Transcript The Origin of Ocean Basins

Chapter 3
The Origin of
Ocean Basins
©2003 Jones and Bartlett Publishers
Introduction
•Last week: Physiography of ocean floor- what it looks like.
- Earth’s compositional & physical structure.
- bathymetric provinces: continental margins, deep-ocean basins,
and mid-ocean ridges.
- properties of crustal material (basalt vs granite).
- isostasy.
- Methods of probing sea-floor: seismic reflection, refraction, etc.
•This week: Sea-floor spreading and global plate
tectonics- how the ocean floor (and continents) got
this way.
Present Day Earth
Early theory
3-1 Continental Drift
• German meteorologist Alfred Wegner 1915
• Geologic and paleontological evidence
– Continuity of rock and structures
– Continents seemed to fit together
– Similar fossils on opposite continents
Wegner: Pangaea 200 to 300 Millions of
Years Before the Present
3-1 Continental Drift
• Continental drift proposed as hypothesis
– Supercontinent of Pangea
– Continental crust ‘plowed through’ basalt
– Fresh basalt extruded in widening gaps
• Problems with Wegner’s hypothesis: granite
cannot displace basalt!
– Granite less dense than basalt
– Granite r = 2.7 – 2.8 g/cm3
– Basalt r = 2.9 g/cm3
3-2 Sea-Floor Spreading
Geologists in the 1960’s Harry Hess (USA), Fred
Vine and Drummond Mathews (Great Britain).
Sea floor spreading demonstrates that the sea
floor moves apart at the oceanic ridges and new
oceanic crust is added to the edges.
Let’s look at physical evidence that lead to
development of this hypothesis.
3-2 Sea-Floor Spreading
3-2 Sea-Floor Spreading
3-2 Sea-Floor Spreading
heat
3-2 Sea-Floor Spreading
Deep
Sediment
layer
shallow
Sediment
layer
3-2 Sea-Floor Spreading
3-2 Sea-Floor Spreading
• Whereas oceanic ridges indicate tension, continental mountains
indicate compressional forces are squeezing the land together.
– Examples: Appalachians, Rockies, Alps and Himalayas.
Sedimentary Rocks Squeezed by Compression
3-2 Sea-Floor Spreading
The
geomagnetic
field is the
magnetic field of
the Earth.
3-2 Sea-Floor Spreading
• Magnetometers detect and measure Earth’s
magnetic field.
3-2 Sea-Floor Spreading
• Moving across the ocean floor perpendicularly to the oceanic
ridges, magnetometers record alternating strong (positive) and
weak (negative) magnetic measurements (called magnetic
anomalies) in response to the influence of the sea floor rocks.
3-2 Sea-Floor Spreading
• Magnetic anomalies form parallel bands arranged
symmetrically about the axis of the oceanic ridge.
3-2 Sea-Floor Spreading
• As basaltic rocks crystallize, some
minerals align themselves with Earth’s
magnetic field, as it exists at that time,
imparting a permanent magnetic field,
called paleomagnetism, to the rock.
• Periodically Earth’s magnetic field
polarity (direction) reverses poles.
Geomagnetic Polarity
Reversals
History of Geomagnetic Polarity Reversals
3-2 Sea-Floor Spreading
• Rocks forming at the ridge crest record the
magnetism existing at the time they solidify.
3-2 Sea-Floor Spreading
• Sea floor increases in age and is more deeply buried by
sediment away from the ridge because sediments have had
a longer time to collect.
3-2 Sea-Floor Spreading
• Rates of sea-floor spreading vary from 1 to 10 cm per year for
each side of the ridge and can be determined by dating magnetic
anomaly stripes of the sea floor and measuring their distance
from the ridge crest.
3-3 Global Plate Tectonics
• Because Earth’s size has not changed,
expansion of the crust in one area requires
destruction of the crust elsewhere.
- Where and how is crust being destroyed?
- let’s look at the evidence.
3-3 Global Plate Tectonics
• Seismicity is the frequency, magnitude and
distribution of earthquakes.
3-3 Global Plate Tectonics
• Subduction is the process at a deep-sea trench
whereby one part of the sea floor plunges below
another and down into the asthenosphere.
3-3 Global Plate Tectonics
• Benioff Zone is an area of increasingly deeper
seismic activity, inclined from the trench
downward in the direction of the island arc.
South Figi Basin and Cross Section
Showing Benioff Zone
3-3 Global Plate Tectonics
From: Pinet, Table 2.2 (Chapt. 2).
ROCK TYPE
TEXTURE
DENSITY
ASSOCIATION
(g/cm3)
Granite
Coarse
2.7 – 2.8
Continental
Crust
Basalt
Fine/coarse
2.9
Ocean crust
Andesite
Fine
2.8+
Volcanic arcs,
Andean type
mts.
3-3 Global Plate Tectonics
3-3 Global Plate Tectonics
3-3 Global Plate Tectonics
Movement of plates is caused by thermal
convection of the “plastic” rocks of the
asthenosphere which drag along the overlying
lithospheric plates, and gravity which pulls
submerged plate downward.
Driving Mechanisms for Plate Motions
3-3 Global Plate Tectonics
3-3 Global Plate Tectonics
• Mantle plumes originate deep within the
asthenosphere as molten rock which rises
and melts through the lithospheric plate
forming a large volcanic mass at a “hot
spot”.
Mantle Plume
3-3 Global Plate Tectonics
3-3 Global Plate Tectonics
The Wilson Cycle
Rift valley forms as continent begins to split. Example:
Afrcan Rift Lakes.
Sea-floor basalts begin forming and continents diverge.
Example: Red Sea.
Broad ocean basins widen, trenches develop, subduction
begins. Example: Atlantic Ocean.
Subduction eliminates much of sea-floor and oceanic ridge.
Example: Pacific Ocean.
Last of sea-floor is eliminated, continents collide forming
mountain chain. Example: Mediterranean Sea.
Convergence of continental plates and uplifting to form
mountain range. Example: Himalayas.
3-4 Transform Faults
The San Andreas fault in
southern California is a
transform fault that
connects the sea-floor
spreading ridge of the
Gulf of California with
the spreading ridge off
Oregon and Washington.
• If these plate motions
continue, Baja will
splinter off California.
3-4 Transform Faults
Because the San Andreas fault has an
irregular trace, strike-slip motion can
cause local compression or tension.
Fault Geometry
3-4 Transform Faults
3-4 Juvenile Ocean Basin
3-4 Juvenile Ocean Basin
Hot, salty groundwater is dissolving metals from
the rocks and depositing them as metal sulfides
in dense brine pools like the Atlantis II Deep.
Atlantis II Deep
3-4 Collision of continents
3-4 Collision of continents
3-4 Collision of continents
Geophysical2Surveying
END 2-5
OF LECTURE
The Physiography of the
2-2 Ocean Floor
Midoceanic
Ridge Province:
-continuous
submarine mountain
range.
-covers ~1/3 of the
ocean floor &
extends ~ 60,000
km around Earth.
-Features include:
*Rift valley: opposite
sides of ridge pulled apart
form valley in center.
*Transform fault: offset
ridge segments- active.
*Fracture zone: inactive
TF moved out into ocean
basins.
3-2 Sea-Floor Spreading
• TENSION
–Pulling apart, stretching force.
–Examples: seafloor spreading at midocean ridges.
• COMPRESSION
–Squeezing together
–Examples: collision of crust to form
mountains.
• COMPRESSION
3-3 Global Plate Tectonics
• Two groupings of seismic events.
– Along ridges and transform faults
– Along margins of N. & S. America, arc around Pacific Ocean S.
Asian mainland through Himalayas, etc. These areas are called
Subduction zones
3-3 Global Plate Tectonics
• Two groupings of seismic events.
– Along ridges and transform faults
• Shallow, relatively weak earthquakes.
• Seismicity due to volcanism and faulting.
• Region of formation of new lithosphere.
– Subduction zone
3-3 Global Plate Tectonics
• Two groupings of seismic events.
– Along ridges and transform faults
• Shallow, relatively weak earthquakes.
• Seismicity due to volcanism.
• Region of formation of new lithosphere.
– Subduction zone
• Shallow to deep strong earthquakes.
• Seismicity due to Tectonism (deformation, or buckling, folding,
faulting crushing of lithosphere).
• Region of destruction of lithosphere.
• Convergent plate boundary.
3-3 Global Plate Tectonics
Wilson Cycle refers to the sequence of
events leading to the formation,
expansion, contracting and eventual
elimination of ocean basins.
• Stages in basin history are:
– Embryonic - rift valley forms as continent begins to
split.
– Juvenile - sea floor basalts begin forming as
continental fragments diverge.
– Mature - broad ocean basin widens, trenches
eventually develop and subduction begins.
– Declining - subduction eliminates much of sea floor
and oceanic ridge.
– Terminal - last of the sea floor is eliminated and
continents collide forming a continental mountain
chain.
3-4 Juvenile Ocean Basin
Hot, salty groundwater is dissolving metals from
the rocks and depositing them as metal sulfides
in dense brine pools like the Atlantis II Deep.
Atlantis II Deep