Chem 1103: Lecture 6: The Origins of Ocean Basins: Chap. 3
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Density
Pressure
 P=gh
 Isostacy - Balance of an object floating on a fluid medium
Differences between continents and ocean basins
 Elevation of earths surface display bimodal distribution with 29% above sea level and
must of the rest at 4-5 km below
 Continental crust
 Granite
 Light color
 Low density 2.7-2.8 g/cm3
 Igneous rock
 Rich in aluminum, silicon, and oxygen
 Continents are thick 30-40 km and have low density and rise high above the supporting
mantle
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 Ocean basins
 sea floor deeper than 2000 m
 Basalt
 Dark colored
 Higher density 2.9 g/cm3
 Volcanic rock rich in silicon, oxygen, and magnesium
 Sea floor is thin 4-10 km has greater density and does not rise as high
 Moho – boundary below rocks of crust and mantle (denser than 3.3 g/cm3)
Plate movement – use foam blocks
o Tension
o Compression
o Shear
 Continental Drift
o Based on fit of continental outlines, fossil and geologic evidence
o Alfred Wegener – proposed continents are part of super continent – Pangea
o His theory of continental drift was rejected because oceanic crust is too dense and
strong for continents to move
o But he was right
 Sea floor spreading
o Sea floor moves apart at oceanic ridges and new oceanic crust is added there
o Rift valleys
 Along the oceanic ridge crests indicate tension
 Bounded by normal faults
 Floored by recently erupted basaltic lava flows
 Axis of the ridge is offset by transform (strike-slip) faults which produce
lateral displacement
o Continental mountains indicate compression – squeezing land together
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Geomagnetic field
o Magnetic field of the Earth
o Magnetometers detect and measure the Earth’s magnetic field
o Variations in magnetic field (weaker or stronger) occur in the rocks are called
magnetic anomalies
o Magnetic anomalies can be measured with magnetometers
o Magnetic anomalies and the types of rocks causing them form parallel bands
arranged symmetrically about axis of ocean ridges
o As basaltic rocks crystallized, 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
o Periodically Earth’s magnetic field polarity reverses poles
o Field polarity as it is today is called normal polarity. North pole is positive and
South pole negative
o Opposite polarity is called reverse polarity poles are opposite
Because of their paleomagnetism, rocks of the sea floor influence the magnetic field
recorded by magnetometers
o Rocks on the sea floor with normal polarity paleomagnetism locally reinforce
Earth’s magnetic field making it stronger and producing a positive anomaly
o Rocks on the sea floor with reverse paleomagnetism locally weaken Earth’s
magnetic field, producing a negative anomaly
o Rocks forming at the ridge crest record the magnetism existing at the time they
solidify
 The next rifting divides them with part being added to each side of the
widening sea floor
 This produces symmetrical bands of magnetic anomalies across the ocean
basin
o Sea floor increases in age away from the ridge and is more deeply curried by
sediment because sediments have had a longer time to collect
o 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 the sea floor and measuring its distance
from the ridge crest
o Continents are moved by the expanding sea floor
Global Plate Tectonics
o Because the Earth’s size is constant, expansion of the curst in one area requires
destruction of the crust elsewhere
 Currently, the pacific ocean basin is shrinking as other ocean basins
expand
 Destruction of sea floor occurs in subduction zones
 Seismicity is the frequency. Magnitude, and distribution of earthquakes
 Earthquakes are concentrated along oceanic ridges, transform faults,
trences and island arcs
 Tectonism refers to the deformation of Earth’s crust
 Benioff Zone is an area of increasingly deeper seismic activity, inclined
from the trench downward in the direction of the island arc
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Subduction is the process at a trench whereby one part of the sea floor
plunges below another and down into the asthenosphere
 As the rocks scrape past each other, they generate earthquakes
 In the asthenosphere the sea fl.oor melts and the molten material
rises, melting through the overlying plate, forming the andesitic
volcanoes of the island arc
o Earth’s surface is composed of a series of lithospheric plates. Plate edges extend
through the lithosphere and are defined by seismicity
 Plate edges are trenches, oceanic ridges and transform faults
 Seismicity and volcanism are concentrated along plate boundaries
 Movement of plates is caused by thermal convection of the “plastic” rocks
of the asthenosphere which drag along the overlying lithospheric plates
 Mantle plumes originate deep within the asthenosphere as molten rock
which rises and melts through the lithosphereic plate forming a large
volcanic mass at a “hot spot”
 “hot spots” are in the interior of a plate, not at the edges
 plate motion progressively moves the volcanic mass away from the
mantle plume and a new volcano develops above the “hot spot”
 Gradually a linear series of volcanoes from indicating past
direction of plate motion
 Only volcanoes closest to the “hot spot” are still active
o Wilson cycle refers to the sequence of events leading to the formation, expansion,
contraction, 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 sections
diverge
 Mature – broad ocean basin widens, trenches develop and
subduction begins
 Declining – subduction eliminates much of the sea floor and
oceanic ridge
 Terminal – last of the sea floor is eliminated and continents collide
forming a continental mountain range
Fault Geometry
o San Andreas Fault is a transform fault separating two segments of a ridge, one in
the Gulf of California and the other far to the northwest
o The fault is a complex system of faults which branch outward as a network
o Lateral motion along the fault system can locally result in compression or tension
causing the land to buckle upward or drop downward, respectively
o Southern California and Baja California are moving northwestward and will
become detached form North America in about 15 million years
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Lecture 6: Chem 1113
Ex 3. Fig 3.7 – caption is reversed
Do a couple of plate boundaries
 Arabian
 Afghanistan
 India
 Red Sea
Bring:
1. Basalt and granite and scale
2. Plate tectonics foam
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