Practice Exam 2 (marine geo)

A bathymetric map shows a broad, comparatively smooth ridge axis with subdued abyssal-hill relief. Is the ridge more likely fast- or slow-spreading? Give three observations or mechanisms that support your answer.

Relatively fast-spreading. Support with the smooth/broad axial morphology, less pronounced deep axial valley, and

subdued fault-related/abyssal-hill relief. Explain that greater magma supply lets a larger fraction of plate separation be

accommodated by magmatic accretion.

Why are faults and fissures common near mid-ocean ridges? Define the difference between a fissure and a fault.

New crust forms and cools at a divergent boundary, while extension creates fractures. A fissure is an open crack; a fault is a

fracture with displacement.

Draw and label a horst and graben. Which is raised and which is down-dropped?

Horst = raised block bounded by faults; graben = down-dropped block between faults. Label the faults and relative vertical

motion.

Explain how abyssal hills can form and name two processes that can modify them after active faulting ends.

Ridge-related faulting produces horst-and-graben relief as crust forms and stretches. Later modification can include mass

wasting, sedimentation, and occasional off-axis volcanism.

Compare a fast-spreading ridge with a slow-spreading ridge using three morphological differences.

Fast: smoother/broader axial high, less pronounced rift valley, typically subdued fault/abyssal-hill relief. Slow: rugged axis,

prominent axial valley, larger/more prominent fault-related relief and hills.

Why is “there are transform faults” not enough evidence to conclude that a ridge is fast- or slow-spreading?

Transform faults occur in ridge systems generally. The answer needs morphology that discriminates spreading style, such

as axial-valley shape and abyssal-hill relief.

Describe the path of seawater through a hydrothermal system from recharge to discharge.

Cold seawater enters permeable crust, descends and heats near a heat source, reacts with basalt and changes chemistry,

then hot buoyant fluid rises through focused pathways and discharges at the seafloor.

Why do black smoker chimneys form metal-sulfide deposits?

Hot fluids carry dissolved metals and sulfur-bearing components. Mixing with cold seawater changes conditions and causes

minerals to precipitate, building sulfide-rich chimneys/deposits.

Compare pillow lava with sheet flows. What does each morphology suggest about lava emplacement?

Pillow lava forms rounded lobes as lava cools rapidly underwater. Sheet flows are broader and smoother, reflecting lava

spreading in more continuous sheets. Describe shapes and connect them to cooling and flow behavior.

What evidence could be used together to investigate a submarine eruption at a ridge or seamount?

Use the combination presented in lecture: changes in plume/fluids or particles, observations of new lava morphology, and

other monitoring evidence. Explain why each observation supports an eruption or hydrothermal disturbance rather than

claiming one signal alone proves it.

Explain why seamount abundance greater than 400 m tends to increase with spreading rate, according to the lecture.

The lecture links faster spreading with greater magma supply, which can support more seamount construction. Mention

that the observed composition is generally MORB-like.

Contrast decompression melting at a mid-ocean ridge with water-assisted melting at an island arc.

At a ridge, mantle rises and pressure decreases, allowing partial melting. At an arc, a subducting slab releases water into

the mantle wedge; water lowers the solidus and promotes partial melting.

Draw a simple hotspot chain and label the active volcanic center, plate-motion direction, and relative ages of volcanoes.

Show the plate moving over a relatively stationary hotspot. The youngest/active volcano is near the hotspot; volcanoes

become older with distance in the direction the plate has carried them.

Put the Hawaiian volcanic stages in order and give one characteristic of each.

Pre-shield → shield-building → post-shield → rejuvenation in some volcanoes. Pre-shield: early alkalic volcanism; shield:

large-volume construction and tholeiitic basalt; post-shield: waning activity and more alkaline/differentiated lavas; rejuvenation:

renewed highly alkalic activity after a period of inactivity.

What is one limitation of applying the simple Hawaii model to every ocean-island chain?

Not all chains show the same age/plate-motion relationship. The lecture notes that onset ages of magmatism in some

examples do not correlate simply with plate motion.

Draw an island-arc cross-section and label trench, forearc, accretionary prism (if present), volcanic arc, backarc,

subducting slab, and mantle wedge.

Arrange features from ocean toward overriding plate: trench, forearc, volcanic arc, backarc. Draw the oceanic slab

descending beneath the overriding plate, a mantle wedge above the slab, and an accretionary prism at the forearc if the

margin is accreting.

Explain the sequence that causes melting beneath a volcanic arc

Subduction carries hydrous material downward; hydrous phases break down and release water; water enters the mantle

wedge and lowers its solidus; partial melting produces magma that rises to form the arc.

Predict how arc width changes between a steeply dipping slab and a shallowly dipping slab. Explain why

A steeper slab tends to produce a narrower arc because it reaches the relevant depth over a shorter horizontal distance. A

shallow slab travels farther horizontally and tends to produce a wider arc.

Sketch qualitative topography, gravity anomaly, and heat-flow trends across trench → forearc → volcanic arc → backarc.

Topography: deep trench, forearc, high volcanic arc, backarc that may be a basin. Gravity: strong negative anomaly near

trench, becoming more positive toward/around the arc with regional variation. Heat flow: relatively low near trench/forearc and

higher toward the arc/backarc.

How can earthquake locations help infer the geometry of a subducting slab?

Plot earthquake locations and depths in cross-section. Their distribution can outline the descending slab and show whether it dips steeply or shallowly.

Explain how a backarc basin can form and identify the feature that may remain from the former arc.

Extension stretches the overriding plate behind the volcanic arc; rifting may progress to seafloor spreading, creating a

backarc basin. A remnant arc may remain.

Distinguish accreting from non-accreting subduction margins.

At an accreting margin, sediment is scraped from the incoming plate and added to the overriding plate, building an

accretionary prism. At a non-accreting margin, sediment is more likely subducted or bypasses the margin.

Distinguish phreatic and phreatomagmatic eruptions.

Phreatic eruptions are steam-driven explosions caused by heated groundwater and need not eject new magma.

Phreatomagmatic eruptions involve direct interaction between magma and external water, causing explosive fragmentation.

You are shown a photograph of a volcanic deposit. List four observations to make before proposing its origin.

Describe grain size, sorting, layering/stratification, clast shape/composition, and/or spatial position relative to the source.

Then distinguish these observations from the interpretation.

A map shows several overlapping ridge segments. What are overlapping spreading centers, and why should they not automatically be described as stable transform offsets?

They are overlapping ridge-axis segments/discontinuities that can reorganize and interact. Unlike major first-order

transform offsets, they are not necessarily stable long-lived offsets.