:

• Embryonic: Triple junction, thermal doming above plume heads, failed rifts, flood basalts
• Juvenile: Early shelves and incipient oceanic crust
• Shelves contain most of the world's oil, gas, and world fisheries

Plate Reconfiguration:

• Red sea opening pushes the Arabian plate north toward Europe along the Dead Sea fault zone
• Dead Sea fault: a transform plate boundary between faster moving Arabian and African plates
• It is an endorheic basin
• Salt diapirs: Salt has moved upward thru overlying layers of rock, are common w/in continental shelves
• Aretet: wall offset by transform slip along plate boundary between African plate and Arabian plate
• Case Example: East African rift including Ethiopia, Red Sea, Jordan and Turkey

Development of "Passive Margins" of Continents:

• Stage 1: East African Rift with continent under extension; the crust is thinned and forms a rift valley
• Stage 2: Red Sea with Continents tearing in two where Continent edges are faulted and uplifted and Basalt eruptions from oceanic crust

Formation of the Red Sea:

• Red Sea: a juvenile basin as it's warm, sheltered, biologically active, and full of minerals
• Stage 3: Atlantic Ocean with continental sediments blanket the subsiding "passive" margins to form continental shelves; the ocean widens and a mid-oceanic ridge develops
• Rock is a naturally formed, consolidated material composed of grains of one or more minerals
• A mineral is naturally occurring, inorganic, crystalline solid that has a specific composition, which are the building blocks of rocks
• Silicate minerals are the most important group of minerals
• Continental crust is intermediate or felsic, made of feldspar and silica
• Oceanic crust is mafic, made of magnesium and iron (ferric) with <50% silica
• Mantle is ultramafic with less than 40% silica

Rocks Melt into Magma:

• They undergo decompression
• They are heated (rare)
• They get wet (flux-induced melting)
• Melting is part melting where more silica rich minerals usually melt before silica-poor minerals
• Magma is more silica-rich than the melting rock
• Wet magmas are higher in silica and more viscous than dry magmas
• High viscosity magma is light in color and low in density due to silica
• Less viscous magma is dark in color and high in density do to very low silica
• Magma is Crystal mush with dissolved gas held in by pressure
• Igneous rocks: either cool at earth's surface or underground and are the product of cooled magma
• Triple junction forms where mantle plumes weaken the crust
• An aulacogen is the failed arm of a three-way rift
• Horsts and grabens describe the structure within a rift
• A mid-ocean ridge is found in juvenile ocean Basins
• Low viscosity lava creates shield volcanoes, and are fairly safe
• High viscosity lava creates stratovolcanoes and is dangerous/explosive
• A passive margin is the edge of a continent that gently meets the ocean.

Rate of Seafloor Spreading is Controlled By:

• Slab pull: one control on the rate of seafloor spreading
• Subducting older oceanic lithosphere exerts greater tension on the mid-ocean ridge compared to subducting younger oceanic lithosphere
• Ridge push: the control on the rate of sea floor spreading thermal uplift on the Mid ocean Ridge causes gravitational sliding of the plates off of the ridge
• Rate of spreading gives slightly different ridge morphologies.
• Fsdt is a board shape
• Ways and locations to study mid-ocean ridges and seafloor spreading

Study of Mid-Ocean Ridges and Seafloor Spreading:

• Direct observation with submersibles
• Study of ancient MORS (Oophiollites) – which resembles green skin that looks like snake remnants of ancient oceanic crust and mid-ocean ridges now exposed on land due to ocean closure
• Rocks become more greenish with age
• Ophiolites are chunks of ocean floor that get thrust up onto land, accretion of abducted ocean crust
• Iceland where the mid-atlantic ridge is exposed on land
• The Iceland plume has lifted the mid-atlantic ridge above sea level and is responsible for breaking up Greenland from British Isles 40 mya and releasing flood basalts
• The Mid atlantic ridge on Iceland is the Reykjanes ridge, which is moving to the northwest away from Iceland plumes

More of the Mid-Atlantic Ridge on Iceland

• Sheeted dikes form where basalt is repeatedly injected
• Pillow lavas form where lava erupts underwater
• Layered Gabbro forms where basaltic magma cools slowly in an axial magma chamber Building OCeanic crust of mid ocean ridges

The Onion Model for Oceanic Crust of Mid-Ocean Ridges:

• Pillow basalts erupted on the surface
• Intrusion of sheeted dikes occur at depth
• Settling of gabbro from magma chambers.
• Lakagigar is the site of a much larger fissure eruption
• Eruption of Lakagigar in 1783 lead to:
• Half of Iceland's livestock dying
• Famine among the human population with 69% of humans perishing
• Destruction of 21 villages
• Successive lava flows thicken the oceanic crust. The Eyjafjallajokull volcano sits underneath a small ice cap, making it dangerous
• Katla: erupted over 20x over the past 1,100 years spreading to Scotland
• Volcanic ash is shattered rock and are dangerous for plane safety
• Tephra is anything thrown out of a volcano (eg. Ash bombs)
• Fjallsjokull is the outlet glacier of Vatnajokull Ice Cap
• Jokulhaups are large floods caused by volcanic eruptions under Vatnajokull ice cap
• Tuyas are flat-topped Moberg mountains resulting from volcanic eruptions under ice caps.
• The entire ocean floor is draped in sediment from these locations:
  • Primary productivity
  • “Terrigenous" sediment
• A Phreatomagmatic: eruption occurs under ice sheets involving steam and magma
• Columnar basalt are rocks that aren't erupted under water, but form when lava cools slowly creating hexagonal columns
• Absolute plate motion (APM) vs relative plate motion (RPM)

Plate Motion Types:

• APM: Describes the movement of the tectonic plate relative to a fixed reference point such as a stationary mantle plume
• RPM: Describes how one plate moves compared to another plate
• Iceland is currently being pulled apart in a north-south direction
• The Afar triangle: separates African, Somali, Arabian plates
• There are 3 ways plates interact with each other around the Pacific Rim:
• Ocean crust slides against ocean crust, which creates island arcs Japan, indonesia
• Ocean crust moves against continental crust generating magmatic arcs West coasts North/South America
• Oceanic or continental crust plates simply slide past each other ex. San Andreas fault, California
• The angle of subduction causes underthrusting or rollback
• Hazards produced by Arc volcanoes
  • Sector collapse
  • Lateral blast
  • Ash clouds
  • Pyroclastic flows from Nuee Ardente
  • Ignimbrites caused by Megathrust

More Hazards caused by Arc Volcanoes

• Earthquakes and Crustal Flexure subsided and uplifted crust which creates varied tectonic geomorphology.
• Japan and Aleutian Islands are affected by Tsunamis and soil liquefaction during arc volcanoes
• Transform faults: creeping and locked segments ex. The San Andreas Fault
• Magmatic Arc Volcanoes The Andes
• The tethys ocean and alpine-himalayan mountain belt are a result of orogenesis in high areas and obduction
• Crustal shortening in the swiss alps is due to volcanoes of Italy
• Plate indenters and the escape of tectonics
• The Himalayas are a common plate indentor
• Vertical ash clouds: viscous lava traps a gas that dissolves and explodes
• Japan consists of 3 plates and is the home of the “Megathrust Earthquake.”
• Tsunamis: movement along fault lines / crustal flexure
• Major earthquakes can also be triggered: collapsing accretionary wedges
• Earthquake location: the detection of 'seismic gaps' around the Pacific Rim
• Cascadia Subduction Zone includes three microplates forced under North America.
• Mount Rainier is a classic Cascade Range stratovolcano.
• Effects of earthquakes depend on its location
• Liquefaction: happens when soft sediment starts to behave like a liquid, when grains become 'supported' by water
• To measure the effects of an earthquake, Mercalli scale = subjective, based on a damage survey
• Pacific Ring ocean closure is a continuation of a process that started in the Alpine-Himalayan Mountain Belt
• The Tethys is a dead ocean that closed during the breakup of Pangea
• Oceans close by subduction until continents come together to be a suture
• Orogeny/Orogenesis is the processes of the building of mountains
• A mountain range itself is an orogen
• Alpine-Himalayan Orogeny records the closure of the Tethys ocean and crumpling together of former ocean rocks

Eruption Types

• Strombolian eruption: the mild fountaining of lava.
• Remember these 5 plate tectonic settings:
• When plates carrying light, low density continental crust collide, they form a crumple zone marked by high mountains and foo oceans that died as continents collided
• When a plate of oceanic crust collides w a continent, the heavier oceanic plate is driven into a subduction zone forming an oceanic crust,
• When 2 plates of oceanic crust collide, the heavier (older) one is pushed down forming an arc of volcanoes
• A transform plate boundary is a setting where two plates simply slide past each other

Tectonic Settings:

• Divergent plate boundaries create continental plates that are being torn apart by rifting, resulting in a young ocean as it thins overtime
• Volcanoes in collisional zones are found on the overriding plate
• The oceanic lithosphere ages is affected by depth
• Associated w volcanoes above subduction zones can cause:
  • Ash with a high angle of decent
  • Stiff lava and steepsided shapes
  • Gas to dissolve and move
  • Lateral and steep blasts can lead to Lahar

Aspects of Orogenys

• Ophiolites, nappes and overthrusts
• Tranform faults and metamorphic rock
• Granite Rock
• The Tethys was the predecessor to the Mediterranean The "zipping" up of the Tethys ocean continues today at Indonesia
• The Tethys started to close because of the opening of the Atlantic If a mid ocean ridge is subducted underneath continental crust: the continental crust rises
• The Matterhorn isn't made of African rock
• Strombolian eruption style: small magma release
• Obduction: the overthrusting over of continental crust
• Kathmandu is under threat from liquefaction from physical displacements caused by earthquakes

Volcanic Eruptions:

• Plinian eruption style: large explosive events with huge amounts of ash
• Mineralogy is the study of minerals as a discipline while minteral exploration examines ores and prospecting to develop and grow resources as apart of the green economy
• Rock cycle: sedimentary, igneous, and metamorphic
• Rock: a naturally formed
• Mineral: a naturlly occuring substance

Identifying Minerals:

• Identifying minerals takes place by observing their color and their streak
• Crystalloraphy is the analysis of crystal shapes
• Mineral cleavage: the shape of mineral when broken
• Tendency for minerals to break in regular plain
• Lustre: the way light is reflected
• Viteous lustre: reflects glass
• Hardness: scratchability
• Non-silicate minerals: Carbonates, evaporites, ores
• Ores: are th foundation of the mineral resorsce industry

Mineral Concentrations

• Metails concentrate in older rocks
• Magma composition changes during with crystallization
• Melting is always partial
• Melt is siliceous rich
• The hotter the temperature crystallizes mafic Sedumetry rocks are a result of breakdown of surface rocks: ex. transportation
• Modes of transportation: Sediment and Water
• Sedimentary structures: Evidence of surphace processes
• Ichofossils: evidence of life, trace fossil
• Chemical weathering: breakdown of minerals by water
• Avaporites: formed in sea water
• Metamorphic: product of alateration
• Analongy: can only turn to ice once
• Contact metamorphosis: marble
• Regional: canbilite
• Hydrothermal: serpiintite
• Gneiss: is a high grade metamorphic
• Mylonte: product of intense shear
• Migmalite: once a gneiss turns to melt

Explore magma types at convergent boundaries and stratovolcanoes. Investigate continental rifting stages and the Dead Sea fault zone. Also, learn about passive margin development and salt diapir formation.