Download AIM: Introduce you to scientific study of the world`s oceans and seas

Mid-ocean ridges (MORs)
•Stand 2.5-4 km above abyssal plains, and may
be ~1000 km across
•Axial valleys are 600 m – 2 km lower than ridge
mountains
•Axial valley/ridge mountains abut oceanic
fracture zones
•Numerous volcanoes have quiescent eruptions
•Axial valleys & ridge mountains composed of
basalts in distinctive pillow forms
MORs continued
•Near axial trough, sediments and sedimentary
rocks are thin or absent
•Sediment thickness increases with distance
from axial trough, but never exceeds 1.3 km
•Age of oldest sediments increases with distance
from axial trough
•Sedimentary rocks are cut by faults, but MORs
are not like continental mountains
Oceanic fracture zones (FZ’s)
•Rectilinear (straight), often for 1000’s of km
across ocean basin
•Narrow, usually <100 km and may be <10 km
wide
•Vertical relief across a FZ may be several km
•Axial valleys offest as much as 1000’s of km
across FZ’s
•When FZ’s extend into abyssal plains, they
are usually covered sedimentary rock
Earthquakes in MOR-FZ systems
•Earthquakes occur in
axial valleys, especially
on faults between
valleys and adjacent
ridge mountains
•Earthquakes occur along
fracture zones between
offset axial valley
segments
MOR
FZ
Deep-ocean trenches (DOT’s)
•Arcuate
•>6 km and often ~10 km deep
•Narrow, typically ~5 km wide at base
•Have a asymmetric V shape, with a gentle
seaward slope, narrow base, and steeper slope
rising to volcanic islands or continental
volcanoes
•Volcanoes may explosive, i.e. Mt. St. Helens
or Mt. Pinatubo
Earthquakes near DOTs
Earthquakes occur along an inclined, planar
zone that extends into the earth beneath
the landward side of the DOT
Dipping zone of
seismic activity =
Benioff-Wadati
Zone
Why do continents and oceans differ so
much?
• To find the answer, need to consider the
character of earth’s interior
• In a first pass at examining Earth’s interior,
we note:
–Earth’s interior is warmer than its exterior
–Rock at earth’s interior of earth is denser
than rock at surface
Interior of earth is warmer than
the exterior
•In boreholes and
tunnels, temperature
rises with depth
•Heat flows steadily
from earth’s interior
•Molten rock
originates inside
earth - how?
Rocks inside earth are denser
than rock at surface
•Density of typical crustal rock is low, 2.7 g/cm3
•Average density of earth is moderate to high,
5.5 g/cm3
•Interior of earth must contain high density
material - estimated to be 10-11 g/cm3
•Measurements of Earth’s moment of inertia
confirm that earth’s mass is concentrated near
its center
These two observations led earth
scientists to the hypothesis of isostacy
• Isostacy = floating balance, in which rigid
crust floats on an underlying mantle
• Mantle is more deformable and more dense
than crust
• Crust floats on mantle by
– Airy mechanism - all crust has similar density.
Where elevation is higher, crust is thicker
– Pratt mechanism - crustal density differs. Where
elevation is higher, crust is less dense
What causes differences in average
elevation of continents and oceans?
• Crustal thickness?
– Thickness of continental crust = 25-40 km
– Thickness of oceanic crust = 5-7 km
• Crustal density?
– At surface, continental crustal rocks have low
density (often about 2.6-2.7 g/cm3), but lower
continental crust is more dense
– Aggregate density of continental crust is 2.8 g/cm3
– Aggregate density of oceaic crust is 2.9 g/cm3
• Variation in crustal thickness is more
significant factor in explaining two elevations
Earth is opaque. How can I state
confidently what is the character of
rock at depth? How can one
determine more information about
earth’s crust and its relationship to
earth’s interior?
Earthquakes and seismology