Open Tasks and Questions
39
Other problems have to do not so much with processes, but with a better grip on
historical reconstruction.
For the marine geologist interested in reconstructing the history of the oceans,
reliable paleogeographic maps showing the distribution of land and water through
geologic time are perhaps the most pressing need. E. C. Bullard et aI., in 1965,
demonstrated how to reassemble correctly the drifting continents and continental
fragments in a sphere (Fig. 1.19). Bullard's co-workers and others since have greatly
extended this type of work and have supplied a series of maps with ancient positions
of continental masses. These, of course, are extremely useful for historical geology,
both continental and marine. However, we know that mountain building and other
processes active at the transitions between oceanic and continental crust have
changed significant details of the continental configurations. Some of these changes
are crucial in deciding, for example, whether there was a connection between one
ocean basin and another. It will take many years of compilation, fieldwork, and
detailed reassembly just to provide the kind of paleogeographic base maps necessary
for explaining the distribution of ancient fossils, for instance.
These tasks are difficult enough for the time back to the Permian. For earlier
periods the challenge takes on intimidating dimensions.
Continuous improvement of the time scale of magnetic reversals, for example, is
an important task, because this scale forms the basis for discovering rates of change
in Earth history, including continental drift or sedimentary processes. A more difficult
task in this context is to determine the duration of magnetic reversals (less
than 10 000 years mostly) and why the reversals occur in the first place. Or, the same
question put differently, why reversals did not occur for considerable time spans, as,
for example, in the middle Cretaceous (see Fig. 9.22). Was the release of plumes from
the lower mantle (which apparently expressed itself millions of years later as outpouring of basalts on the seafloor) in any way related to the cessation of magnetic
reversals? If so, why should this be? Reversals, then, pose problems a-plenty - and
not just for migrating birds using the magnetic field to orient themselves.
Many fundamental questions have been raised (see Fig. 1.20). What determines
sea-floor spreading rates? What is the reason for changing spreading directions and
rates? What is the significance of the enormous clusters of islands and seamounts in
the South Pacific (between the Bikini Atoll, some 2000 km southeast of the Marianas
stretching parallel and inside the andesite line (Fig. A6.1) for about 8000 km to the
Tuamotu Archipelago)? Presumably, they have to do with details of mantle processes
which we are far from grasping as in other tectonic and volcanic intra-plate
situations.
Clearly, all the major features on the thin skin of planet Earth, the crust, must
ultimately owe much to such mantle processes. What does convection in the mantle
actually look like? Are upper and lower mantle convection largely decoupled or not?
What is the role of downgoing slabs in mixing the mantle? What about the rising
plumes? How important are they within the convection scenario? Where is their
source? How much is the material modified on the way up? How stable are the
plumes through geologic time? Why do they have quite different expressions on the
surface, ranging from oceanic plateaus to island groups?
39
Other problems have to do not so much with processes, but with a better grip on
historical reconstruction.
For the marine geologist interested in reconstructing the history of the oceans,
reliable paleogeographic maps showing the distribution of land and water through
geologic time are perhaps the most pressing need. E. C. Bullard et aI., in 1965,
demonstrated how to reassemble correctly the drifting continents and continental
fragments in a sphere (Fig. 1.19). Bullard's co-workers and others since have greatly
extended this type of work and have supplied a series of maps with ancient positions
of continental masses. These, of course, are extremely useful for historical geology,
both continental and marine. However, we know that mountain building and other
processes active at the transitions between oceanic and continental crust have
changed significant details of the continental configurations. Some of these changes
are crucial in deciding, for example, whether there was a connection between one
ocean basin and another. It will take many years of compilation, fieldwork, and
detailed reassembly just to provide the kind of paleogeographic base maps necessary
for explaining the distribution of ancient fossils, for instance.
These tasks are difficult enough for the time back to the Permian. For earlier
periods the challenge takes on intimidating dimensions.
Continuous improvement of the time scale of magnetic reversals, for example, is
an important task, because this scale forms the basis for discovering rates of change
in Earth history, including continental drift or sedimentary processes. A more difficult
task in this context is to determine the duration of magnetic reversals (less
than 10 000 years mostly) and why the reversals occur in the first place. Or, the same
question put differently, why reversals did not occur for considerable time spans, as,
for example, in the middle Cretaceous (see Fig. 9.22). Was the release of plumes from
the lower mantle (which apparently expressed itself millions of years later as outpouring of basalts on the seafloor) in any way related to the cessation of magnetic
reversals? If so, why should this be? Reversals, then, pose problems a-plenty - and
not just for migrating birds using the magnetic field to orient themselves.
Many fundamental questions have been raised (see Fig. 1.20). What determines
sea-floor spreading rates? What is the reason for changing spreading directions and
rates? What is the significance of the enormous clusters of islands and seamounts in
the South Pacific (between the Bikini Atoll, some 2000 km southeast of the Marianas
stretching parallel and inside the andesite line (Fig. A6.1) for about 8000 km to the
Tuamotu Archipelago)? Presumably, they have to do with details of mantle processes
which we are far from grasping as in other tectonic and volcanic intra-plate
situations.
Clearly, all the major features on the thin skin of planet Earth, the crust, must
ultimately owe much to such mantle processes. What does convection in the mantle
actually look like? Are upper and lower mantle convection largely decoupled or not?
What is the role of downgoing slabs in mixing the mantle? What about the rising
plumes? How important are they within the convection scenario? Where is their
source? How much is the material modified on the way up? How stable are the
plumes through geologic time? Why do they have quite different expressions on the
surface, ranging from oceanic plateaus to island groups?