Seamounts, Island Chains, and Hot Spots 29
the plates has its own particular motion, which can be read from the magnetism of the
sea floor, as we shall see. The quantitative development of these concepts was initiated in the late 1960s by W. J. Morgan, D. P. McKenzie and R. L. Parker; X.
LePichon; and by B. Isacks, I. Oliver, and L. R. Sykes.
The motions are generally uniform and do not result in deformation of the plates;
hence they can be described as rotations on a sphere, according to a theorem of the
famous mathematician, Leonhard Euler (1707-1783). The fracture zones provide
traces for the latitudinal circles around the pole of rotation (which need not coincide
with that of the rotation of the Earth, see Fig. 0.4). Thus, the pole of rotation can be
determined for each plate. Geometry requires that spreading rates must increase away
from the pole of rotation for separating plates, and this is indeed observed. It will be
noted in Fig. 1.13 that a plate can contain both oceanic and continental lithosphere. In
fact, the continents share the motions of the mobile ocean floor. Thus, the continents
do drift, as Wegener had supposed, but not by plowing through the mantle magma.
Since Morgan proposed his scheme of plates making up the globe, new plates
have been discovered, and several modifications of plate geometry and kinematics
made. One recent model that summarizes progress in this field of research (c. de
Mets et aI., 1990, Geophys. J. 101) distinguishes 12 major plates including Philippine
(PH), Cocos (North of Nazca), and Caribbean Plates, and separate a North American
Plate from a South American one and an Indian Plate from an Australian one by
transition zones with diffuse deformation. The position of the boundary of the North
American Plate in the Arctic realm is likewise not well defined. More than a hundred
mantle plumes have now been defined by different authors, spread around the oceans
and on continents as well. These hot spots (Fig. 1.13 b) ar of interest in the context of
plate movements, as we shall see later.
1.7 Seamounts, Island Chains, and Hot Spots
With few exceptions, oceanic islands are made of volcanic rock, with or without a
crown of reef carbonate. A crown of reef carbonate, of course, can only be precipitated in shallow water because it depends on algal growth. Thus, if a seamount is
found with a top of reef carbonate, and deeply submerged below the present sea level,
it must have sunk. Such seamounts are common in the western Pacific.
It has been said that the discovery of flat-topped seamounts held the key to the
new understanding of the origin of ocean basins. Flat-topped seamounts were first
described in the 1940s by H. H. Hess (Fig. 0.1) in the central Pacific. Hess proposed
that these table mounts, the guyots, as he named them, had formed as volcanic
islands, were truncated by wave erosion, and then sank to their present depths. He
also initially thought they might be of Precambrian age, with lots of time available for
subsidence. However, no rocks older than Cretaceous were ever dredged from the
guyots.
In essence, Hess' hypothesis of guyot formation was an extrapolation of Charles
Darwin's hypothesis of atoll formation (see Chap. 7). The idea of seamount subsidence was easily reconciled with Hess' later concept of sea-floor spreading
the plates has its own particular motion, which can be read from the magnetism of the
sea floor, as we shall see. The quantitative development of these concepts was initiated in the late 1960s by W. J. Morgan, D. P. McKenzie and R. L. Parker; X.
LePichon; and by B. Isacks, I. Oliver, and L. R. Sykes.
The motions are generally uniform and do not result in deformation of the plates;
hence they can be described as rotations on a sphere, according to a theorem of the
famous mathematician, Leonhard Euler (1707-1783). The fracture zones provide
traces for the latitudinal circles around the pole of rotation (which need not coincide
with that of the rotation of the Earth, see Fig. 0.4). Thus, the pole of rotation can be
determined for each plate. Geometry requires that spreading rates must increase away
from the pole of rotation for separating plates, and this is indeed observed. It will be
noted in Fig. 1.13 that a plate can contain both oceanic and continental lithosphere. In
fact, the continents share the motions of the mobile ocean floor. Thus, the continents
do drift, as Wegener had supposed, but not by plowing through the mantle magma.
Since Morgan proposed his scheme of plates making up the globe, new plates
have been discovered, and several modifications of plate geometry and kinematics
made. One recent model that summarizes progress in this field of research (c. de
Mets et aI., 1990, Geophys. J. 101) distinguishes 12 major plates including Philippine
(PH), Cocos (North of Nazca), and Caribbean Plates, and separate a North American
Plate from a South American one and an Indian Plate from an Australian one by
transition zones with diffuse deformation. The position of the boundary of the North
American Plate in the Arctic realm is likewise not well defined. More than a hundred
mantle plumes have now been defined by different authors, spread around the oceans
and on continents as well. These hot spots (Fig. 1.13 b) ar of interest in the context of
plate movements, as we shall see later.
1.7 Seamounts, Island Chains, and Hot Spots
With few exceptions, oceanic islands are made of volcanic rock, with or without a
crown of reef carbonate. A crown of reef carbonate, of course, can only be precipitated in shallow water because it depends on algal growth. Thus, if a seamount is
found with a top of reef carbonate, and deeply submerged below the present sea level,
it must have sunk. Such seamounts are common in the western Pacific.
It has been said that the discovery of flat-topped seamounts held the key to the
new understanding of the origin of ocean basins. Flat-topped seamounts were first
described in the 1940s by H. H. Hess (Fig. 0.1) in the central Pacific. Hess proposed
that these table mounts, the guyots, as he named them, had formed as volcanic
islands, were truncated by wave erosion, and then sank to their present depths. He
also initially thought they might be of Precambrian age, with lots of time available for
subsidence. However, no rocks older than Cretaceous were ever dredged from the
guyots.
In essence, Hess' hypothesis of guyot formation was an extrapolation of Charles
Darwin's hypothesis of atoll formation (see Chap. 7). The idea of seamount subsidence was easily reconciled with Hess' later concept of sea-floor spreading