Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few centimeters to several meters, it is a general term for a zone of chemical precipitation and hardening formed at or near the surface of sedimentary bodies through pedogenic and non-pedogenic processes. It is formed by the accumulation of soluble minerals deposited by mineral-bearing waters that move upward, downward, or laterally by capillary action assisted in arid settings by evaporation. Minerals found in duricrust include silica, iron and gypsum. Duricrusts need to be formed in absolute accumulation, therefore they must have a source and precipitation. Duricrust is studied during missions to Mars because it may help prove the planet once had more water. Duricrust was found on Mars at the Viking 2 landing site, a similar structure, nicknamed "Snow Queen," was found under the Phoenix landing site. Phoenix's duricrust was confirmed to be water-based. Caliche DILL, H. G. WEBER, B. and BOTZ, R. Metalliferous duricrusts - markers of weathering: A mineralogical and climatic-geomorphological approach to supergene Pb-Zn-Cu-Sb-P mineralization on different parent materials.- Neues Jahrbuch für Mineralogie Abhandlungen, 190: 123-195 Description NASA: The Sands of Mars
The sub-Cambrian peneplain is an ancient flat, erosion surface, exhumed and exposed by erosion from under Cambrian strata over large swathes of Fennoscandia. Eastward, where this peneplain dips below Cambrian and other Lower Paleozoic cover rocks; the exposed parts of this peneplain are extraordinarily flat with relief of less than 20 m. The overlying cover rocks demonstrate that the peneplain was flooded by shallow seas during the Early Paleozoic. Being the oldest identifiable peneplain in its area the Sub-Cambrian peneplain qualifies as a primary peneplain; the surface was first identified Arvid Högbom in a 1910 publication with Sten Rudberg producing in 1954 the first extensive map. This mapping has been improved upon by Karna Lidmar-Bergström since the 1980s; the Sub-Cambrian peneplain extends as an continuous belt along the eastern coast of Sweden for some 700 km from north to south. Near Stockholm and Hudiksvall the peneplain is densely dissected by joint valleys and at the High Coast is the Sub-Cambrian peneplain is both uplifted and eroded.
More inland the peneplain can be traced at the crestal region of the South Swedish Dome where it is dissected by joint valleys. The Sub-Cambrian peneplain in the crestal region of the South Swedish Dome is the highest step in a piedmonttreppen system seen in Småland. In southern Sweden the peneplain surfaces tilt away from the crest of South Swedish Dome, to the northwest in Västergötland, to the northeast in Östergötland and to the east in eastern Småland. At this last region the sub-Cambrian peneplain is truncated to the west by a well defined and prominent scarp that separates it from the South Småland peneplain to the west. In the Central Swedish lowland the peneplain extends further west being 450 km wide from west to east. In Bohuslän, at the northern end of the Swedish West Coast, there is some uncertainty over whether the hilltops are remnants of the peneplain. A similar situation occurs in central Halland. Further west, parts of the Paleic surface in Norway have been interpreted to be part of the peneplain, tectonically uplifted and is disrupted by NNE-SSW trending faults.
Near the 1,100 m high Hardangervidda plateau in Norway is the Sub-Cambrian peneplain has been uplifted at least thousand meters, albeit Hardangervidda itself is part of a much younger peneplain formed in the Miocene epoch. At Stöttingfjället in northern Sweden the peneplain occur, as result of tectonic uplift, at about 650 meters giving origin to a series of water gaps including those of Ångermanälven, Indalsälven and Ljusnan. In northwestern Finland the Ostrobothnian Plain is a continuation of the peneplain. To the east the Sub-Cambrian peneplain continues as an unconformity beneath the East European Platform. On a grand-scale the peneplain is not flat as it has been deformed; this deformation is an isostatic response to erosion and the load of Phanerozoic sediments that rests above much of the peneplain. The peneplain is characterized by a general lack of inselbergs. One exception to this is the island Blå Jungfrun in the Baltic Sea, an ancient inselberg formed in Precambrian time and buried in sandstone after its formation.
Blå Jungfrun remained buried until erosion of the East European Platform freed it in geologically recent times. Interpretations of Jotnian sandstone imply that much of the Baltic Shield have had faint relief since the Mesoproterozoic, but no exhumed peneplain from this period has been preserved; the low relief terrain on which the Jotnian sandstone deposited was disturbed by the Sveconorwegian orogeny in western Sweden about 1,000 million years ago and begun to erode again into a terrain of subdued relief. The peneplain formed after 600 million years prior to the Cambrian trangression; the basement rocks forming the peneplain surface were exhumed from depths were the temperature was in excess of 100° C prior to the formation of peneplain. Karna Lidmar-Bergström and co-workers assume the peneplain formed though a cycle of erosion with a preceding brief valley phase and that it grades down to a former sea level. Due to the absence of land vegetation in Precambrian times sheet wash is thought to have been an important process of erosion leading to the formation of extensive pediments.
Sheet wash would have hindered the formation of deep weathering profiles. Indeed, at the places the substrate of the Sub-Cambrian peneplain is kaolinized it never exceeds a few meters in depth; the flatness of the peneplain meant that during the Cambrian transgression large areas were swiftly flooded forming large and shallow inland seas in changing configurations. The new relief formed on top of Cambrian sediments smoothed out irregularities in the peneplain. Early Cambrian sandstones overlying the peneplain in southern Norway and Bornholm have never been recycled; this means the parent rocks of the sandstone were eroded and the sediment reworked and weathered reaching sedimentary maturity with no other in-between step or hiatus. The source areas for these sandstones are local rocks from the Transscandinavian Igneous Belt or the Sveconorwegian and Gothian orogens. Baltic Klint Muddus plains Norrland terrain
Mean sea level is an average level of the surface of one or more of Earth's oceans from which heights such as elevation may be measured. MSL is a type of vertical datum – a standardised geodetic datum –, used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and aircraft flight levels. A common and straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location. Sea levels can be affected by many factors and are known to have varied over geological time scales; however 20th century and current millennium sea level rise is caused by global warming, careful measurement of variations in MSL can offer insights into ongoing climate change. The term above sea level refers to above mean sea level. Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level. Instantaneous sea level varies quite a lot on several scales of space.
This is because the sea is in constant motion, affected by the tides, atmospheric pressure, local gravitational differences, salinity and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point. Still-water level or still-water sea level is the level of the sea with motions such as wind waves averaged out. MSL implies the SWL further averaged over a period of time such that changes due to, e.g. the tides have zero mean. Global MSL refers to a spatial average over the entire ocean. One measures the values of MSL in respect to the land. In the UK, the Ordnance Datum is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921, the vertical datum was MSL at the Victoria Liverpool. Since the times of the Russian Empire, in Russia and other former its parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 1.230m below the average sea level. In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level, it is used for main part of Africa as official sea level. As for Spain, the reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references are made to the Amsterdam Peil elevation, which dates back to the 1690s. Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008. Height above mean sea level is the elevation or altitude of an object, relative to the average sea level datum, it is used in aviation, where some heights are recorded and reported with respect to mean sea level, in the atmospheric sciences, land surveying.
An alternative is to base height measurements on an ellipsoid of the entire Earth, what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is used to define heights; the alternative is to use a geoid-based vertical datum such as NAVD88. When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines; the elevation of a mountain denotes the highest point or summit and is illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both. In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol. To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field.
In reality, due to currents, air pressure variations and salinity variations, etc. this does not occur, not as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as ocean surface topography, it varies globally in a range of ± 2 m. Adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month Metonic cycle and the 223-month eclipse cycle on the tides. Several terms are used to describe the changing relationships between sea level and dry land; when the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps; the term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. The term "isostatic" refers to changes in
The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago. The Miocene was named by Charles Lyell; the Miocene is followed by the Pliocene. As the earth went from the Oligocene through the Miocene and into the Pliocene, the climate cooled towards a series of ice ages; the Miocene boundaries are not marked by a single distinct global event but consist rather of regionally defined boundaries between the warmer Oligocene and the cooler Pliocene Epoch. The Apes first evolved and diversified during the early Miocene, becoming widespread in the Old World. By the end of this epoch and the start of the following one, the ancestors of humans had split away from the ancestors of the chimpanzees to follow their own evolutionary path during the final Messinian stage of the Miocene; as in the Oligocene before it, grasslands continued to forests to dwindle in extent. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems.
The plants and animals of the Miocene were recognizably modern. Mammals and birds were well-established. Whales and kelp spread; the Miocene is of particular interest to geologists and palaeoclimatologists as major phases of the geology of the Himalaya occurred during the Miocene, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere. The Miocene faunal stages from youngest to oldest are named according to the International Commission on Stratigraphy: Regionally, other systems are used, based on characteristic land mammals. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula. Mountain building took place in western North America and East Asia. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines.
Well studied continental exposures occur in Argentina. India continued creating dramatic new mountain ranges; the Tethys Seaway continued to shrink and disappeared as Africa collided with Eurasia in the Turkish–Arabian region between 19 and 12 Ma. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea near the end of the Miocene; the global trend was towards increasing aridity caused by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the late Miocene was responsible for the shrinking of tropical rain forests in that region, Australia got drier as it entered a zone of low rainfall in the Late Miocene. During the Oligocene and Early Miocene the coast of northern Brazil, south-central Peru, central Chile and large swathes of inland Patagonia were subject to a marine transgression; the transgressions in the west coast of South America is thought to be caused by a regional phenomenon while the rising central segment of the Andes represents an exception.
While there are numerous registers of Oligo-Miocene transgressions around the world it is doubtful that these correlate. It is thought that the Oligo-Miocene transgression in Patagonia could have temporarily linked the Pacific and Atlantic Oceans, as inferred from the findings of marine invertebrate fossils of both Atlantic and Pacific affinity in La Cascada Formation. Connection would have occurred through narrow epicontinental seaways that formed channels in a dissected topography; the Antarctic Plate started to subduct beneath South America 14 million years ago in the Miocene, forming the Chile Triple Junction. At first the Antarctic Plate subducted only in the southernmost tip of Patagonia, meaning that the Chile Triple Junction lay near the Strait of Magellan; as the southern part of Nazca Plate and the Chile Rise became consumed by subduction the more northerly regions of the Antarctic Plate begun to subduct beneath Patagonia so that the Chile Triple Junction advanced to the north over time.
The asthenospheric window associated to the triple junction disturbed previous patterns of mantle convection beneath Patagonia inducing an uplift of ca. 1 km that reversed the Oligocene–Miocene transgression. Climates remained moderately warm, although the slow global cooling that led to the Pleistocene glaciations continued. Although a long-term cooling trend was well underway, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene; the Miocene warming b
In geology and geomorphology a base level is the lower limit for an erosion process. The modern term was introduced by John Wesley Powell in 1875; the term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory. The "ultimate base level" is the plane that results from projection of the sea level under landmasses, it is to this base level that topography tends to approach due to erosion forming a peneplain close to the end of a cycle of erosion. There are lesser structural base levels where erosion is delayed by resistant rocks. Examples of this include karst regions underlain by insoluble rock. Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins. An example of this is the Messinian salinity crisis, in which the Mediterranean Sea dried up making the base level drop more than 1000 m below sea level; the height of a base level influences the position of deltas and river terraces. Together with river discharge and sediment flux the position of the base level influences the gradient and bed conditions in rivers.
A relative drop in base level can trigger re-adjustments in river profiles including knickpoint migration and abandonment of terraces leaving them "hanging". Base level fall is known to result in progradation of deltas and river sediment at lakes or sea. If the base level falls below the continental shelf, rivers may form a plain of braided rivers until headward erosion penetrates enough inland from the shelfbreak; when base levels are stable or rising rivers may aggrade. Rising base levels may drown the lower courses of rivers creating rias; this happened in the Nile during the Zanclean flood when its lower course became, in a short time, a large estuary extending up to 900 km inland from the Mediterranean coast. Base level change may be related to the following factors: Sea level change Tectonic movement River capture Extensive sedimentation
Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth's surface. Geomorphologists seek to understand why landscapes look the way they do, to understand landform history and dynamics and to predict changes through a combination of field observations, physical experiments and numerical modeling. Geomorphologists work within disciplines such as physical geography, geodesy, engineering geology, archaeology and geotechnical engineering; this broad base of interests contributes to many research interests within the field. Earth's surface is modified by a combination of surface processes that shape landscapes, geologic processes that cause tectonic uplift and subsidence, shape the coastal geography. Surface processes comprise the action of water, ice and living things on the surface of the Earth, along with chemical reactions that form soils and alter material properties, the stability and rate of change of topography under the force of gravity, other factors, such as human alteration of the landscape.
Many of these factors are mediated by climate. Geologic processes include the uplift of mountain ranges, the growth of volcanoes, isostatic changes in land surface elevation, the formation of deep sedimentary basins where the surface of the Earth drops and is filled with material eroded from other parts of the landscape; the Earth's surface and its topography therefore are an intersection of climatic and biologic action with geologic processes, or alternatively stated, the intersection of the Earth's lithosphere with its hydrosphere and biosphere. The broad-scale topographies of the Earth illustrate this intersection of surface and subsurface action. Mountain belts are uplifted due to geologic processes. Denudation of these high uplifted regions produces sediment, transported and deposited elsewhere within the landscape or off the coast. On progressively smaller scales, similar ideas apply, where individual landforms evolve in response to the balance of additive processes and subtractive processes.
These processes directly affect each other: ice sheets and sediment are all loads that change topography through flexural isostasy. Topography can modify the local climate, for example through orographic precipitation, which in turn modifies the topography by changing the hydrologic regime in which it evolves. Many geomorphologists are interested in the potential for feedbacks between climate and tectonics, mediated by geomorphic processes. In addition to these broad-scale questions, geomorphologists address issues that are more specific and/or more local. Glacial geomorphologists investigate glacial deposits such as moraines and proglacial lakes, as well as glacial erosional features, to build chronologies of both small glaciers and large ice sheets and understand their motions and effects upon the landscape. Fluvial geomorphologists focus on rivers, how they transport sediment, migrate across the landscape, cut into bedrock, respond to environmental and tectonic changes, interact with humans.
Soils geomorphologists investigate soil profiles and chemistry to learn about the history of a particular landscape and understand how climate and rock interact. Other geomorphologists study how hillslopes change. Still others investigate the relationships between geomorphology; because geomorphology is defined to comprise everything related to the surface of the Earth and its modification, it is a broad field with many facets. Geomorphologists use a wide range of techniques in their work; these may include fieldwork and field data collection, the interpretation of remotely sensed data, geochemical analyses, the numerical modelling of the physics of landscapes. Geomorphologists may rely on geochronology, using dating methods to measure the rate of changes to the surface. Terrain measurement techniques are vital to quantitatively describe the form of the Earth's surface, include differential GPS, remotely sensed digital terrain models and laser scanning, to quantify, to generate illustrations and maps.
Practical applications of geomorphology include hazard assessment, river control and stream restoration, coastal protection. Planetary geomorphology studies landforms on other terrestrial planets such as Mars. Indications of effects of wind, glacial, mass wasting, meteor impact and volcanic processes are studied; this effort not only helps better understand the geologic and atmospheric history of those planets but extends geomorphological study of the Earth. Planetary geomorphologists use Earth analogues to aid in their study of surfaces of other planets. Other than some notable exceptions in antiquity, geomorphology is a young science, growing along with interest in other aspects of the earth sciences in the mid-19th century; this section provides a brief outline of some of the major figures and events in its development. The study of landforms and the evolution of the Earth's surface can be dated back to scholars of Classical Greece. Herodotus argued from observations of soils that the Nile delta was growing into the Mediterranean Sea, estimated its age.
Aristotle speculated that due to sediment transport into the sea those seas would fill while the land lowered. He claimed that this would mean that land and water would swap places, whereupon the proc
Climatic geomorphology is the study of the role of climate in shaping landforms and the earth-surface processes. An approach used in climatic geomorphology is to study relict landforms to infer ancient climates. Being concerned about past climates climatic geomorphology considered sometimes to be an aspect of historical geology. Since landscape features in one region might have evolved under climates different than today, studying climatically disparate regions might help understand present-day landscapes. For example, Julius Büdel studied both cold-climate processes in Svalbard and weathering processes in tropical India to understand the origin of the relief of Central Europe, which he argued was a palimpsest of landforms formed at different times and under different climates; the various subbranches of climatic geomorhpology focus on specific climatic environments. Desert geomorphology or the geomorphology of arid and semi-arid lands shares many landsforms and processes with more humid regions.
One distinctive feature is the sparse or lacking vegetation cover, which influences fluvial and slope processes, related to wind and salt activity. Early work on desert geomorphology was done by Western explorers of the colonies of their respective countries in Africa, in frontier regions of their own countries or in the deserts of foreign countries such as the Ottoman Empire, the Russian Empire and China. Since the 1970s desert geomorphology in Earth has served to find analogues to Martian landscapes; as a discipline periglacial geomorphology is close but different to Quaternary science and geocryology. Periglacial geomorphology is concerned with non-glacial cold-climate landforms in areas with and without permafrost. Albeit the definition of what a periglacial zone is not clear-cut a conservative estimate is that a quarter of Earth's land surface has periglacial conditions. Beyond this quarter an additional quarter or fifth or Earth's land surface had periglacial conditions at some time during the Pleistocene.
In periglacial geomorphology noted researchers include Johan Gunnar Andersson, Walery Łoziński, Anders Rapp and Jean Tricart. If the tropics is defined as the area between 35° N and 35° S about 60% of Earth's surface lies within this zone. During most of the 20th century tropical geomorphology was neglected due to a bias towards temperate climates, when dealt with it was highlighted as "exotic". Tropical geomorphology do differ from other areas in the intensities and rates at which surface processes operate, not by the type of processes; the tropics are characterized by particular climates, that may be humid. Relative to temperate zones the tropics contain areas of high temperatures, high rainfall intensities and high evapotranspiration all of which are climatic features relevant for surface processes. Another characteristic, not related to present-day climate per see, is that a large portion of the tropics have a low relief, inherited from the continent of Gondwana. Julius Büdel, Pierre Birot and Jean Tricart have suggested that tropical rivers are dominated by fine-grained suspended load derived from advanced chemical weathering, this would make them less erosive than rivers elsewhere.
Some landforms thought as tropical like bornhardts are more related to lithology and rock structure than climate. Climatic geomorphologists have devised various schemes that divide Earth's surface into various morphoclimatic zones. However, only some processes and landforms can be associated with particular climates, meaning that they are zonal. Despite this, azonal processes and landforms might still take on particular characteristics when developing under the influence of particular climates; when identified, morphoclimatic zones do lack sharp boundaries and tend to grade from one type to another resulting in that only the core of the zone has all expected attributes. Influential morphoclimatic zoning schemes are those of Julius Büdel and of Jean Tricart and André Cailleux. Büdel's schemes stresses planation and valley-cutting in relation to climate, arguing the valley-cutting is dominant in subpolar regions while planation is so in the tropics; as such this scheme is concerned not only with processes but with end-products of geomorphic activity.
The scheme of Tricart and Cailleux emphasizes the relationship between geomorphology and vegetation. An early attempt at morphoclimatic zoning is that of Albrecht Penck in 1910, who divided Earth in three zones depending on the evaporation-precipitation ratios. A 1994 review argues that only the concepts of desert, periglacial and a few coastal morphoclimatic zones are justified; these zones amounts to about half of Earth's land surface, the remaining half cannot be explained in simple terms by climate-landform interactions. The limitations of morphoclimatic zoning were discussed by Siegfried Passarge in 1926 who considered vegetation and the extent of weathered material as having more direct impact than climate in many parts of the World. According to M. A. Summerfield large-scale zoning of the relief of Earth's surface is better explained on the basis of plate tectonics than on climate. An example of this are the Scandinavian Mountains whose plateau areas and valleys relate to the history of uplift and not to climate.
Piotr Migoń has questioned the validity of certain morphoclimatic zonation schemes since they are named after processes, like planation, that might not occurring at all in large swathes of the zone. Referring to the 1977 scheme of Büdel Migoń states: Is i