Stein, 2003., pp. 301. The concept that the lithosphere cools with time such that isotherms deepen with the square root of age has many observable consequences. The simplest is that ocean depth should vary with age, which make sense, because spreading centers are ridges precisely because the ocean deepens on either side. To model this effect, we consider the mass in two columns, one at the ridge and one at age t, and invoke the idea of isostasy, which means that the masses in two columns balance (Figi.5.3-6).
It contracts, so its volume decreases, resulting in an increase in density. The cooling and shrinking of the lithosphere result in an increase in its density and so, as a result of isostasy, it subsides into the asthenosphere and ocean depth increases away from the ridge, from about 2–3 km at oceanic ridges to about 5–6 km for abyssal plains. Indeed, one of the more remarkable observations of ocean-floor bathymetry is that ocean floor of similar age always occurs at similar depths beneath sea level (Figure 10). The relationship between mean oceanic depth ( d in metres) and lithosphere age ( t in Ma) can be expressed as: d = 2500 + 350 t ½ ( 1 ) If the depth of the ocean floor can be determined, then the approximate age of the volcanic rocks from which it formed may also be estimated, and vice versa.
The instrument has no connection to the sea surface and is recovered by acoustic or time release.
V (Sea)=1525 m/s V (Sediments)= 1800 m/s V (Metamorphic) =2500
Profiles Pr 1, Pr 2 and the northern part of Pr 4 cross the area of Crete (Bohnhoff et al., 2001), profiles Pr 3, southern part of Pr 4 and Pr 5 (Makris and Broenner, 2001; Broenner, 2003) cross the Mediterranean Ridge. Numerical values indicate the Vp velocities in kilometers per second.
Figure 2. DART tsunameters are now used in a few strategic locations in the deep ocean to measure tsunami waves unaltered by shoaling and refl ections at a coast. The tsunameters sense the pressure induced on the ocean bottom by passing tsunami waves, transmit these data by acoustic modem to an oceansurface buoy moored nearby, and from the buoy the data are sent via satellite to the warning centers.
Solid Earth Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Today’s class: Oceanic Lithosphere Reading: Fowler Chapter 9 , pp.391-416
<ul><ul><li>Oceanic Crust and Continental Crust </li></ul></ul><ul><ul><li>Total Sediment Thickness Database </li></ul></ul><ul><ul><li>Depth of Sea Floor versus Age of Sea Floor </li></ul></ul><ul><ul><li>Marine Seismic Acquisition </li></ul></ul><ul><ul><li>OBS and Streamer </li></ul></ul>Summary Lecture 12
Chapter Exam Next Tuesday Class Fowler Chapters 8 and 9 , pp.391-416 Assignment Prepare your questions for Chapter 9n and upload them to class page before the exam. April 21, 2007
Exposed Oceanic Crust <ul><li>Often regarded as examples of oceanic crust. However, because they are now tectonically emplaced on land, they are atypical and might not represent n normal oceanic crust. Ophiolites are probably samples of young oceanic crust produced in back-arc basins, or fore-arc basins, associated with subduction zones (Fowler, pp.405). </li></ul>Oman ophiolite harzburgite upper mantle both from: http://www.bris.ac.uk/Depts/Geol/vft/oman.html
pp. 339 of Stein, 2003 modified after McClousky et al., 2000
Why does the Ocean Depth with lithospheric age deepens with distance from the spreading ridges? Recall
Depth for Ocean Age for Lithosphere If a body cools, what happens to its density?
MULTICHANNEL SEISMIC EQUIPMENT P.Farcy Farcy, B. , B. Marsset Marsset, H. , H. Nouze Nouze
Ocean Bottom Seismograph http://www.geopro.com/obs.html The OBS is cased in a glass sphere which can be deployed up to water depths of 6700m, recording seismic signals directly on the seafloor with a gimbal mounted 3C-geophone and a deep sea hydrophone.
V Sea =1525 m/s V sediments = 1800 m/s V Metamorphic =2500 m/s
Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface , Nedimovic et al., NATURE |VOL 424 24 JULY 2003
Nedimovic et al., NATURE |VOL 424 | 24 JULY 2003 |
W ide A perture R eflection R efraction P rofiling
WARP Offshore <ul><li>The basic principle of WARRP is utilizing the amplitude burst when P-wave energy is totally reflected beyond the critical angle of incidence. To record at wide angles large offsets are required, much larger than conventional seismic arrays can provide. </li></ul>
OBS application with a vertical streamer http://www.geopro.com