Ocean floor:

1
Ocean floor Technological progress has
enabled a better mapping of the seafloor direct
observations and sampling, bathymetry, internal
architecture, structure and geophysical
properties
The discovery made ca 40-50 years ago showing
that oceanic lithosphere is generated by the
magmatic and tectonic activity along the
Mid-Ocean-Ridges is one of the most central
elements of Plate-Tectonics.
The Mid-Ocean-Ridges have a length of ca 60 000
km they are a formidable system with major
importance for the driving force of the plates
and in the enery-budget of the plate-tectonics.
2
Mid-Ocean-Ridges Have these characteristics

• TOPOGRAPHY (1000 km) broad ridges with narrow
• central rifts.

• BASALT VOLCANISITY mostly tholeitic composition

3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)

• SEISMIC ACTIVITY (mainly shallow earthquakes)

6. MAGNETIC ANOMALIES oriented parallel with
the ridges
3
Observations show that the width of a spreading
ridge is proportional with the spreading
velocity, illustrated below (NB scale h/v 1/60)
A spreading centre comprises a rift-valley
between normal faults. The rift is often sharply
defined as a narrow (10-30km) zone. The
lithosphere is at its thinnest above such a
rift over en slik rift, and in many models, the
astenosphere is considered to reach
the seafloor. The crust and lithosphere
thicken away from the rift. This is compensated
by Isostasy and the crust uplifted in the
rift-shoulders.
4
TOPOGRAPHY and gravity along mid-ocean-ridges
provide important information about processes
which are responsible for their formation. The
high topography is a result of thermal expansion
and lower density (?). i.e. a mass-deficiency/volu
me which is isostatically compensated by the
topography (mid-ocean-ridges float high).
Curve-fitting or empirical measurements show
that the ocean depth (D) is D
avt d0 t ocean floor age in million
years d0 water depth at the spreading
ridge ( 2.5 km) a constant 0.336 NB
This equation is purely empirical and says
nothing about the processes.
5
DEPTH D avt d0 t - age depth- d0 2.5
km) constant- a 0.336
What is the water depth above 16 myr old ocean
floor? D 0.336v16 2.5km 3.8 km
The graph shows depth of the ocean as a function
of age out from a ridge. Empirical studies show
that the subsidence follows another curve D
6.4 - 3,2e-t/62,8 when the ocean floor gets older
than 70 myr. Again this is pure curve-fitting and
not quantified from any process. So how Can
this be done quantified using the physical laws?
For t lt 70 mill år --gt D avt d0
For t gt 70 mill år --gt D 6.4 - 3,2e-t/62,8
Havdyp
Alder
6
Depth of oceans (z), ???coeff. of thermal
expansion 3 x 10-5K-1 w- water depth ?- density
or mantle(m) (3.2) water (w) t- time T-
temperature, T11280oC, Ts0oC ?- thermal
diffusivity
1) Density as function of temperature
2) Column A at compensation
3) Column B at compensation
4) (2), (3) and isostasy, see Stüwe p157
5) (4) first term after , finds derivative with
respect to z and wrights into integral and it
gets form which says that water depth depends on
the density sturcture as a function of depth
6) Inserting (1) into (5) where T(z) is the
unknown (determined from heat conduction equatio
n see Stüwe p 96)
7) Inserting heat conduction equation in (6),
which simplifies to
8) and to
9) After taking constanst out of the integrall.
If we introduce the constant n in (10)
10) We can take all the constants out to the
integral and get
11) Integral of the errorfunction is not know the
0 and z but is know for integration with Limit
infinity, it is
12) Substutuing this integral into (11) we get an
expression for the water depth
NB! w at the spreading ridge
13) Which after inserting standard values for all
the constanst give
14) The water depth in oceans is proportional to
the square root of the age and 5.91 times 10-5
7
Mid-Ocean-Ridges Have these characteristics

• TOPOGRAPHY (1000 km) broad ridges with narrow
• central rifts.

• TOPOGRAPHY (1000 km) broad ridges with narrow
• central rifts.

• BASALT VOLCANISITY mostly tholeitic composition

• BASALT VOLCANISITY mostly tholeitic composition

3. HIGH HEAT-FLOW
3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)

• SEISMIC ACTIVITY (mainly shallow earthquakes)

6. MAGNETIC ANOMALIES oriented parallel with
the ridges
8
ELEVATED HEAT-FLOW
It is very hard to obtain accurate heat-flow
measurements! Heat-flow is given in
heat-flow-units defined as milliwatt/m2. A
simplified empirical formula for heat flow (Q)
is Q 473 t -1/2 t - age, t gt 120 Ma Q
33.5 67e -t/62.8 Also a purely empirical
expression, how can we quantify ? Q
k(T1-Ts)????x1/2 k-conductivity
T-temperatures, u rifting rate ? diffusivity
x-distance
9
HEAT-FLOW
Q -k(TdT-T)/dz -k dT/dz (rate of flow
pr unit area up through plate) where k -
thermal conductivity (Wm-1 oC-1) T - temp (T
(z dz) gt Tz z - thickness of plate
T dT
z dz
a
k - thermal conductivity (Wm-1 oC-1)
z
T
Flow of heat

• Consider a small volume of height dz and
  cross-section a
• Change in temperature dT in time dt depends
• Flow og heat across the surface (net heat-flow in
  or out)
• Heat generated in the volume
• Thermal capasity (spesific heat) of the material
• One dimensional heat conduction equation
• r - density, cP - spesific heat,
• A - heat production pr unit time
• Temp is assumed to be function of time and depth
  only, can be expanded to 3-d

10
3-dimentional heat conduction equation
or using differential operator notation
(Laplacianoperator)
Also considering movement of small volume of
material with velocity uz
conduction term production term, advection term
11
NEGATIV GRAVITY ANOMALI (melts)
The free air anomaly gf gobs- glat -
gh where glat g(l) geq(1 asin2l
bsin4l) gh g0(1 - 2h/R) l
latitude The Bouger anomaly gb gf - dgb
dgt gobs- glat gh- dgb
dgter Where dgb 2pGrh (bouger correction) G -
gravitational constant (6,673 x 10-11
m3kg-1s-2) dgt - terrain correction (deviations
from horizontal) h - height r - density
12
NEGATIV GRAVITY ANOMALI (melts)
13
Mid-Ocean-Ridges Have these characteristics

• TOPOGRAPHY (1000 km) broad ridges with narrow
• central rifts.

• TOPOGRAPHY (1000 km) broad ridges with narrow
• central rifts.

• BASALT VOLCANISITY mostly tholeitic composition

3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)

• SEISMIC ACTIVITY (mainly shallow earthquakes)

• SEISMIC ACTIVITY (mainly shallow earthquakes)

6. MAGNETIC ANOMALIES oriented parallel with
the ridges
14
SEISMIC ACTIVITY (mainly shallow earthquakes)
Earthquakes last week Jan- first week Feb, 2004
Fast spreading East-Pasific Rise
Intermediate spreading rate
Mid-Atlantic Ridge and Southeast Indian Ridge
15
Earthquakes along the Mid-Atlantic Ridge near
the Azores
16
Mid-Ocean-Ridges Have these characteristics

• TOPOGRAPHY (1000 km) broad ridges with narrow
• central rifts.

• TOPOGRAPHY (1000 km) broad ridges with narrow
• central rifts.

• BASALT VOLCANISITY mostly tholeitic composition

3. HIGH HEAT-FLOW
4. NEGATIVE GRAVITY ANOMALIES (melts)

• SEISMIC ACTIVITY (mainly shallow earthquakes)

6. MAGNETIC ANOMALIES oriented parallel with
the ridges
6. MAGNETIC ANOMALIES oriented parallel with
the ridges
17
MAGNETIC ANOMALIES
We know that the earth is a magnetic dipol, with
magnetic north and south. The magnetic field
varies in both intensity and orientation, but
over time (105yr) the magnetic poles coinside
with the rotation poles i.e. geographical north
and south poles. Consequently the magnetic Field
is vertical near the poles and horisontal near
equator!
18
Reversals of the magentic field leads to periods
of normal (present) and reverse magnetisation
During seafloor- Spreading, the newly formed
crust will function as a magnet tape-recorder
where the alternating Normal and
reverse Magnetizations will be preserved as
intensity variations
19
The reversals produces periods of normal and
(present) and reverse magnetization, which is
preserved in the geo-record
Reversals may also be studied on land in
volcanic or sedimentary rocks. The reversals may
be calibrated against mot stratigraphy and
radiometric age-determinations and
magnetostratigraphy, is a dating method if the
anomaly-sequence may be identified.
20
The figure shows the theoretical distribution of
anomalies in a spreading ridge where the
introduction of new magnetic material occur in a
zone with width from 0 til 10 km. Even in a
relatively broad volcanic zone there is an
identifiable magnetic anomaly-pattern The
magnetic anomalies are among the best evidence
for seafloor spreading. It is hard to explain
this pattern in other ways, and there is no
other physio-chemical process than reversal that
can explain the change in polarity.
21
The tectono-magmatic processes along spreading
ridges gives a relatively uniform architecture
of the oceanic lithosphere in time and space.
Supra-custals, (basalts and sediments)
Sheeted dyke complex
Isotropic varied textured gabbros
Layered gabbros
Ultramafic cumulates
Ultramafic mantle tectonites
Magma composition Tholteitic MORB (Mid-Ocean-Ridg
e-Basalt) Formed by relatively high degree of
partial melting at relatively shallow level in
the astenosphere
22
Intra-oceanic suspect/exotic terranes
Modern oceans contains large anomalies which
have a different origin than spreading at
ridges. These include 1) Pieces of continents 2)
Oceanic islands 3) Hot-spot traces and islands 4)
Arc and back-arc compexes 5) Transform
complexes Such terranes may end up inside suture
zones of orogenic belts, in which case they
represent suspect and/or exotic elements of the
mountain belt. SUSPECT TERRANES
TECTONOSTRATIGRAPHIC TERRANES THAT HAVE UNSETTLED
AFFINITY/ ORIGIN WITH RESPECT TO THE CONTINENT
WHERE IT ENDS UP AFTER AN OROGENY EXOTIC
TERRANES TECTONOSTRATIGRAPHIC TERRANES THAT HAVE
OUTBOARD ORIGIN WITH RESPECT TO THE CONTINENT
WHERE IT ENDS UP AFTER AN OROGENY. EXAMPLES
OPHIOLITES AND ISLAND ARC COMPLEXES, CONTINETAL
FRAGMENTS WITH AN ORIGIN IN ANOTHER CONTINENT
23
Reconstruction of former plate motions. Based on
magnetic anomaly-patterns in the oceans it is
possible to reconstruct the face of the earth
back in time for the period we have oceanic
lithospere preserved
We can see that the oldest ocean floor is ca
180Ma, how Can we reconstruct plate-motions
before mid-Jurassic time?
180
155
130
24
Age of ocean floor,with plate-reconstructions for
the past 130 Ma. Notice that the reconstruction
also shows oceanic lithosphere that was destroyed
during this time-span (from www.geodynamic.no)