Inverted Relief
Encyclopedia
Inverted relief is a landscape that is part of a planet's surface, e.g. Mars, that contains positive landforms, i.e. hills and ridges, that were once depressions in its surface. On Earth, inversion of relief can occur whenever either the sediments or other strata underlying a depression, typically the floor of a floodplain
or fluvial
valley, becomes more resistant to erosion than the strata in which the valley has been cut. Then differential erosion preferentially removes the less resistant strata underlying the walls of the depressions and adjacent uplands, leaving the floor of the depression as a topographic high.
, accumulated within depressions, i.e. stream valleys and lake basins. Later, erosion
— possibly by wind, which can't move coarse-grained sediments — removed fine-grained sediments comprising the surround surface layers, but left behind the more resistant coarse-grained sediments as a ridge. Second, another process by which inverted relief might be created would be the filling of a fluvial
valley with either lava
or welded tuff. A layer composed of either of these volcanic rocks would resist erosion while the surrounding surface is eroded away to create a ridge out of what was formally a valley. Finally, inverted topography might be formed by the cementation by mineral
s dissolved in water of sediments that once either filled or underlay a topographic depression such as a lake or fluvial valley. In terrestrial environments, such cementation often occurs within stream valleys as the result of the formation of duricrust
s, i.e. silcrete
or ferricrete
, by pedogenic processes. As in case of lava, the cemented sediments would resist erosion while the surrounding countryside is eroded away to create a ridge or hill where a depression once existed.
A classic example of terrestrial inverted relief is Table Mountain, Tuolumne County, California
. In case of Table Mountain, multiple lavas flows filled an ancient fluvial
valley that cut westward through the across the central Sierra Nevada to the Central Valley about 10.5 million years ago. These Miocene
lava flows filled this ancient river valley with a thick sequence of high-K, trachyandesite lavas that are significantly more resistant to erosion than the Mesozoic
siltstone
and other rock in which the valley was cut. As a result, subsequent differential erosion left these volcanic rocks as sinuous ridge, which now stands well above landscape underlain by more deeply eroded Mesozoic rocks.
Inverted relief in the form of sinuous and meandering ridges, which are indicative of ancient, inverted fluvial channels, is argued to be evidence of water flowing on the Martian surface in the past. Examples of such ridges, which might represent ancient fluvial channels, are shown below from various parts of Mars. Because of sinuous ridges, which might be ancient fluvial channels, and minerals that indicated the past presence of water, Miyamoto Crater was proposed in 2010 as a potential location to be searched for evidence of life on Mars.
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
or fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
valley, becomes more resistant to erosion than the strata in which the valley has been cut. Then differential erosion preferentially removes the less resistant strata underlying the walls of the depressions and adjacent uplands, leaving the floor of the depression as a topographic high.
Formation
Multiple processes can cause the floor of a depression to become more resistant to erosion than its surrounding slopes and uplands. First, coarse-grained sediments, i.e. gravelGravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
, accumulated within depressions, i.e. stream valleys and lake basins. Later, erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
— possibly by wind, which can't move coarse-grained sediments — removed fine-grained sediments comprising the surround surface layers, but left behind the more resistant coarse-grained sediments as a ridge. Second, another process by which inverted relief might be created would be the filling of a fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
valley with either lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
or welded tuff. A layer composed of either of these volcanic rocks would resist erosion while the surrounding surface is eroded away to create a ridge out of what was formally a valley. Finally, inverted topography might be formed by the cementation by mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s dissolved in water of sediments that once either filled or underlay a topographic depression such as a lake or fluvial valley. In terrestrial environments, such cementation often occurs within stream valleys as the result of the formation of duricrust
Duricrust
Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few millimeters or centimeters to several meters....
s, i.e. silcrete
Silcrete
Silcrete is an indurated soil duricrust formed when silica is dissolved and resolidifies as a cement. It is a hard and resistant material, and though different in origin and nature, appears similar to quartzite...
or ferricrete
Ferricrete
Ferricrete is a hard, erosion-resistant layer of material at the land surface that consists of near surface sediments cemented by iron oxide in to a duricrust. Ferricretes contains sediments and other non-indigenous materials, which have been transported from outside the immediate area in which it...
, by pedogenic processes. As in case of lava, the cemented sediments would resist erosion while the surrounding countryside is eroded away to create a ridge or hill where a depression once existed.
A classic example of terrestrial inverted relief is Table Mountain, Tuolumne County, California
Table Mountain (Tuolumne County, California)
Table Mountain is a mountainous landform in Tuolumne County, California. Nine million years ago, lava erupted from the east and filled an ancient river bed. Subsequently, the surface around the river eroded, while the hard lava protected the river from erosion...
. In case of Table Mountain, multiple lavas flows filled an ancient fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
valley that cut westward through the across the central Sierra Nevada to the Central Valley about 10.5 million years ago. These Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
lava flows filled this ancient river valley with a thick sequence of high-K, trachyandesite lavas that are significantly more resistant to erosion than the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
and other rock in which the valley was cut. As a result, subsequent differential erosion left these volcanic rocks as sinuous ridge, which now stands well above landscape underlain by more deeply eroded Mesozoic rocks.
Inverted relief in the form of sinuous and meandering ridges, which are indicative of ancient, inverted fluvial channels, is argued to be evidence of water flowing on the Martian surface in the past. Examples of such ridges, which might represent ancient fluvial channels, are shown below from various parts of Mars. Because of sinuous ridges, which might be ancient fluvial channels, and minerals that indicated the past presence of water, Miyamoto Crater was proposed in 2010 as a potential location to be searched for evidence of life on Mars.