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Ocean floor: Technological progress has enabled a better mapping of the seafloor: direct observations and sampling, bathymetry, internal architecture,

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Presentation on theme: "Ocean floor: Technological progress has enabled a better mapping of the seafloor: direct observations and sampling, bathymetry, internal architecture,"— Presentation transcript:

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 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. The discovery made ca 40 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.

2 Mid-Ocean-Ridges Have these characteristics: 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. 2.BASALT VOLCANISITY mostly tholeitic composition 3.HIGH HEAT-FLOW 4.NEGATIVE GRAVITY ANOMALIES (melts) 5.SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges ridges

3 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/volume which is isostatically compensated by the topography (mid-ocean-ridges ”float” high). Empirical measurements show that the ocean depth (D) is: D = a √ t + d 0 D = a √ t + d 0 t: ocean floor age in million years; t: ocean floor age in million years; d 0 : water depth at the spreading ridge (≈ 2.5 km) d 0 : water depth at the spreading ridge (≈ 2.5 km) a: constant = 0.336 a: constant = 0.336

4 DEPTH: D = a √ t + d 0 t - age; depth- d 0 ≈ 2.5 km); constant- a = 0.336 What is the water depth above 16 myr old ocean floor? D = 0.336√16 + 2.5km = 3.8 km Havdyp For t For t D = a √ t + d 0 For t > 70 mill år --> D = 6.4 - 3,2e -t/62,8 The graph shows depth of the ocean as a function of age out from a ridge. Emperical 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. These are purely descriptive formula and do describe processes Alder

5 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.

6 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. Mid-Ocean-Ridges Have these characteristics: 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. 2.BASALT VOLCANISITY mostly tholeitic composition 3.HIGH HEAT-FLOW 4.NEGATIVE GRAVITY ANOMALIES (melts) 5.SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges ridges 2.BASALT VOLCANISITY mostly tholeitic composition 3.HIGH HEAT-FLOW

7 ELEVATED HEAT-FLOW It is very hard to obtain accurate heat-flow measurements! Heat-flow is given in heat-flow-units defined as milliwatt/m 2. A simplified empirical formula for heat flow (Q) is: Q = 473 t -1/2 t - age, t > 120 Ma t - age, t > 120 Ma Q = 33.5 + 67e -t/62.8  ! Also descriptive formula describing observations

8 HEAT-FLOW: Q = -k[(T+  T-T)/] Q = -k[(T+  T-T)/dz] = -k  T/  z = -k  T/  z (rate of flow pr unit area up through plate) where k - thermal conductivity (Wm -1 o C -1 ) T - temp (T T - temp (T (z +  z) > T z z - thickness of plate a   z z +  z Flow of heat k - thermal conductivity (Wm -1 o C -1 ) Consider a small volume of height  z and cross-section “a” Change in temperature  T in time  t depends 1)Flow og heat across the surface (net heat-flow in or out) 2)Heat generated in the volume 3)Thermal capasity (spesific heat) of the material One dimensional heat conduction equation:  - density, c P - 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  full explanation of heat-flow equations see document: heatflow.pdf(based on Fowler’ book: Solid Earth or the file: heatflow on course homepages.

9 3-dimentional heat equation: or [using differential operator notation (Laplacian operator)] Also considering movement of small volume of material with velocity u z conduction term; production term, advection term

10 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. Mid-Ocean-Ridges Have these characteristics: 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. 2.BASALT VOLCANISITY mostly tholeitic composition 3.HIGH HEAT-FLOW 4.NEGATIVE GRAVITY ANOMALIES (melts) 5.SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges ridges 4.NEGATIVE GRAVITY ANOMALIES (melts)

11 NEGATIV GRAVITY ANOMALI (melts) The reference gravity formula for a given  = latitude (  and  are constants) is: g( )=g e ( 1 +  sin 2 +  sin 4  The Bouger anomaly : g b = g f -  g b +  g t = g obs - g(  +  g f -  g b +  g ter = g obs - g(  +  g f -  g b +  g terWhere: g e = gravitational acceleration 9.7803185 ms -1 g obs - observed Free air anomaly g f = g obs - g( ) +  g f g 0 - gravity at sea level  g b = 2  G  h (bouger correction) G - constant of gravitation (6,673 x 10 -11 m 3 kg -1 s -2 )  g t - terrain correction (deviations from infinite horizontal surface)  g f = g 0 - g(h)= 2h/R g 0 (free air correction) h - height  - density R- earth radius (See Fowler’s textbook Solid Earth, chapter 5)

12 NEGATIV GRAVITY ANOMALI (melts)    u  anomaly expresses deficiency/excess of mass below the measuring point, assuming average mass and no topography at the reference level

13 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. Mid-Ocean-Ridges Have these characteristics: 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. 2.BASALT VOLCANISITY mostly tholeitic composition 3.HIGH HEAT-FLOW 4.NEGATIVE GRAVITY ANOMALIES (melts) 5.SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges ridges 5.SEISMIC ACTIVITY (mainly shallow earthquakes)

14 SEISMIC ACTIVITY (mainly shallow earthquakes with magnitude < 7) Earthquakes last week Jan- first week Feb, 2004 Fast spreading East-Pasific Rise Intermediate spreading rate Mid-Atlantic Ridge Mid-Atlantic Ridge and Southeast Indian Ridge

15 Earthquakes along the Mid-Atlantic Ridge near the Azores

16 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. Mid-Ocean-Ridges Have these characteristics: 1.TOPOGRAPHY (≈1000 km) broad ridges with narrow central rifts. 2.BASALT VOLCANISITY mostly tholeitic composition 3.HIGH HEAT-FLOW 4.NEGATIVE GRAVITY ANOMALIES (melts) 5.SEISMIC ACTIVITY (mainly shallow earthquakes) 6. MAGNETIC ANOMALIES oriented parallel with the ridges ridges 6. MAGNETIC ANOMALIES oriented parallel with the ridges 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 (10 5 yr) 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 Animated simulation of of magnetic field reversal http://www.psc.edu/science/glatzmaier.html

20 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.

21 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.

22 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-Ridge-Basalt) Formed by relatively high degree of partial melting at relatively shallow level in the astenosphere

23 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 SUSPECT TERRANES : TECTONOSTRATIGRAPHIC TERRANES THAT HAVE UNSETTLED AFFINITY/ ORIGIN WITH RESPECT TO THE CONTINENT WHERE IT ENDS UP AFTER AN OROGENY EXOTIC TERRANES 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

24 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 180155130 We can see that the oldest ocean floor is ca ≈180Ma, how Can we reconstruct plate-motions before mid-Jurassic time?


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