Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 68

Chapter 4
PALEOMAGNETISM AND THE POSSIBLE EFFECT
OF DEPTH OF BURIAL AND NON-HYDROSTATIC STRESSES.
SIGNIFICANCE OF SUBMAIHNE LINEAR MAGNETIC
ANOMALIES
CONTENTS
With age and depth of burial the eharacteristics of rocks usually change.
Rock magnetization is not exempt from that rule. The Deccan traps have
been deeply buried and permeated by hot groundwater, as their thorough
alteration reveals. At. the same time, they were probably also subjected
to strong oriented stresses. It is suggested that the magnetization may
change systematically and regionally under such conditions, and that the
“drift” of India is for this reason a misinterpretation. It is suggested that
such and other changes of rock magnetization my be more common than
is usually assumed, and that the seafloor linear anomalies are one type
of secondary magnetization.
When first we read about geological stratigraphy, we think
it quite natural that there exist rocks from the various earth
ages, and that it has been possible to arrange them into a rela-
tive time sequence. It may, then, seem strange to put the ques-
tion: Why do older rocks than, say, 50—70 million years still
exist on the present surface of the earth?
But the answer to just such a question leads to a clarifying
restatement of known facts: The old rocks exist because, for
geological time intervals, they have been sheltered against de-
nudation by a cover of younger rocks or by submergence. The
covering by younger rocks means that the older ones have
been kept under pressure, and pressurized groundwater at some
elevated temperature for geological time intervals.
A well-known reflection of this is found in sedimentary rocks.
The youngest are still little consolidated, but with increasing
age, the diagenesis, the hardness, and the recrystallization in-
crease, and when we come to an age of 1000 My, practically
all the original sediments are coarse-crystalline, — an indica-