Transcript AIM: Introduce you to scientific study of the world's

Rocks of different origins and ages occur in
three fundamentally different geological
provinces
•Mountain belts
•Cratons or shield areas
•Rift systems
– Have paired deep valleys and flanking mountain ranges
– Boundaries between valleys and flanking ranges are
steeply-inclined faults
– Faults accommodate crustal elongation
– Rifts experience extensive igneous activity
– Volcanoes in rift systems are rarely explosive;
volcanism is quiescent
Rocks of different origins and ages occur in
three fundamentally different geological
provinces
•Mountain belts
•Cratons or shield areas
•Rift systems
– Consist of linked collections of deep valleys and moderately high
flanking mountain ranges
– Steeply-inclined faults mark the boundaries between deep valleys
and flanking ranges
– Faults accommodate crustal elongation
– Valleys and flanking ranges are often the locus of extensive
igneous activity
– Active volcanoes in rift systems are rarely explosive; volcanism is
quiescent
Bulk chemical composition of continents
difficult to determine accurately, but we
estimate:
•Continental rock is relatively enriched in
–SiO2 Al2O3
K2O
Na2O
CaO
•Continental rock is relatively depleted in
–
FeO
MgO
Taking the elevation of 1 km below
sea level as edge of continents,
roughly 40% of earth’s crust is
continental
Ocean basins
•Four ocean basins on earth at present
•Ocean floors have a restricted suite of rock
types
–Unconsolidated sediments (clays, oozes, etc.)
–Fine-grained sedimentary rocks
–Extrusive igneous rocks - basalts
–Intrusive igneous rocks - gabbros
–Foundation is serpentinized peridotites, which
are part of the earth’s mantle
Ocean basins
•Four ocean basins on earth at present
– Atlantic, Pacific, Indian, & Arctic
•Find a relatively restricted suite of rock types
on ocean floors
– Unconsolidated sediments
– Fine-grained sedimentary rocks (clays, oozes, etc.)
– Extrusive igneous rocks of a particular chemical
composition - basalts
– Intrusive igneous rocks of the same particular
chemical composition - gabbros
– All are underlain by serpentinized peridotites, which
are part of the earth’s mantle
Ocean floors are so dominated by basalts
that the bulk chemical composition of
oceanic rock is relatively easy to estimate:
•Ocean floor rock is relatively enriched in
– Al2O3
CaO
FeO
MgO
•Ocean floor rock is relatively depleted in
–
SiO2
K2O
Na2O
Oceanic rocks are uniformly younger
than 200 Ma old
While this sounds like an immense length
of time, it is only about 5% of earth’s
history
Unique geomorphologic features in ocean
basins
•Continental shelves
–70-80 km wide regions where water depth
<200 m
–Underlain by thick accumulations of
unconsolidated sediments and young
sedimentary rocks
–Sediment derived from adjacent continent
–Strata lie on a basement of continental rock
–Are the submerged fringes of continents
Unique geomorphologic features in ocean
basins
•Continental shelves
– Typically 70-80 km wide; water depth is typically <200 m
– Often have very thick accumulations of unconsolidated
sediments and young sedimentary rocks
– Detritus or sediment is derived from the adjacent continent
– Sedimentary rocks lie on a basement of regular, continental
rock. Basement may be igneous, metamorphic, or old
sedimentary rocks, depending upon what crops out above
shoreline.
– Continental shelves are the submerged fringes of continents
•Continental slopes
•Continental rises
•Abyssal plains
Unique geomorphologic features in ocean
basins
•Continental shelves
•Continental slopes
–Seaward of continental shelf, water depths
increase from 200 m to 3-4 km
–Average surface slope is 4-5
–Underlain by sedimentary rocks continuous with
those in continental shelves
–Basement still is typical continental rock
Unique geomorphologic features in ocean
basins
•Continental shelves
•Continental slopes
–Continental slopes are found seaward of continental shelves
–Water depths increase from 200 m to 3-4 km (5-10 km along
‘active’ continental margins)
–Average surface slope is 4-5°; slope is very gentle but is still 20x
average dip of continental shelves
–Underlain by young sedimentary rocks that are continuous with
those found in continental shelves
–Basement still is typical continental rock
•Continental rises
•Abyssal plains
Unique geomorphologic features in ocean
basins
•Continental shelves
•Continental slope
•Continental rises
–Seaward of continental rise
–Water depths usually about 3-4 km
–Average surface slope is ~1°
–Underlain by strata continuous with those in
continental slopes
–Basement can be either granites or basalts
and gabbros
Unique geomorphologic features in ocean
basins
•Continental shelves
•Continental slope
•Continental rises
– Found seaward of continental rise
– Water depths usually about 3-4 km (5-10 km along
‘active’ continental margins)
– Average surface slope is ~1°
– Underlain by young sedimentary rocks that are
continuous with those found in continental shelves
– Basement may be either continental rock (granites)
or oceanic rock (basalts and gabbros)
•Abyssal plains
Unique geomorphologic features in ocean
basins
•Continental shelves
•Continental slopes
•Continental rises
•Abyssal plains
– Lie beneath 3.5-5 km of water
– Nearly level, often featureless plains
• Abyssal hills rise <1 km above abyssal plains
• Seamounts rise >1 km above abyssal plains
– Sea floor sedimentary rocks are distinct from those
found near continents
– Basement is basalts or gabbros
Unique geomorphologic features in ocean
basins
•Continental shelves
•Continental slopes
•Continental rises
•Abyssal plains
– Nearly level floors of ocean basins lie beneath 3.5-5 km of
water
– Largely featureless, but with some local relief
– Abyssal hills rise < 1km above abyssal plains
– Seamounts rise > 1 km above abyssal plains
– Underlain by young sedimentary rocks that are distinct from
those found fringing the continents
– Basement is oceanic rock (basalts or gabbros)
Continental shelves, slopes and rises are
dissected by submarine canyons
• Deep, V-shaped valleys
• Some submarine canyons abut or connect with rivers
that flow off continents
–Turbidity currents derived from rivers erode a
continuation of the river
• Some submarine canyons are ‘headless’
–Headless canyons often have periodic spacing
–inferred to where escaping from sediment weakens
sediments and sedimentary rocks, leading to slope
failure
Continental shelves, slopes and rises are
dissected by submarine canyons
• Deep, V-shaped valleys eroded into shelves, slopes
and, to a lesser degree, rises
• Some submarine canyons abut or connect with rivers
that flow off continents
–Sediment-charged river water in turbidity currents flows
down canyon, eroding a continuation of the river
• Some submarine canyons are ‘headless’
–Headless canyons often have periodic spacing along the
continental margin
–Are inferred form where a high water table leads to
weakening of sediments and sedimentary rocks, which causes
slope failure
The remaining geomorphic features
in ocean basins (found in the abyssal
plains) correlate with and correspond
to three distinct geological provinces
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
Mid-ocean ridges (MORs)
•Stand 2.5-4 km above abyssal plains, and may
be ~1000 km across
•Characterized by high relief; steep-sided axial
valleys or troughs are 600 m – 2 km lower than
adjacent ridge mountains
•In plan, straight sections of axial valley/ridge
mountains abut against oceanic fracture zones
•Axial valleys and ridge mountains dotted by
active volcanoes that erupt often, with a
quiescent eruption style
•Axial valleys and ridge mountains composed of
volcanic rock - basalts in distinctive pillow
forms
Mid-ocean ridges (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
Mid-ocean ridges (MORs), continued
•In vicinity of ridge mountains or axial valleys,
rare unconsolidated sediments and sedimentary
rocks occur in thin, flat-lying layers
•Thickness of sediment cover increases with
distance from axial trough, but never exceeds
1.3 km
•Sedimentary rocks may be cut by faults but
are not deformed like sedimentary rocks in
continental mountains
•Age of the oldest sedimentary rock (lying on
basalt) is greater at increasing distance from
axial trough