Geoscience Reference
In-Depth Information
ocean sediment and the basalt lavas; as the volcano grows, the foot of the massive
pile thrusts outwards. These detachments tend to slope upwards away from the
centre of volcanism. The upper parts of the pile have bedding planes that slope
downwards away from the centre. Eventually, failure occurs in the upper part and
massive landslips move out into the surrounding ocean. The landslip headwall
becomes a high cliff, up to 1000 m high. Often failure is catastrophic, spawning
tsunami, with the debris transported out as deep-water turbidites. Examples of
this can be seen in many volcanic islands. Some of the older Hawaiian islands
have huge debris fields offshore with giant blocks of slumped rock. In the Atlantic
there is concern about potential failure in the Canary Isles, which may be a major
present-day natural hazard.
Problems
1. (a) Assuming isostatic equilibrium and Airy-type compensation, calculate the thick-
ness of the oceanic crust if the continents averaged 50 km thick.
(b) What is the minimum possible thickness of the continental crust? (Use densities
of sea water, crust and mantle of 1.03
×
10
3
, 2.9
×
10
3
and 3.3
×
10
3
kg m
−
3
; the
ocean-basin depth is 5 km.)
2. Determine the oceanic crustal structure for the wide-angle reflection-refraction data
shown in Fig. 9.7(a).
(a) Use the normal-incidence two-way travel times to estimate the depth of the seabed.
(Use 1.5 km s
−
1
for the velocity of sound in sea water.)
(b) Use the slope-intercept method to estimate the following: (i) an upper crustal P-
wave velocity (use first arrivals at less than 10 km distance), (ii) the lower-crustal P-
wave velocity (use P3), (iii) the upper-mantle velocity, (iv) an upper-crustal S-wave
velocity (use S-wave arrivals at 10-15 km distance), (v) the lower-crustal S-wave
velocity (use S3), (vi) the upper-mantle S-wave velocity and (vii) the thickness of
the oceanic crust at this location.
(c) Compare the crustal thickness obtained in (vii) with the value obtained using
normal-incidence two-way times.
(d) Calculate the ratios of P-wave velocity to S-wave velocity. Do these values fall
within the expected range?
3. Derive and plot the relationship between continental crustal thickness and ocean depth.
Assume isostatic equilibrium and Airy-type compensation.
4. If the magma chamber on the East Pacific Rise at 12
◦
N described in the example
in Section 9.4 were filled with molten basaltic magma with seismic P-wave velocity
3kms
−
1
,how wide would it be? At the other extreme, if it contained only 10% partial
melt, what would its width be? Comment on the likelihood of detection of such extreme
magma chambers by seismic-reflection and -refraction experiments.
5. What would be the likelihood of delineating the magma chambers of Problem 4 by
seismic methods if the dominant frequency of your signal were (a) 5 Hz, (b) 15 Hz and
(c) 50 Hz?
