مروری بر خصوصیات لندفرم های کارستی در لایه‌های گچی

Introduction
Evaporite formations, many of which include thick gypsumunits, are covered 8% of the Earth's surface and underlie about 25% of the continental surface of the Earth. Gypsum (CaSO4.2H2O) is readily soluble in water. Therefore, karstic landscapes on gypsum terrains develop and are degraded much faster than they do on limestone or carbonate rocks .Because of its high solubility and dissolution rate, gypsum karst can evolve on a human time scale rather than a geological time scale. In spite of its widespreadoccurrence and its relevant importance from the applied perspective (hazards, impacts, and resources), gypsum karst has received scarce attention andremains comparatively understudied. From the 1990s, gypsum karst study began in earnest worldwide and in Iran, these studies is very young and rarely documentation can be found has studied gypsum Karst especially. It can be said this branch of karst science still in its infancy. In fact, the lack of codified documentation about gypsum karst in the Persian language has been main motivation for writing this article. Actually, the goal of this paper is to describe gypsum kart, its landforms, its problems and its importance for human life.
 
Methodology
This study started with searching Gypsum karst in Science direct and Elsevier sites and fallowing with searching words such as Gypsum karst landform, Gypsum Karren, Gypsum sinkhole.  Due to limitation resources, most of article were used in writing this paper. Most of the reviewed articles have been published in journals such as Geomorphology, Carbonates and Evaporites, Earth Surface Processes and Landforms, International Journal of Speleology، Engineering Geology, and Environmental Geology. Because of these articles were not similar, authors were unable to perform a systematic review or meta-analysis on the articles. So in this research, we have tried to provide a narrative review of the gypsum karst concepts, gypsum landforms and their problems.
Results and discussion
The reviewed articles have studied on various aspects of gypsum karst including geomorphology, geology, environmental hazards, civil engineering, and hydrogeology. However, even when the gypsum karst applied aspect studies, geomorphological survey is an essential part of them. Therefore, based on case studies and those are particularly focused on karst gypsum geomorphology, authors have provided an overview of the gypsum karst concepts, gypsum Karst outcrops in the world and Iran, Gypsum and carbonate Karst differences, and then have discussed about surface gypsum karst landforms (such as Sinkhole, Karren, Poljes, Tumuli and Polygons, Escarpments and Landslides, Blind valleys and Springs, and morpho-structures generated by interstratal dissolution of gypsum and subsidence of the overlying rocks), and their problems.
 
Conclusion
Due to the high rate of dissolution in the gypsum formation, occurrence mapping of the gypsum formations in surface and underlying has high importance, Because even if these area have not specific gypsum karst features currently, they can be susceptible to dissolution with natural and human factors such as irrigation, water harvesting, construction and dewatering of dams, mining, or even minor changes in climate at a short time. Study of gypsum karst features during the time also is very essential, because they can show significant changes in short periods of times. In addition to the gypsum surface karstification, Interstratal karstification can cause cap rock collapse sinkhole, subsidence, and also lead to creation of suitable aquifer in such area.
Although gypsum karst has many features in common withcarbonate karst, it shows significant peculiarities mainly due tolithology type and characteristics of stratigraphic formations (sedimentary environments of them).  Rapiddissolution may substantially weaken the gypsiferous rockmass at a human timescale. Consequently, karst morphologies form and evolve much faster in gypsum than in carbonate rocks. Moreover Gypsum has a lower mechanical strength and a moreductile rheology than carbonate rocks whereas carbonate rocks are very resistant and brittle. Gypsiferous formations commonly include more soluble salts (e.g., halite, sylvite, and glauberite), and thin marl units. Interstratal dissolution of interbedded salts induces the development of fractures and breccia’s in the overlying and interbedded gypsiferous sequence, providing pathways for groundwater flows. While carbonate formation often are seen the thick and dense layers.
Study of Gypsum landforms indicate that the differences in form and size of them are as a result of combination various factors processes including dissolution, reprecipitation, recrystallization, changes in the volume of the outer layer of the rocks, other weathering process, and gypsum lithology and stratigraphic formations characteristics. This means that, in spite of dominating the dissolution in gypsum karst landscapes the other processes can have a significant role in development of gypsum landforms. We should not neglect tectonic evolution and climatic conditions, because they also play an effective role in creating a variety of landforms and development of karstification in such areas.
In environmental management, surface gypsum Karst landforms can be separated into two general categories. The first category is hazardous landforms and the second category is surface landforms without hazard. Sinkholes, subsidence and gypsum cliffs and earthquakes locate in the first category because they can cause many hazards for man-made Constructions and human life. The second category includes Karen , Tumuli and Polygons, Poljes, blind valley, springs, and Breccia pipes that not only are hazardous, but also sometimes can be beneficial (for example, gypsum karst springs as sources of groundwater and breccia pipes can be used as construction materials). Of course, those sinkholes that intersect the water table or occur in groundwater discharge zones that create constant or seasonal lakes locate in the second category.
In general it can be said, Understanding the geomorphology, hydrology, and underlying geology are essential for developing and managing gypsum karst areas. Local or national planning based on careful investigation, can permit safe development of gypsum karst areas. So due to the gypsum formations are covered 80000 square of kilometers of the Iran’s surface (however we have not exact information about gypsum formations underlie the continental surface of Iran) identification of gypsum landforms, mapping of them and providing susceptibility and hazard maps of them (specially sinkholes and subsidence), and also local hydrogeology survey are crucial for development and public safety in these region.