consider an octupole term equal to 10% (resp. 20%) of the
dipolar component, the maximum error committed is *7°
(resp. 13°) in the mid-latitudes. Finally, a last source of error is
related to the preservation of the magnetic signal in the sedimentary series. Indeed, during diagenesis, the magnetic carriers
can undergo a decrease in the inclination of the magnetic carriers. The inclination measured is therefore less than the actual
inclination of the magnetic field that prevailed at the time this
detrital remanent magnetization was acquired. This inclination
flattening can be a source of error in paleoreconstructions
(Cogné et al. 2013).
The Topography of the Earth
Mountain ranges and high plateaus play an important role in
atmospheric circulation. The uplift of the Himalayas and the
Tibetan plateau is undoubtedly the most characteristic
example of the relationship between topography and the
evolution of atmospheric circulation and climate. The hypsometric curve of the Earth reveals that today, reliefs higher
than 2 km represent about 10% of the land area. Still, this
curve reflects the orogenic context of the Earth that has
prevailed since the beginning of the Cenozoic only, and one
expects that this curve evolved through time.
The great areas of high-altitude terrain (*>2 km) typically developed during the formation phases of a supercontinent (for example, in the Carboniferous, during the
formation of the supercontinent Pangea), while periods of
break-up of the supercontinents (the Mesozoic for example)
are characterized by more modest reliefs (although low
reliefs of less than 2 km in general, can form in case of
continental rifting). In the oceans, seafloor reliefs are mostly
dominated by ocean ridges and some high oceanic plateaus
rising above the abyssal plains.
The altitude of the continents, plains, collision mountain
ranges (Himalayas, Alps) and the zones of the East African
Dome result first of all from the (quasi-) isostatic equilibrium
of the lithospheric column marked by density heterogeneities
on the underlying asthenosphere. The bathymetry of the
ocean floor is controlled by the cooling and by the progressive thickening of the oceanic lithosphere formed at the
dorsal ridges in isostatic equilibrium on the asthenosphere.
However this scheme does not apply everywhere, suggesting other mechanisms. Some reliefs are not at isostatic
equilibrium, as is the case, for example, of volcanic islands
that develop on an oceanic crust (like Hawaii) or continental
crusts previously subjected to the weight of an ice cap (like
the Scandinavian region). The charge brings about a flexure
whose wavelength is related to the elastic rheology of the
crust. After the rapid melting of an ice cap, the deformation
gradually fades over time with a rate depending on the
viscosity of the underlying mantle.
Topography can also be controlled by dynamic processes
2 related to movement of matter and heat transfer
within the viscous mantle. Upward or downward movements
of mantle matter or dipping of a lithospheric plate into the
mantle create mass anomalies that can induce long-wave
crustal deformations. This is known as dynamic topography
(Husson 2006) as opposed to isostatic topography. The uplift
of the southern African plateau and the Colorado plateau
during the Cenozoic is explained by movements of mass
anomalies in the mantle. Dynamic topography also helps to
explain the history of the Western Interior Seaway that
connected the Arctic Ocean to the Gulf of Mexico across the
North American continent during the Cretaceous, but also
the flooding followed by the exondation of part of Australia
at the same time, or the flooding of the Sunda shelf in
Indonesia during the Pleistocene (Sarr et al. 2019).
Restoring the past topography of the Earth is undoubtedly
the most difficult part of paleogeographic reconstructions. It
involves determining the spatial expanse of the terrain, its
age and altitude. The reliefs bring about deformations and/or
structural and petrological markers which, when fixed in
time, are used by geologists to constrain the tectonic event.
These markers are not always easy to detect because more
recent events often mask earlier events. A phase of continental accretion caused the India-Asia collision in Southeast
Asia during the Triassic period, but this orogeny remains
uncertain, partly because this collision erased part of the
previous geological history. Determining the paleoaltitude of
mountains is therefore crucial in order to model
pre-Quaternary paleoclimates. Several methods (flora, sediment, oxygen isotopes, cosmogenic isotopes) have been
developed, but only the most commonly used will be
depicted here. It is very important to differentiate between
absolute methods, which makes it possible to estimate the
paleoaltitude of a relief at a given time and methods estimating the vertical velocity of the rocks in these mountains.
The relative methods reflect a balance between the vertical
movements linked to a geodynamic event (a collision for
example) and those due to erosion which denudes the surface, favoring the rise of deep-set rocks by isostatic
readjustment.
To measure an altitude or to estimate a paleoaltitude, a
reference level is essential. The surface of the oceans provides this reference, but this fluctuates over geological time.
However, the amplitudes in these variations are less than the
uncertainty obtained on paleoelevations, regardless of the
methods used. Nonetheless, eustatic variations are crucial
when examining the regions between the continental shelf (>
−200 m) and the vast lowland plains (<200 m).
2
Although dynamic, collision mountain ranges and intracontinental
rifting are essentially isostatic equilibrium processes.
2 The Changing Face of the Earth Throughout the Ages
29
dipolar component, the maximum error committed is *7°
(resp. 13°) in the mid-latitudes. Finally, a last source of error is
related to the preservation of the magnetic signal in the sedimentary series. Indeed, during diagenesis, the magnetic carriers
can undergo a decrease in the inclination of the magnetic carriers. The inclination measured is therefore less than the actual
inclination of the magnetic field that prevailed at the time this
detrital remanent magnetization was acquired. This inclination
flattening can be a source of error in paleoreconstructions
(Cogné et al. 2013).
The Topography of the Earth
Mountain ranges and high plateaus play an important role in
atmospheric circulation. The uplift of the Himalayas and the
Tibetan plateau is undoubtedly the most characteristic
example of the relationship between topography and the
evolution of atmospheric circulation and climate. The hypsometric curve of the Earth reveals that today, reliefs higher
than 2 km represent about 10% of the land area. Still, this
curve reflects the orogenic context of the Earth that has
prevailed since the beginning of the Cenozoic only, and one
expects that this curve evolved through time.
The great areas of high-altitude terrain (*>2 km) typically developed during the formation phases of a supercontinent (for example, in the Carboniferous, during the
formation of the supercontinent Pangea), while periods of
break-up of the supercontinents (the Mesozoic for example)
are characterized by more modest reliefs (although low
reliefs of less than 2 km in general, can form in case of
continental rifting). In the oceans, seafloor reliefs are mostly
dominated by ocean ridges and some high oceanic plateaus
rising above the abyssal plains.
The altitude of the continents, plains, collision mountain
ranges (Himalayas, Alps) and the zones of the East African
Dome result first of all from the (quasi-) isostatic equilibrium
of the lithospheric column marked by density heterogeneities
on the underlying asthenosphere. The bathymetry of the
ocean floor is controlled by the cooling and by the progressive thickening of the oceanic lithosphere formed at the
dorsal ridges in isostatic equilibrium on the asthenosphere.
However this scheme does not apply everywhere, suggesting other mechanisms. Some reliefs are not at isostatic
equilibrium, as is the case, for example, of volcanic islands
that develop on an oceanic crust (like Hawaii) or continental
crusts previously subjected to the weight of an ice cap (like
the Scandinavian region). The charge brings about a flexure
whose wavelength is related to the elastic rheology of the
crust. After the rapid melting of an ice cap, the deformation
gradually fades over time with a rate depending on the
viscosity of the underlying mantle.
Topography can also be controlled by dynamic processes
2 related to movement of matter and heat transfer
within the viscous mantle. Upward or downward movements
of mantle matter or dipping of a lithospheric plate into the
mantle create mass anomalies that can induce long-wave
crustal deformations. This is known as dynamic topography
(Husson 2006) as opposed to isostatic topography. The uplift
of the southern African plateau and the Colorado plateau
during the Cenozoic is explained by movements of mass
anomalies in the mantle. Dynamic topography also helps to
explain the history of the Western Interior Seaway that
connected the Arctic Ocean to the Gulf of Mexico across the
North American continent during the Cretaceous, but also
the flooding followed by the exondation of part of Australia
at the same time, or the flooding of the Sunda shelf in
Indonesia during the Pleistocene (Sarr et al. 2019).
Restoring the past topography of the Earth is undoubtedly
the most difficult part of paleogeographic reconstructions. It
involves determining the spatial expanse of the terrain, its
age and altitude. The reliefs bring about deformations and/or
structural and petrological markers which, when fixed in
time, are used by geologists to constrain the tectonic event.
These markers are not always easy to detect because more
recent events often mask earlier events. A phase of continental accretion caused the India-Asia collision in Southeast
Asia during the Triassic period, but this orogeny remains
uncertain, partly because this collision erased part of the
previous geological history. Determining the paleoaltitude of
mountains is therefore crucial in order to model
pre-Quaternary paleoclimates. Several methods (flora, sediment, oxygen isotopes, cosmogenic isotopes) have been
developed, but only the most commonly used will be
depicted here. It is very important to differentiate between
absolute methods, which makes it possible to estimate the
paleoaltitude of a relief at a given time and methods estimating the vertical velocity of the rocks in these mountains.
The relative methods reflect a balance between the vertical
movements linked to a geodynamic event (a collision for
example) and those due to erosion which denudes the surface, favoring the rise of deep-set rocks by isostatic
readjustment.
To measure an altitude or to estimate a paleoaltitude, a
reference level is essential. The surface of the oceans provides this reference, but this fluctuates over geological time.
However, the amplitudes in these variations are less than the
uncertainty obtained on paleoelevations, regardless of the
methods used. Nonetheless, eustatic variations are crucial
when examining the regions between the continental shelf (>
−200 m) and the vast lowland plains (<200 m).
2
Although dynamic, collision mountain ranges and intracontinental
rifting are essentially isostatic equilibrium processes.
2 The Changing Face of the Earth Throughout the Ages
29