Teaching Earth Sciences - Earth Science Teachers' Association

Peneplains and plate tectonics
Mark Hayward
Abstract
Regional-scale erosion surfaces known as peneplains
preoccupied geomorphologists for about sixty years of
the twentieth century. By the time the theory of plate
tectonics had become embedded in the Earth sciences,
the peneplain concept had virtually disappeared
from view. Recent research, however, indicates that
the concept, albeit in a revised form, could have an
important role to play in understanding landforms on
a large scale in the context of plate tectonics.
Introduction
It is surprising that two recent physical geography texts
(Holden, 2008 and McKnight & Hess, 2008) devote no
space to landforms at the regional scale. The latter, an
American text, includes a map of world landform regions,
but states that the pattern is random and will not be
considered any further! The other text, a widely used
British one at undergraduate level, covers plate tectonics
briefly. The explanation of regional-scale surface features
is an important aspect of geomorphology that has been
quite neglected. The theory of plate tectonics provides a
framework to explain the distribution of landform regions.
The peneplain concept provided geomorphologists with
a phenomenon that can be mapped and correlated from
place to place at a regional scale (morphostratigraphy).
The quantitative revolution in geography from the
1960s onwards led to an emphasis on smaller-scale
process studies. This was a paradigm shift away from the
dominance of Davisian denudation chronology, which
underpinned the peneplain concept. Meanwhile, a better
understanding of tectonics undermined the eustatic
view of world-wide changes in base level that many
geomorphologists believed in.
It is not intended to argue for a return to W M Davis’s cycle
of youth-maturity-old age that dominated geomorphology
for half a century towards the end of the last millennium.
This article will focus on one of his important concepts,
the peneplain, and summarise some recent research by
Earth scientists, while setting it alongside arguably its
stratigraphic counterpart, the unconformity. Google
Scholar was used to search for relevant recent research,
using the keywords peneplain, planation surface and
unconformity. Most ‘hits’ were in the form of abstracts,
but several were pdf files. Unless otherwise stated, the
term peneplain will be used in a broad sense, following
Fairbridge & Finkl (1980): ‘a polygenetic surface of low
relief’: in other words, an extensive lowland formed by
weathering and erosion.
Do peneplains exist?
Peneplains were once fundamental to geomorphological
analysis (Figure 1). Phillips (2002) however, states that
‘despite more than a century of effort, no convincing
example of a contemporary peneplain has been identified,
and the identification of relict peneplains is uncertain and
controversial’. He sets out to demonstrate mathematically
that the relationship between erosion and deposition rates
and uplift or erosion are ‘dynamically unstable’. Therefore,
tectonic stability alone is insufficient to account for the
lack of peneplains, and must be considered together with,
for example, changes in sea level, climate and isostatic
changes.
Nevertheless, whether considering large-scale surface
features of the Earth, or extensive unconformities in the
geological record, evidence of widespread erosion surfaces,
or planation surfaces (therefore peneplains in Fairbridge’s
sense) is incontrovertible.
Examples of peneplains in recent research
Coltori et al (2007) write that ‘Planation surfaces are
an old-fashioned topic in geomorphology, but they are
nevertheless important where they make up much of
the landscape’. Furthermore, ‘These surfaces indicate
that eastern Africa underwent long episodes of tectonic
quiescence during which erosion processes were able
to planate the surface at altitudes not too far from sea
level’; in other words they are peneplains in the Davisian
sense. Successive planation leads to stepped topography.
Peulvast & Claudino Sales (2004) have re-evaluated
stepped planation surfaces in North East Brazil, relating
them to continental rifting and Atlantic opening in the
Cretaceous. Campbell et al (2006) document the Neogene
www.esta-uk.net Vol 35 No 1 2010 Teaching Earth Sciences 43