GEOS 1004 - Divergent Boundaries

Oceans are the product of…

divergent boundaries

What was the first scientific exploration of the oceans called?

HMS Challenger

When did the HMS Challenger take place?

1872 - 1876

How long was the journey of the HMS Challenger?

127,500 km (79,200 mi)

The HMS Challenger measured…

ocean depths with weighted lines (a few hundred data points for 3/4 of the earth)

Sound waves can measure…

the bottom of the ocean thousands of times per second (allows us to map the ocean in great detail)

Passive Margins: ?

continental shelves, slopes, and rises (all 3 of which are continental crust)

Sedimentation (passive margins process): ?

sediment mixed with the water creates water with a higher density

Marianas-Type (active margins): ?

produces an arc of volcanic islands

Andean-Type (active margins): ?

produces an arc of continental volcanoes

Trenches and Volcanic Arcs (active margins process): ?

continental crust extends to the trench

Thick piles of sediments and high pressure rocks are…

active margins processes

Plains: ?

not much going on (very slow processes)

Highs: ?

islands, atolls, seamounts, guyots (formed from isolated volcanoes, eroded by wave action)

How are Highs formed?

once the volcano dies out, the island begins to erode away: fringing coral reef → barrier reef → lagoon and atoll

Mid-Ocean Ridges: ?

spreading centers

Basic Features of Mid-Ocean Ridges: ?

thin crust = high heat flow; small volcanoes, shallow earthquakes; young rocks at the center, older on each side

At slow spreading rates…

a prominent central rift valley develops along the ridge crest, and the topography of the ridge is typically rugged

Along fast spreading centers…

medial rift valleys do not develop, and the topography is comparatively smooth

Unique Rock Sequence: ?

1 - deep ocean sediments; 2 - pillow basalts; 3 - parallel (sheeted) dikes; 4 - serpentinite (gabbro); 5 - mantle peridotite and related rocks

What is an Ophiolite Suite?

found in mountain ranges worldwide; they make up the ocean floor

As plates separate, buoyant mantle…

partially melts and dikes connect magma chamber to the surface

Massive sulfide ore deposits are found…

in the mountain belts worldwide

How do Volcanogenic Massive Sulfides work?

seawater is heated at the spreading center and precipitates fine-grained metal sulfides (looks like a chimney belching black smoke); could lead to seafloor mining (for these minerals)

True or False: Pre-1950 oceans were thought to be permanent.

true

True or False: There are rocks that are older than the Mesozoic era in the oceans.

false

Continental Rocks: ?

a thick sedimentary veneer over complicated rocks

Oceanic Rocks: ?

thin sediments, straightforward rocks

Oceanic Sedimentation - near continents: ?

fine-grained detrital rocks

Oceanic Sedimentation - away from continents: ?

limestones are in shallow water, cherts are in deep water

Process of Oceanic Sedimentation far away from continents: ?

the shelves are in shallow waters → the shelves are made mostly of calcium carbonate shells → forms limestone; silica shells are only found in deep waters → forms cherts

Process for Rifting and New Oceans: ?

begin as continental upwarps —> upwarps collapse into rift valleys —> rift valleys deepen to admit sea water —> linear seas widen into oceans

Active Rifting - multiple stages shown in East Africa: ?

linear seas, continental rift valleys, early stage upwarps and volcanoes

Failed Rifting: ?

rifting can slow, pause, restart, or stop entirely (continental mass spreads apart, forms a basin, and the basin fills with sediment)

Oceanic plateaus appear similar to _____ _______ on continents (may be tied to ___ ____ _________ and rifting)

flood basalts; hot spot formation

Most intense volcanism follows a…

continental breakup

Ocean Trenches and Seafloor Subduction are largely in the…

Circum-Pacific Belt

Recycling at subduction zones —> ?

creation of spreading centers

Subduction varies —> it depends on…

how long the ocean rocks have cooled (when the plate is still warm, it is more buoyant, and the surface area in which earthquakes will occur is larger) —> may impact the frequency and size of benioff-zone earthquakes

At Subducting Spreading Centers…

plate boundaries change type