Marine Geology Exam #2

How does water get into the crust? What happens once it enters?

Faulting allows for the water to get into the crust, conduction occurs heating the water.

What is the significance of Chlorine being in magma

evidence that sea water has infiltrated magma (hydrothermal circulation is occurring) 

Define “V, F, PB” for Faulting and Fissuring at Spreading Centers

V: Volcanism (at edge of spreading center)
F: Zone of Active Fissuring  
PB: Plate Boundary

What happens between the PB

• between them there is faulting and movement importantly deformation - active zone

• Only Extensional Faults 

• Fault Initiation 

• Consecutive faults

Fissures

• Vertical at surface 

• Absolute tensile strength

• Average length: 170 m (30-650m) on EPR @ 9N 

• Depth is estimated at 400+-100m 

• Widths typically 0.2-3m 

• Mark where the deformation is beginning

Growth faults

layers grow as the faulting is occurring (thicker on down drop side) 

Faulting and Grabens at Fast Spreading

• Growth Faults

• Normal faults closer to plate boundaries

• Generating faulted crust @ mid ocean ridge -> once they hit PB turn into Abyssal Hills 80% seafloor (slower spreading rate generates high, wider and longer hills), decay with time 

“Domes”

Made of Serpentine -> mantle rock that is brought to the surface, not symmetrical w/ spread "Asymmetric spreading” 50% of oceanic crust along slow spreading ridges is asymmetrical more earthquakes 

Faulting and Grabens at Fast Spreading Formation

• No volcanism = detachment zone 

• Volcanism = core complexes (heat melts and weakens the mantle) 

• Not at Fast -> AMC 

MORs: Cracks and Crevices

• MOR magma supply occurs along vertical magma conduits through which buoyant magma rises to form new oceanic crust

• The thin lines to left/right of the rising magma are extinct magma conduits (not connected to an active magma source) but that are still fractures in oceanic crust

• Extinct conduits, alongside cracks forming due to cooling/accretion of newborn crust, causes the oceanic crust at MOR systems to be highly porous

Sub-seafloor Crustal Aquifer

• At such high pressures, seawater has no choice but to fill every available space and thus fills these cracks and crevices to comprise the sub-seafloor ocean crustal aquifer

  • Water not only moves vertically toward the seafloor surface but also horizontally, especially along ridge axes.

  • Horizontal flow is actually faster than vertical flow, in many cases

• 1-2% of ocean seawater is flowing through the crust at any given time

  • In the sub-seafloor crustal aquifer, seawater is closer to heat source and thus warms compared to seawater

Hydrothermal Fluids

• Hot and thus buoyant (low density), causing them to rise

• Formed from chemically modified seawater, due to hot interactions with rocks, so that they have low oxygen and high metal content

Hydrothermal Vent Formation

Hydrothermal fluids do not physically contact the magma. Instead, they react with hot oceanic crust

“Black Smoke” 

Black color forms when anoxic vent fluid is mixed with oxygenated seawater, causing some of the dissolved iron to precipitate into solid minerals that are black colored

What makes up the chimney material, through which the vent fluid flows?

Sulfide minerals that also precipitate from vent fluid solution upon meeting seawater

Hydrothermal Vents at MORs

• Within the sub-seafloor crustal aquifer, large-scale seawater convection cells are formed that support the production of many hydrothermal vent sites along a single section of ridge axis with favorable heat supply and porosity

• Cold seawater penetrates DEEP (here, 2-3 km!) and returns to surface heated

• The spacing of these cells affected by MOR segmentation/discontinuities

• One hydrothermal vent per ~10 km of MOR axis (on Avg) 

• only 10% of the MOR mountain chain has been explored

Volcanic arc vents

1. At oceanic-oceanic subduction zones, oceanic crust is subducted and melted, which produces a volcanic arc.

2. Potential for shallow magma sources to be present to heat water

3. Need fractures in these volcanic arcs to percolate seawater!

Back-arc spreading centers

At subduction zones, the subducting slab “pulls on the edge of the overlying plate until it splits open” to form a zone of expansion, a back-arc spreading center

1. This spreading center is on the other side of a volcanic arc from the trench

2. Caused by the convection of melted crustal material

Oceanic hotspot vents

1. Hotspot volcanoes also bring the ingredients of hydrothermal vents together: shallow heat source and fractured crust

   1. Many hotspot-associated hydrothermal vents have lower temperature vent fluids (<100ºC)

   2. These volcanic vent systems can be particularly important because they are so shallow, so it is easier for them to affect the critical surface ocean

Terrestrial hotspots

Hotspots are intraplate features, but they can form under continental crust as well (think yellow stone)

Microbiology of Vents

• vents are microbial oases, with 107-109 cells g-1 rock on the outside of active and inactive vent structures

• home to many hyperthermophilic Bacteria and Archaea, as well as acidophiles. hottest growing microbes on record both from marine vents

• controlling factors on what microbes appear where are gradients: 

  • TEMPERATURE, CHEMISTRY and GEOLOGY/GEOCHEMISTRY

Why mine hydrothermal vents

• Vents a major source of polymetallic sulphides, which contain copper, zinc, gold, silver

• Different than ferromanganese nodules on abyssal plain, which contain nickel, copper, manganese, and rare earth metals

Between 1985 and 2011, there were responses to ____ suspected magmatic events; ___ found evidence for eruption.

35, 22

What is the rate of the spreading centers for the MOR eruptions

1.1 to 9.0 cm/yr.

1986 MOR eruption

Hydrothermal event plume detected along the Cleft segment of Juan de Fuca ridge. First young lavas of known age (series of pillow mounds)

Event Plumes (megaplumes)

• Large discharges of chemically distinct fluids that rise 100’s of meters to form discrete, stable eddies. Range in volume from ~1–150 km3.

• 1986 Event Plume dissipated in ~60 days. Heat output equaled annual output of ≤2000 high-temperature vents. 

1991 MOR eruption

In March, divers in Alvin at 9°50’N found newly dead, charred tubeworms and new hydrothermal vents. 210Po age dating suggests 2–4 weeks before arrival. (Uranium has a ½ life of 4.5 million years)

What was used to date the MOR eruption of 1991 and why?

• Time scales return to secular equilibrium after 5x half-life of the daughter nuclide 

• PO 210 to Pb 206 = 2 years (roughly) per ½ life 

• Helps determine if there are multiple lava flows within a shorter time period (where seds may not be present between) 

1993 MOR eruption

Earthquake swarm detected on CoAxial segment of Juan de Fuca ridge. First “response” expedition found event plumes, new hydrothermal vents, and still-cooling lava. In the first month of real-time acoustic monitoring using the U.S. Navy Sound Surveillance System (SOSUS)

1998 MOR eruption

Eruption at Axial seamount and along Juan de Fuca ridge segment partially buried pressure-temperature sensor. First in situ record of lava emplacement. Pressure tells you the height of the sea floor (water pressure) -> once on the floor can help determine sea floor movement. 

2006 MOR eruption

Several Ocean Bottom Seismometers (OBS) would not release from the seafloor at 9°50’N. Response cruises found them trapped in new lava. First time a second eruption was observed at the same MOR location.

“FAMOUS”

French-American Mid-Ocean Undersea Study first ever MOR exploration by manned submersibles (71-74) 

Types of Lava Flows

• Pillow 

• Lobate Flow 

• Ropy Sheet Flow 

• Jumbled Sheet Flow 

• Hackly

3 main types of lava flows (range-ish)

Pillow is bulbous and Sheet is flat sheet, Lobate is in between the two of them 

Pillow Lava

uniquely subaqueous (only form under water) -> waters high heat capacity causes these flows (rapid cooling) -> Fast cooling, low flow, high slope  

 Hyaloclastite

rapid quenching by water

Seamounts vs. Islands

• Seamount: submarine volcanic edifice; can range from tens of meters to several kilometers high. Commonly have steep outer slopes, relatively flat tops, and collapse pits/calderas.

• Island: a topographic high that rises above sea level.

Guyot

Flat-topped submarine volcanic mountain; flat top is associated with erosion near sea level followed by subsidence

Seamount Distribution and Spreading Rate

• Seamounts are not distributed evenly; the lecture shows marked asymmetry in their distribution.

• The number of seamounts >400 m tall increases with spreading rate.

• Faster spreading → greater magma supply → more opportunity to build large seamounts.

• Seamounts near mid-ocean ridges are generally similar in composition to MORB (mid-ocean-ridge basalt).

Faster spreading is associated with greater magma supply from mantle melting beneath the ridge. More magma allows more/larger volcanic edifices to form, so the number of large seamounts increases.

Why Do Seamounts Form?

Hypothesis 1 — “Leaks” from the large melting region:

• Small volcanic eruptions may occur within/near the large melting region associated with a mid-ocean ridge.

• Explains their MORB-like composition and increase with spreading rate.

• Does not fully explain why some seamounts form linear chains.

Hypothesis 2 — Small-scale hot spots:

• Some researchers propose that seamounts are small-scale hot spots, which helps explain linear chains.

Mid-Ocean Ridge Mantle Melting Mechanism

Decompression melting
Mantle rises → pressure decreases → partial melting

Ocean-island hot spot Mantle Melting Mechanism

Hot/upwelling mantle + decompression

Hot plume rises and produces melting beneath the plate

Island Arc Mantle Melting Mechanism

Water-assisted melting
Slab dehydration releases H2O → lowers mantle-wedge solidus

Hot Spot Plumes and Volcanic Chains

• A hot spot plume is a region of anomalously hot mantle rising toward the surface.

• The Hawaii model treats the hot spot as relatively stationary while the tectonic plate moves over it.

• New volcanoes form over the active hot spot; older volcanoes are carried away as the plate moves.

• Therefore, volcanic age generally increases with distance from the active volcanic center.

Hawaii Model — Ocean Island Evolution

Pre-shield: early volcanic activity; alkali basalt.

• Shield-building: major construction stage; very large volcanic volume; dominated by tholeiitic basalt.

• Post-shield: volcanic activity decreases; erosion and subsidence become increasingly important; more

alkaline/differentiated lavas occur.

• Post-erosional / rejuvenation: volcanism can restart after a long inactive interval; the lecture gives ~0.5–3 million

years for some Hawaiian volcanoes.

• Rejuvenation-stage lavas are highly alkalic and geochemically indicate a deep source; some deposits contain garnet-bearing xenoliths.

Pre-shield → Shield → Post-shield → Rejuvenation/post-erosional

Deviations from the Hawaii Model

Not every ocean-island chain behaves exactly like the simple Hawaii model.

age of onset of magmatism does not correlate simply with plate motion.

Do not automatically assume every volcanic chain is a perfect stationary hot spot with constant plate

velocity.

Basic Island-Arc Cross Section

Oceanic plate → trench → forearc → volcanic arc → backarc.

• Trench = major topographic low.

• Volcanic arc = topographic high.

• Forearc lies between trench and volcanic arc.

• Backarc lies behind the volcanic arc and may contain a backarc basin.

Why Does an Island Arc Form?

Subduction → dehydration → H■O enters mantle wedge → solidus decreases → partial melting → magma rises → volcanic arc

• Subducting oceanic crust contains hydrous minerals.

• With increasing pressure and temperature, hydrous phases break down.

• Water is released into the mantle wedge.

• Water lowers the melting temperature (solidus) of the mantle.

• Partial melting produces magma, which rises to form the volcanic arc.

Volcanic Arc Location

• The volcanic arc is NOT directly above the trench.

• There is a forearc region between the trench and the volcanic arc.

• The slab reaches the depths/conditions where dehydration and mantle melting occur farther beneath the overriding plate.

Subduction Angle vs. Volcanic-Arc Width

• Steeper slab → narrower volcanic arc.

• Shallower slab → wider volcanic arc.

• A steep slab reaches the relevant depth over a shorter horizontal distance.

• A shallow slab travels farther horizontally before reaching the same depth.

Earthquakes and the Subducting Slab

• Earthquake locations can trace the geometry of a descending slab.

• A dipping zone of earthquakes can therefore reveal the position and angle of the subducting plate.

Topography Across an Island Arc

• Trench: deep topographic low.• Forearc: relatively low region between trench and arc.

• Forearc: relatively low region between trench and arc.

• Volcanic arc: topographic high.

• Backarc: can become lower where a backarc basin develops.

Gravity Anomaly Across an Island Arc

• A strong negative gravity anomaly is associated with the deep trench.

• Gravity anomaly can become positive around/near the volcanic arc.

• The gravity pattern changes across the trench–arc–backarc system

 Heat Flow Across an Island Arc

• Heat flow is relatively low near the trench/forearc.

• Heat flow is higher toward the volcanic arc/backarc region.

• The forearc is associated with relatively cool material and the subducting slab, while the arc/backarc is associated with hotter mantle and magma generation.

low near trench → increasing toward volcanic arc/backarc.

Backarc Basins

• Backarc basins form behind volcanic arcs through extension.

• A volcanic arc can undergo rifting.

• Rifting can progress to seafloor spreading.

• A spreading center can develop, producing a backarc basin and separating a remnant arc from the active arc.

Sequence: Arc → rifting → spreading → backarc basin + remnant arc

Accreting Margins

Sediment is scraped from the subducting plate. Builds an accretionary prism. 

Non-Accreting Margins

More sediment is carried down with the subducting plate. Sediment subduction/bypass dominates.

Hydrothermal Systems and Massive Sulfide Deposits

• Cold seawater enters permeable oceanic crust and moves downward.

• The fluid is progressively heated as it moves deeper.

• Fluid–rock reactions alter the chemistry; the lecture shows loss of Mg, O, and sulfate and enrichment in metals and

magmatic gases at depth.

• Very hot fluid becomes buoyant and rises through focused conduits.

• When hot fluid mixes with cold seawater, metals precipitate and can form massive sulfide deposits.

Black smoker 

Very hot; lecture diagram shows >350°C focused discharge. Metal-rich fluids; dark sulfide precipitates. 

White smoker

Cooler; lecture diagram shows <300°C.
Different fluid chemistry and mineral precipitation.

Island evolution

The sequence of volcanic construction, erosion, subsidence, and possible reef development that changes an island through time.

Pre-shield stage

The early stage of volcanic growth before the main shield-building phase; eruptions begin constructing the volcanic edifice.

Shield-building stage

The main constructional phase, when large volumes of generally low-viscosity lava build a broad, gently sloping shield volcano.

Shield volcano

A broad volcano with gentle slopes, built mainly by repeated flows of low-viscosity lava.

Low-viscosity lava

Lava that flows relatively easily and can travel far from its vent, helping build broad volcanic shields.

Post-shield stage

A later stage when the main shield-building phase wanes and volcanic activity becomes less voluminous or changes in composition.

Erosion and subsidence stage

The stage when an island is worn down by erosion while the volcanic edifice and oceanic lithosphere sink relative to sea level.

Rejuvenated stage

A later, renewed period of volcanism that can occur after a prolonged interval of reduced or absent activity; it does not occur on every island.

Coral-reef stage

A stage in which reef growth may track the island’s gradual subsidence and erosion, depending on environmental conditions.

Atoll stage

A ring-shaped reef surrounding a lagoon, often associated with the subsidence of a volcanic island after the central island has been greatly reduced or submerged.

Island subsidence

The gradual sinking of an island relative to sea level, including as the oceanic lithosphere cools and the volcanic load settles.

Island erosion

The removal and redistribution of island material by waves, streams, mass wasting, and other surface processes.

Island lifetime

The time an island remains above sea level; the lecture compares lifetime with island area and with the type of sediment/material.

Island area and lifetime

The lecture plots maximum island area against island lifetime; use the graph to assess how island size relates to persistence above sea level rather than assuming a universal lifetime.

Consolidated sediment

Sediment that has been compacted or cemented into a more coherent material; it generally resists erosion better than loose sediment.

Unconsolidated sediment

Loose sediment that has not been strongly compacted or cemented and can be more easily eroded or remobilized.

Phreatic eruption

An explosive eruption driven by heated groundwater or other non-magmatic water flashing to steam; the explosion does not require new magma to reach the surface.

Phreatomagmatic eruption

An explosive eruption caused by direct interaction between magma and non-magmatic water, such as groundwater or seawater.

Key control on phreatic/phreatomagmatic explosions

How water behaves as it is heated, especially its rapid expansion and change in density/phase under the local pressure conditions.

Water density during heating

Water’s density changes strongly with temperature and pressure; rapid heating and expansion can contribute to explosive behavior.

Critical point of water

The temperature-pressure condition above which liquid and vapor are no longer distinct phases; the lecture’s water-property plots emphasize that pressure affects water behavior during heating.

Surtseyan eruption

A shallow-water or shoreline explosive eruption in which magma interacts with water, producing steam-driven explosions and abundant fragmented material.

Magma-water interaction

Contact between hot magma and water can rapidly vaporize water, fragment magma, and generate explosive eruptions.

Volcanic island growth cycle

A conceptual sequence from early volcanic construction through shield growth, waning activity, erosion/subsidence, possible rejuvenation, and later reef/atoll development.

Island-arc sedimentation

The distribution of sediment around an island arc is shaped by volcanic sources, wind, ocean currents, slope failures, and gravity-driven transport.

Volcaniclastic sediment

Sediment made from volcanic material, including ash, pumice, and fragmented lava or older volcanic rocks.

Ash fall

Fine volcanic particles transported through the atmosphere and deposited over land or sea, sometimes far from the eruption.

Coarse ash

Relatively large ash particles that tend to fall closer to their source than fine ash, although wind and eruption conditions affect their distribution.

Main provinces of coarse ash

The lecture’s map illustrates geographic regions where coarse volcanic ash is distributed; interpret it as a pattern of deposition influenced by volcanic sources and transport.

Pyroclastic flow

A hot, fast-moving mixture of volcanic gas, ash, and rock fragments that travels along the ground and can move down volcanic slopes.

Pyroclastic-flow deposit

Material left behind after a pyroclastic flow loses energy and settles; deposits can blanket broad areas and may be thickest along flow pathways.

Sediment gravity flow

A downslope movement of sediment driven by gravity, often when sediment-water mixtures become denser than surrounding seawater.

Turbidity current

A sediment-laden density current that flows downslope underwater and can transport sediment into deep basins.

Debris flow

A gravity-driven flow containing a high concentration of sediment and larger clasts; it can move rapidly down slopes or submarine channels.

Grain size and sediment transport

Coarser or denser particles tend to settle sooner, while finer particles can remain suspended longer and travel farther, depending on flow energy and currents.

Sediment dispersal around island arcs

Sediment is distributed unevenly around arcs by wind-blown ash, ocean currents, volcanic flows, submarine slope failures, and basin topography.

Arc flank

The side of an island arc where sediment can be transported away from the volcanic edifice toward adjacent basins.

Mature basin deposition

Sediment accumulates in an established basin as material is delivered from volcanic, erosional, and gravity-flow processes.

Island-arc splitting

Extension can divide a volcanic arc, producing separated arc segments and a basin between them; the lecture contrasts this with sedimentation in a mature basin.

Backarc basin

A basin behind a volcanic arc that may form through extension or spreading; it can receive sediment from the arc and surrounding margins.