When mantle plumes forming hotspots are located in the vicinity of spreading
ridges, this could interfere with the ridge segments and eventually change the
geometry of a spreading ridge as well as the composition of the volcanics erupted
at the ridge axis. Other chapters in this book (such as Chap. 7) will show how this
type of interaction can influence the rock composition and geological structure
when a spreading ridge meets a hotspot.
Why do we Study Hotspots?
Oceanic basins cover about 70 % of the World’s oceans and seas. Along with
spreading ridges, basins are also the sites of some of the major volcanic activity on
Earth (Fig. 9.1). The intraplate regions have deep basins associated with hotspots
where the exchange of matter between the Earth’s mantle and the lithosphere is
taking place. It is estimated that about 30 % of seafloor volcanism is due to hotspot
activity.
It has only been since the 1970s that oceanic basins and their observed major
linear structures were investigated. Before then, most of our efforts to understand
oceanic basins were related to the study of their islands. Early geological studies
had to concentrate on the most easily accessible, subaerial parts of the volcanic
edifices. However, Brousse (Brousse and Maury 1980) and Menard (1986)
accomplished pioneering work in Pacific Island studies.
Apart from being popular holiday destinations, oceanic-island volcanoes such
as Hawaii, Tahiti, or the Canaries are built from magmas that yield valuable
information about the interior of our planet. It is now well established that most of
the Earth’s volcanic activity occurs underneath sea level and often at great depths
([1000 m). Although the plate tectonic theory provides a framework for understanding the volcanic events on spreading ridge axes, it does not explain the
presence of volcanism in ocean basins. Hence, a study of submarine volcanoes
found at a distance from spreading ridges can provide critical information about
the depth at which the mantle begins to melt and how mantle plumes rising
towards the surface could generate ‘‘hotspots’’.
Hotspots have influenced the Earth’s spreading ridge systems and learning
about the processes controlling ‘‘hotspot versus spreading ridge interactions’’
could help us to better understand the global convection system of our planet’s
interior. If the formation of landmasses that went on to build today’s continents
started in the middle of ancient oceans, it would be worthwhile to understand how
present-day mantle plumes form hotspots and oceanic islands (see Chap. 2). At the
beginning of our planet’s existence, emerged landmasses must have been the
exposed summits of large, volcanic edifices, which appeared in the first ‘‘magma
ocean’’. The importance of today’s hotspots is that they produce volcanoes, which
are the surface expression of upwelling convective cells of chemical fluxes having
a correlation with deep mantle material (Morgan 1971, 1972). Jason Morgan’s
model (1971) was based on the idea that lower, primordial mantle material rises in
Introduction
303
ridges, this could interfere with the ridge segments and eventually change the
geometry of a spreading ridge as well as the composition of the volcanics erupted
at the ridge axis. Other chapters in this book (such as Chap. 7) will show how this
type of interaction can influence the rock composition and geological structure
when a spreading ridge meets a hotspot.
Why do we Study Hotspots?
Oceanic basins cover about 70 % of the World’s oceans and seas. Along with
spreading ridges, basins are also the sites of some of the major volcanic activity on
Earth (Fig. 9.1). The intraplate regions have deep basins associated with hotspots
where the exchange of matter between the Earth’s mantle and the lithosphere is
taking place. It is estimated that about 30 % of seafloor volcanism is due to hotspot
activity.
It has only been since the 1970s that oceanic basins and their observed major
linear structures were investigated. Before then, most of our efforts to understand
oceanic basins were related to the study of their islands. Early geological studies
had to concentrate on the most easily accessible, subaerial parts of the volcanic
edifices. However, Brousse (Brousse and Maury 1980) and Menard (1986)
accomplished pioneering work in Pacific Island studies.
Apart from being popular holiday destinations, oceanic-island volcanoes such
as Hawaii, Tahiti, or the Canaries are built from magmas that yield valuable
information about the interior of our planet. It is now well established that most of
the Earth’s volcanic activity occurs underneath sea level and often at great depths
([1000 m). Although the plate tectonic theory provides a framework for understanding the volcanic events on spreading ridge axes, it does not explain the
presence of volcanism in ocean basins. Hence, a study of submarine volcanoes
found at a distance from spreading ridges can provide critical information about
the depth at which the mantle begins to melt and how mantle plumes rising
towards the surface could generate ‘‘hotspots’’.
Hotspots have influenced the Earth’s spreading ridge systems and learning
about the processes controlling ‘‘hotspot versus spreading ridge interactions’’
could help us to better understand the global convection system of our planet’s
interior. If the formation of landmasses that went on to build today’s continents
started in the middle of ancient oceans, it would be worthwhile to understand how
present-day mantle plumes form hotspots and oceanic islands (see Chap. 2). At the
beginning of our planet’s existence, emerged landmasses must have been the
exposed summits of large, volcanic edifices, which appeared in the first ‘‘magma
ocean’’. The importance of today’s hotspots is that they produce volcanoes, which
are the surface expression of upwelling convective cells of chemical fluxes having
a correlation with deep mantle material (Morgan 1971, 1972). Jason Morgan’s
model (1971) was based on the idea that lower, primordial mantle material rises in
Introduction
303