Oceans Flashcards
For Exam 3, May 6th
What are the 3 major zones in the ocean?
1- Continental Margin: the boundary between continental and oceanic crust
2- Deep-ocean Basin: large zone between the continental margin and the spreading ridge
3- Oceanic Ridges: underwater mountain ranges formed by a divergent plate boundary
What defines an active vs passive margin?
Active - an area with lots of tectonic activity, close to a subduction zone or transform boundary
Passive - an area with little tectonic activity, far away from any plate boundary (but closest to divergent)
What are the 3 parts of a continental margin?
Continental…
1- Shelf: thick, gently sloping, wide, shallow flooded extension of the continent
2- Slope: sharp change in slope that marks the change from continental to oceanic crust
3- Rise: gently sloping oceanic material at the base of the slope that accumulates lots of sediments
How do the 3 parts of the continental margin differ in active and passive margins?
Active: smaller continental shelf and abrupt/steep descent of slope into an ocean trench
Passive: larger continental shelf and less steep slope
What are accretionary prisms? How/Where do they form? What are the metamorphic conditions in one?
Sediments scraped off the subducting plate (convergent boundary/active margin) accumulate and form a wedge-shape
Low temp and high pressure creates strange metamorphic rocks like blue schist
What is a turbidity current? What types of features and rocks do they create?
An underwater landslide (caused by storms) that transports great quantities of sediments past the continental slope and onto the rise
Creates deep-sea fans (look similar to deltas or alluvial fans)
Forms turbidites: poorly sorted sedimentary rock with multiple layers of graded bedding, deposited at the bottom of the slope when velocity sharply decreases
Why are ocean trenches deeper in the Pacific Ocean?
More tectonic activity at subduction zones (since this ocean basin is in an older phase of the Wilson’s cycle)
Why are there more islands in the Pacific Ocean?
More tectonic activity means more rising magma and more seamounts, or oceanic volcanoes, which can create islands if they expel magma at a fast enough pace
Forms long linear chains such as the Hawaiian Islands
What are Abyssal Plains and why are they largest in the Atlantic Ocean?
Deep flat ocean bottoms blanked by a thick layer of sediments or turbidites
Largest in the Atlantic because this ocean basin is in an early stage of the Wilson’s cycle, so it’s growing
Why do ridges form at spreading centers?
1- the amount of magma being generated at the rift zone creates a bulge
2- the change in density of the rock as it cools causes it to sink
How does spreading rate influence the shape of the mid-ocean ridge? In other words, how do slow and fast spreading rates look different?
Slower (poorly developed magma chamber): magma doesn’t get as far before it cools and sinks, forming rift valleys and rugged topography
Faster (well developed magma chamber): magma goes farther before cooling and sinking, leading to smoother topography and a less noticeable valley
What are ophiolites?
A preserved land exposure of oceanic crust (which will be mafic) caused by plate tectonics and uplifting or exhuming
What are the general layers of oceanic crust (from top to bottom)?
Unconsolidated sediment, basaltic pillow lava, sheeted dike complex, dense ultramafic rock that marks the top of the upper mantle (gabbro and peridotite)
What is the Wilson’s Cycle?
The birth + death of an ocean basin
Birth: plates pull apart, thinning crust, magma upwelling and long linear faults, fill with water (e.g. Red Sea or East Africa)
Expansion: continue to grow in size (Atlantic)
Death: shrinking in size due to subduction (Pacific)
Why does oceanic crust only date back to about 200 mya?
It gets subducted
What do mantle plumes or hotspots have to do with continental rifting?
Hotter than normal mantle rock will rise and erupt
This concentrated under the supercontinent Pangea because the large land mass traps heat beneath it like a blanket
Why don’t we know much about our oceans?
It’s tough to explore down there because of the intense pressure of the water and the amount of money it takes to fund it
Instead, we focus on zones of interest like convergent plate boundaries and areas with the potential for oil
