MAT is a strict interpolator; it does not attempt to
extract a general relationship between assemblage composition and environmental factors. Its applicability is therefore contingent on appropriate coverage of the
environmental gradient and of the associated assemblage
compositions. Because of no-analog conditions due to
evolution of niche requirements of the constituent species,
assemblage-based transfer functions are typically limited
in their applicability to late Quaternary sediments.
Sea-surface temperature reconstructions via MAT can be
obtained to a precision of 1
C, and the variance among
the SST values of the best analogs can be used to assess
the uncertainty of a fossil estimate.
Bibliography
Hutson, W. H., 1980. The Agulhas current during the late Pleistocene: analysis of modern faunal analogs. Science, 207, 64–66.
MOHOROVIC ˇ IC ´ DISCONTINUITY (MOHO)
Carolina Lithgow-Bertelloni
Department of Earth Sciences, University College
London, London, UK
Synonyms
Crust-mantle transition; Moho
Definition
Moho: The rapid increase in seismic velocity with which
we often identify the boundary between Earth’s crust
and mantle. More specifically, the Moho is the depth at
which the P-wave velocity first increases rapidly or discontinuously to 7.6–8.6 km/s, or if steep velocity gradients are not present, then it is the depth at which the
P-velocity exceeds 7.6 km/s (Steinhart, 1967).
Introduction
The Mohorovičić discontinuity, also known as the Moho,
is a discrete jump in seismic wave velocities – a seismic
discontinuity – that has come to define the boundary
between the crust and the mantle. Its significance stems
from its clear demarcation of the first boundary of the
chemically differentiated solid Earth. This seminal discovery (Mohorovicic, 1910) complemented that of Earth’s
other major chemical boundaries with the nearly contemporaneous discovery of the outer core (Oldham, 1906)
and the inner core (Lehman, 1936) a few decades later.
The seismic discontinuity at the Moho is explained generally as a chemical (compositionally distinct) boundary
separating crust (both oceanic and continental) from the
mantle, although in detail the Moho is a much more complex boundary. This is in contrast with most other seismic
discontinuities in the mantle, which are explained by
changes in the structure of minerals with homogeneous
composition as a function of pressure. The petrologic
Moho is defined as the change from more felsic crust to
more mafic mantle, but it does not always coincide with
the Mohorovičić discontinuity as determined by refraction
velocity studies or reflection studies. The velocity discontinuity may in places occur within the crust as the basalt to
eclogite metamorphic transition (O’Reilly and Griffin,
2013) or within the mantle at the base of a serpentinized
layer. Depending on tectonic setting its thickness
may vary.
Today, mapping of the Moho at global and regional
scales with a variety of geophysical probes, from activesource seismic reflection and refraction studies to
teleseismic studies to gravity inversions, is a crucial aspect
of geological and geophysical research. Drilling through
the Mohorovičić discontinuity remains an elusive goal.
In honor of the centenary of its discovery, an excellent
and comprehensive review of historical progress and
state-of-the-art research was published in December
2013 (Moho: 100 years after Andrija Mohorovic ˇić,
Tectonophysics, 609, 2013).
Historical overview
Andrija Mohorovičić saw a peculiar signal in
seismograms from a shallow earthquake 40 km southeast
of Zagreb, Croatia, in the Kupa Valley on 8 October
1909. Mohorovičić obtained records from seismic stations
within 800 km of the epicenter for both earthquake and
aftershocks. He observed additional primary (P) wave
arrivals with different apparent velocities, at distances
between 400 and 720 km, which he called Ρ. The “normal”
arrival was the direct travel path of the P-wave from hypocenter to receiver. The other was from a wave traveling
along the interface between two media and refracted
towards the receiver at the critical angle (Figure 1). The
latter headwave (P n ) arrived earlier and its apparent velocity, ~7.8 km/s, was much faster than the direct wave.
Mohorovičić surmised that Ρ could be from no deeper than
~50 km and it must represent “. . . a sudden change of the
material which makes up the interior of the earth, because
there a step in the velocity of the seismic wave must exist”
(Mohorovičić, 1910, p. 34). The P n headwave identifies
the seismic discontinuity that marks the compositionally
distinct boundary between crust and mantle. Over the following two decades, contributions from Sir Harold Jeffreys, Victor Conrad, and others showed that the
Mohorovičić discontinuity was ubiquitous.
However, even by the late 1950s, mapping of the Moho
was limited. But from the late 1960s onwards, regional
maps were published for Europe (Morelli et al., 1967),
the Soviet Union (Belyaevsky et al., 1973), and the USA
(Warren and Healy, 1973). Remarkably, these are very
similar to today’s compilations (Thybo et al., 2013).
Drilling to and through the Moho to understand the
nature of the discontinuity and its relation to changes in
other physical properties is difficult. The only direct
attempt is Project Mohole, which ended in 1967
MOHOROVIC ˇ IC ´ DISCONTINUITY (MOHO)
515
extract a general relationship between assemblage composition and environmental factors. Its applicability is therefore contingent on appropriate coverage of the
environmental gradient and of the associated assemblage
compositions. Because of no-analog conditions due to
evolution of niche requirements of the constituent species,
assemblage-based transfer functions are typically limited
in their applicability to late Quaternary sediments.
Sea-surface temperature reconstructions via MAT can be
obtained to a precision of 1
C, and the variance among
the SST values of the best analogs can be used to assess
the uncertainty of a fossil estimate.
Bibliography
Hutson, W. H., 1980. The Agulhas current during the late Pleistocene: analysis of modern faunal analogs. Science, 207, 64–66.
MOHOROVIC ˇ IC ´ DISCONTINUITY (MOHO)
Carolina Lithgow-Bertelloni
Department of Earth Sciences, University College
London, London, UK
Synonyms
Crust-mantle transition; Moho
Definition
Moho: The rapid increase in seismic velocity with which
we often identify the boundary between Earth’s crust
and mantle. More specifically, the Moho is the depth at
which the P-wave velocity first increases rapidly or discontinuously to 7.6–8.6 km/s, or if steep velocity gradients are not present, then it is the depth at which the
P-velocity exceeds 7.6 km/s (Steinhart, 1967).
Introduction
The Mohorovičić discontinuity, also known as the Moho,
is a discrete jump in seismic wave velocities – a seismic
discontinuity – that has come to define the boundary
between the crust and the mantle. Its significance stems
from its clear demarcation of the first boundary of the
chemically differentiated solid Earth. This seminal discovery (Mohorovicic, 1910) complemented that of Earth’s
other major chemical boundaries with the nearly contemporaneous discovery of the outer core (Oldham, 1906)
and the inner core (Lehman, 1936) a few decades later.
The seismic discontinuity at the Moho is explained generally as a chemical (compositionally distinct) boundary
separating crust (both oceanic and continental) from the
mantle, although in detail the Moho is a much more complex boundary. This is in contrast with most other seismic
discontinuities in the mantle, which are explained by
changes in the structure of minerals with homogeneous
composition as a function of pressure. The petrologic
Moho is defined as the change from more felsic crust to
more mafic mantle, but it does not always coincide with
the Mohorovičić discontinuity as determined by refraction
velocity studies or reflection studies. The velocity discontinuity may in places occur within the crust as the basalt to
eclogite metamorphic transition (O’Reilly and Griffin,
2013) or within the mantle at the base of a serpentinized
layer. Depending on tectonic setting its thickness
may vary.
Today, mapping of the Moho at global and regional
scales with a variety of geophysical probes, from activesource seismic reflection and refraction studies to
teleseismic studies to gravity inversions, is a crucial aspect
of geological and geophysical research. Drilling through
the Mohorovičić discontinuity remains an elusive goal.
In honor of the centenary of its discovery, an excellent
and comprehensive review of historical progress and
state-of-the-art research was published in December
2013 (Moho: 100 years after Andrija Mohorovic ˇić,
Tectonophysics, 609, 2013).
Historical overview
Andrija Mohorovičić saw a peculiar signal in
seismograms from a shallow earthquake 40 km southeast
of Zagreb, Croatia, in the Kupa Valley on 8 October
1909. Mohorovičić obtained records from seismic stations
within 800 km of the epicenter for both earthquake and
aftershocks. He observed additional primary (P) wave
arrivals with different apparent velocities, at distances
between 400 and 720 km, which he called Ρ. The “normal”
arrival was the direct travel path of the P-wave from hypocenter to receiver. The other was from a wave traveling
along the interface between two media and refracted
towards the receiver at the critical angle (Figure 1). The
latter headwave (P n ) arrived earlier and its apparent velocity, ~7.8 km/s, was much faster than the direct wave.
Mohorovičić surmised that Ρ could be from no deeper than
~50 km and it must represent “. . . a sudden change of the
material which makes up the interior of the earth, because
there a step in the velocity of the seismic wave must exist”
(Mohorovičić, 1910, p. 34). The P n headwave identifies
the seismic discontinuity that marks the compositionally
distinct boundary between crust and mantle. Over the following two decades, contributions from Sir Harold Jeffreys, Victor Conrad, and others showed that the
Mohorovičić discontinuity was ubiquitous.
However, even by the late 1950s, mapping of the Moho
was limited. But from the late 1960s onwards, regional
maps were published for Europe (Morelli et al., 1967),
the Soviet Union (Belyaevsky et al., 1973), and the USA
(Warren and Healy, 1973). Remarkably, these are very
similar to today’s compilations (Thybo et al., 2013).
Drilling to and through the Moho to understand the
nature of the discontinuity and its relation to changes in
other physical properties is difficult. The only direct
attempt is Project Mohole, which ended in 1967
MOHOROVIC ˇ IC ´ DISCONTINUITY (MOHO)
515