Spacecraft exploration has revealed abundant evidence that volcanic activity, or volcanism, has played a significant role in the geologic evolution of Mars [1]. Scientists have known since the Mariner 9 mission in 1972 that Mars possesses some of the most dramatic volcanic landforms found anywhere within the solar system. These landforms include extensive lava plateau, vast lava plains, and the largest known shield volcanoes (both minor and major) in the Solar System [2,3]. Martian volcanic features range in age from Noachian (>3.7 billion years) to late Amazonian (< 500 million years), indicating that the planet has been volcanically active throughout its history [4], and some speculate it probably still is so today [5]. The volcanoes on Mars appear to be broadly similar in overall morphology (although, often quite different in scale) to volcanic features on Earth, which suggests that Martian eruptive processes are not significantly different from the volcanic styles and processes on Earth [6].

The distribution of volcanoes on Mars is not uniform; there are several regions where central volcanic constructs are concentrated. The western hemisphere of Mars is dominated by a massive volcano-tectonic complex known as the Tharsis region or the Tharsis bulge. This immense, elevated structure is thousands of kilometers in diameter and covers up to 25% of the planet’s surface [7]. It contains the highest elevations on the planet. Three enormous volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons (collectively known as the Tharsis Montes), sit aligned northeast–southwest along the crest of the bulge. The Tharsis bulge contains some of the youngest lava flows on Mars, but the bulge itself is believed to be very ancient[8]. According to Hodges and Moore [9] a complex history of overlapping volcanic events over a long period of time characterizes the Tharsis volcanoes as deduced from their extraordinary larger or complex summit caldera. The extreme massiveness of Tharsis has placed tremendous stresses on the planet's lithosphere. As a result, immense extensional fractures (grabens and rift valleys) radiate outward from Tharsis, extending halfway around the planet [10].

The vast Alba Mons (formerly Alba Patera) occupies the northern part of the Tharsis bulge, whereas the huge shield volcano Olympus Mons lies off the main bulge, just off the western edge of the province. The Alba Mons is a unique volcanic structure, with no counterpart on Earth or elsewhere on Mars. The flanks of the volcano have extremely low slopes characterized by extensive lava flows which can be traced as far north as 61°N and as far south as 26°N, thus making it one of the widest extensive volcanic features in the Solar System [5,11,12]. The huge shield volcano Olympus Mons, the largest volcano on the planet, is located 1200 km northwest of the Tharsis Montes. It was built up by many thousands of individual flows of highly fluid lava

In addition to the large shield volcanoes, Tharsis contains a number of smaller volcanoes called tholi. The tholi are dome-shaped edifices with flanks that are much steeper than the larger Tharsis shields. Ceraunius Tholus Uranius Tholus, and Uranius Mons and Uranius Mons follow a northeast-southwest trend which is the same trend of all three Tharsis Montes. A smaller volcanic center lies several thousand kilometers west of Tharsis in Elysium. The Elysium volcanic complex is about 2,000 kilometers in diameter and consists of three main volcanoes, Elysium Mons, Hecates Tholus, and Albor Tholus. Elysium Mons is the largest volcanic edifice in the province. It is thought to be somewhat different from the Tharsis Montes, in that development of the former involved both lavas and pyroclastics [11].

The main difference between Martian shield volcanoes and those on Earth is in size. For example, the tallest volcano on Mars, Olympus Mons, is nearly 100 times greater in volume than Mauna Loa in Hawaii, the largest shield volcano on Earth. Geologists think one of the reasons volcanoes on Mars were able to grow so large is that Mars has fewer tectonic boundaries in comparison to Earth [13]. Lava from a stationary hot spot was able to accumulate at one location on the surface for many hundreds of millions of years.

Topographically, the northern and southern hemispheres of Mars are strikingly different from each other. This hemispheric dichotomy is a fundamental global geologic feature of the planet. The northern part of the planet is an enormous flat topographic depression with few craters and lies below the conventional ‘zero elevation’ level. About one-third of the planet's surface (mostly in the northern hemisphere) lies 3–6 km lower in elevation than the southern two-thirds. The other two-thirds of the Martian surface are the southern hemisphere which is mountains and highlands, with dense craters and mostly well above zero elevation. This dichotomy is also expressed in two other ways: as a difference in volcanic crater density and crustal thickness between the two hemispheres [5]. Concerning the crustal thickness, topographic and geophysical gravity data indicate that the crust in the southern highlands has a maximum thickness of about 58 km, whereas crust in the northern lowlands "peaks" at around 32 km in thickness [14,15].

The origin and age of the hemispheric dichotomy are still debated. Hypotheses of origin generally fall into two categories: one, the dichotomy was produced by a megaimpact event or several large impacts early in the planet's history [16-18] or two, the dichotomy was produced by crustal thinning in the northern hemisphere by mantle convection, overturning, or other chemical and thermal processes in the planet's interior [19,20]. The Martian dichotomy appears to be extremely old whatever its origin. Crater counting studies of QCDs suggest that the underlying surface in the northern hemisphere is at least as old as the oldest exposed crust in the southern highlands [21]. The northern lowlands were also urged as may have contained a large standing body of water in earlier Mars history [22-24].

Straddling the dichotomy boundary in Mars's western hemisphere is the massive volcano-tectonic province of the Tharsis bulge. Near the equator in the western hemisphere lies an immense system of deep, interconnected canyons and troughs collectively known as the Valles Marineris. The canyon system extends eastward from Tharsis for a length of over 4,000 km, nearly a quarter of the planet's circumference [25].

Scientists have never recorded an active volcano eruption on the surface of Mars [26]. However, the European Space Agency's Mars Express orbiter photographed lava flows interpreted in 2004 to have occurred within the past two million years, suggesting a relatively recent geologic activity [27]. An updated study in 2011 estimated that the youngest lava flows occurred in the last few tens of millions of years [28]. The latter authors consider this age makes it possible that Mars is not yet volcanically extinct.

Because Mars has no oceans and hence no "sea level", a zero-elevation surface had to be selected as a reference level; this is called the areoid [29,30] of Mars, analogous to the terrestrial geoid. In 2001, Mars Orbiter Laser Altimeter data led to a new convention of zero elevation defined as the equipotential surface (gravitational plus rotational) whose average value at the equator is equal to the mean radius of the planet [31]. In practice, today this surface is defined directly from satellite gravity measurements.

Much works have been done on the volcanoes and volcanic activity on the surface of Mars. However, the vast majority of these works focused on the shield volcanoes, the giant impact basins, the big channels and the impact craters, but never really been able to look at the smaller volcanoes on Mars except of few works [4,32, 33].

The present investigation is concerned with the study of smaller volcanoes and volcanic landforms which are formed at or near the ground surface around a volcanic vent by lava flow or fragments of lava thrown out during eruption on the surface of Mars. The study has contributed immensely to the interpretation of different generations of independent collapse calderas and volcanic pipes, and thus confirms that the volcanic activity in the planet Mars is not limited to several shield volcanoes, but includes many volcanic craters that have long been identified as “impact craters” formed by collision of meteorites with the planet.

Previous Related Work

Mars has the greatest diversity of craters of any planet in the Solar System [34]. The craters have been described for a long time as belonging to the” impact crater types” [34-37]. Those which are below about 7 km in diameter were called simple craters; they are bowl-shaped with sharp raised rims and have depth/diameter ratios of about 1/5 [35]. According to this author the simple craters change to more complex types at diameters of roughly 5 to 8 km. Complex craters have central peaks (or peak complexes), relatively flat floors, and terracing or slumping along the inner walls. Complex craters are shallower than simple craters in proportion to their widths, with depth/diameter ratios ranging from 1/5 at the simple-to-complex transition diameter (~7 km) to about 1/30 for a 100-km diameter crater. Another transition occurs at crater diameters of around 130 km as central peaks turn into concentric rings of hills to form multi-ring basins [5].

Over 42,000 of “impact craters” greater than 5 km in diameter have been catalogued on Mars [36] while the number of smaller craters is innumerable. The density of craters on Mars is highest in the southern hemisphere, south of the dichotomy boundary. A new study by SPACE.com Staff [38] recorded more than 635,000 “impact craters” at least 0.6 miles (1 kilometer) wide on Mars [37]. However, only about one thousands of these craters have names which are assigned by the International Astronomical Union [39].

However, several craters on Mars, thought to be created by impacts from space, have later been found by some scientists to be caused by explosive volcanic eruptions. Their assessment is based on images and topographic data from NASA's Mars Odyssey, Mars Global Surveyor and Mars Reconnaissance Orbiter spacecraft, as well as the European Space Agency's Mars Express orbiter.

In Search of Martian Craters by Schmidt [40], the author highlighted the valuable remarks by the scientists Ramsey and Crown. Ramsey who is part of a research team developing techniques to accurately identify and analyze small features on the Martian surface in satellite images said. "In the past, we've focused on the large volcanoes, the giant impact basins, and the big channels," "but we've never really been able to look at the small things on Mars”. The "small things" that Ramsey and his team hope to shed light on are craters - bowl-shaped depressions on a planetary surface that are typically caused by one of two processes. Impact craters form when a meteoroid, asteroid, or comet collides with a planet. Volcanic craters delineate vent areas at the summit of a volcano. David Crown, senior scientist at the Planetary Science Institute in Tucson, Ariz., and a co-investigator on the project also said [40] that "When you get down to small sizes, it's hard to tell the difference between a volcanic crater and an impact crater". Schmidt commented that the researchers hope to come up with criteria that will enable them to distinguish between impact craters and maar craters solely by looking at satellite data. Because the presence of maar craters is a strong indication of water beneath the surface, the ability to identify them on Mars has important implications for understanding the planet's geologic history. "We're interested in craters that are potentially water-driven features, because that means there could be liquid water beneath the surface, which is a big deal for Mars exploration," said Ramsey. In the past, most scientists believed that Mars' volcanic activity was most intense in its early time (its first billion years), and that the last three billion years have been relatively quiet," Ramsey said, but the ability to now see smaller things on the Martian surface may change existing beliefs about the planet's geologic history.

Wilson and Head [41] maintained that explosive volcanic eruptions, potentially involving large amounts of magmatic and entrained water, are thought to have been very important in the Noachian and Hesperian periods earlier in Mars history. Evidence for ancient explosive volcanism within Arabia Terra, Mars has been recently given by Scientists from NASA and the Planetary Science Institute in Tucson, Ariz. [4,32]. These scientists have identified several irregularly shaped craters located within Arabia Terra, Mars, representing a new type of highland volcanic construct and together constitute a previously unrecognized Martian igneous province. Similar to terrestrial supervolcanoes, these low-relief paterae possess a range of geomorphic features related to structural collapse, effusive volcanism and explosive eruptions. Extruded lavas contributed to the formation of enigmatic highland ridged plains in Arabia Terra.

Michalski and Bleacher [32] paid attention to the fact that a series of Martian craters assumed to have been formed by meteorites may actually be extinct volcanoes so massive that, when they were active billions of years ago, they could have buried Mars in ash. They recorded several irregularly shaped craters located within Arabia Terra a geologically ancient region of northern Mars, representing a new type of highland volcanic construct and together constitute a previously unrecognized Martian igneous province. The craters appear as several huge circular pits that resemble Earth's calderas, in which magma beneath a volcano drains after a volcanic eruption, causing the ground above the magma chamber to collapse. The best example on Mars is a feature called Eden patera, a depression about 85 kilometers long, 55 kilometers wide and 1.8 kilometers deep. Michalski and his colleague Bleacher in a report in Nature today1, describe three separate calderas within the depression, along with possible signs of a lake of solidified lava and a volcanic vent where lava could have oozed out.

Michalski et al. [4] interpreted Eden Patera as a complex caldera formed through structural collapse associated with withdrawal or migration of magma at depth, and explosive eruptions. Some of reasons for suspecting that Eden Patera is a collapsed caldera not an impact crater are its irregular shape, an apparent lack of a raised rim or central peak, and lack of impact ejecta. Other features recorded by [4] in northern Arabia Terra that contain evidence for collapse associated with volcanic activity include Siloe Patera, Euphrates Patera and Semeykin Crater. According to the same authors these features constitute a new category of Martian volcano that can be described as plains style caldera complexes, of which Eden Patera is the type-example.

The discovery of a new type of volcanic construct in the Arabia volcanic province [32] fundamentally changes the picture of ancient volcanism and climate evolution on Mars. Other eroded topographic basins in the ancient Martian highlands that have been dismissed as degraded impact craters should be reconsidered as possible volcanic constructs formed in an early phase of widespread, disseminated magmatism on Mars [4].

Emran et al. [33] interpreted surficial geology, including possible geologic history, of Siloe Patera by combining analysis of spectral analysis from TIR data of THEMIS, mineralogical phases (e.g. olivine, phyllosilicate, and hydrated silicates) from NIR data of CRISM, and local morphology from HiRISE and CTX. They came to the conclusion that the presence of faulting consistent with collapsed features and related lavaflow features confirm that Siloe Patera is a collapsed caldera of Late Noachian to Early Hesperian [1], which was modified by subsequent water-related processes.

The present investigation concerns with the topography, topographic elevation, morphology and relative age concept of different types of volcanic calderas and volcanic pipes. 300 volcanoes of the highest resolution on the surface of Mars were chosen for detailed study, of which 150 volcanoes were subjected to a detailed statistical analysis for means of graphical correlation of the morphological and topographical properties of these volcanoes. The study reveals the recognition of six different types of volcanic calderas and two types of volcanic pipes. The relative age of these types of volcanic landforms on the Martian surface is assigned on basis of stratigraphic relationships of different volcanoes.

Methods of Study

This study has been made possible by the use of Google Mars which is a program that allows exploring Mars through official satellite images gathered by different spacecraft orbiting the planet. The program is an application within Google Earth Pro which is currently the standard version of the Google Earth desktop application as of version 7.3.2.5776 (64-bit), Build Date Tuesday, March 5, 2019 12:43:51 AM UTC7.3. The program of Google Mars allows viewers to zoom around the Red Planet in much higher resolution than the simpler browser version and will even render certain locations in 3-D. It includes extremely highresolution images from the Mars Reconnaissance Orbiter's HiRISE camera on the NASA Mars Reconnaissance Orbiter spacecraft, the Context Camera (CTX) on NASA's Mars Reconnaissance Orbiter which offers great details with around 20 feet per pixel, the Narrow Angle Mars Orbiter Camera (MOC) on the NASA Mars Global Surveyor spacecraft,the High Resolution Stereo Camera (HRSC) instrument on the European Space Agency Mars Express spacecraft, the Compact Reconnaissance Imaging Spectrometer for Mars(CRISM) instrument on the NASA Mars Reconnaissance Orbiter spacecraft. Finally, there are many high-resolution panoramic images from various Mars landers, such as the Mars Exploration Rovers, Spirit and Opportunity, that can be viewed in a similar way to Google Street View.

300 satellite images (Visible imagery of Google Mars) of craters on the surface of Mars were carefully examined for resolution, distribution, unique morphology and evidence of volcanic activity. The images include volcanic craters at the ground surface with variable depths, sizes, outer rims and showing good evidence of eruption of volcanic lava and signs of formation of volcanic calderas around clear volcanic vents through the collapse of the volcano's roof and the withdrawal of the lava to the drained magma chamber, as well as craters with a second post caldera re-eruption of lava after re-filling of the magma chamber. They include also the small craters formed by violent, supersonic eruption of deep-origin volcanoes leading to the formation of pipe-like structures that penetrate the surrounding rock and create bowel-shaped, craterlike depression which does not form a large above-ground elevation as typical volcanoes do.

For means of graphical correlation between different types of volcanoes the morphological parameters of 150 volcanoes among the fore-mentioned 300 volcanoes have been subjected to detailed statistical analyses. To make the graphic comparison possible between different types of volcanoes, the electronic profile is rejected and a manual cross section has been made for each volcano across its width, where selected points of elevation were chosen along the width of the volcano in the range of 15 or 16 points which are more or less symmetrically spaced (side symmetry around the center of volcano) across the width of volcano. The results were subjected to the Microsoft Excel 2010 program where a cross-section is drawn representing the topography and topographic elevation of sides/flanks of volcano, the maximum depth of volcanic crater (caldera/ volcanic pipe) below the ground surface, the height of the outer rims above the ground, the maximum height of the mountain volcano above ground surface (above surroundings). The width and diameter of the craters are given for each volcano according to the measurements of the ruler of Google Mars. Due to the great variability in thickness of the outer rim of the volcanic crater, so the value given for the height of the outer rim is the average of this height around the crater. Graphic representation of all these topographic and morphologic features in different types of volcanoes has been given.

Correlation of different parameters in different types of volcanoes is made using bimodal and three-dimensional diagrams. The parasitic post-calderas composite volcanoes or cinder cones which are formed by re-filling of the magma chamber and re-eruption of lava at the bottom surface of some pre-existing large calderas are also described and measured for width and height using the electronic profile.

Results and Discussion

Evidence of Volcanic Origin of the Studied Craters on Mars

Volcanic craters produced by collapse of surface rock into an empty magma chamber after massive volcanic eruptions are called calderas. Craters produced by the collision of a meteorite with the surface of Mars (or another planet or moon) are called impact craters. Evidence of the first case includes presence of lava flows and/or water vapor and hematite near or around the crater, or successive collapse of the roof of the magmatic chamber instead of one stage of collapse, or re-filling of the magma chamber and re-eruption of post caldera volcanic landforms from the same vent at the bottom surface of the pre-existing caldera, or the presence of younger generations of different morphologies and different ages of volcanic eruption inside the same caldera. In the impact craters there is usually a large, rayed (star-shaped) distribution of ejecta blanket (enormous amounts of shattered material resulted from the pulverization of the rock immediately after the strike) around the rim of the resultant crater.

The detailed study of 300 craters in both northern and southern hemispheres of Mars revealed that these craters are volcanic in origin and represented by collapse calderas and volcanic pipes. These volcanic landforms are variable in size, subsurface depth below ground surface, height of outer rim above ground surface, topography of the bottom surface of depression and topographic levels with respect to the conventional 0 datum. They are occurring either independently as free landforms on surface of plains, valleys and plateaus or as parasitic landforms along the flanks of shield volcanoes or at the bottom floor of larger calderas.

The criteria of volcanic origin of the studied craters can be summarizes as below:

1. The collapse calderas represent the most spectacular and abundant volcanic landform among the studied craters. They possess a bowel-shaped depression formed when the volcano collapse into the void left when its magma chamber is emptied to form a large crater. Magmais stored beneath a volcano in a magma chamber. When a massive volcanic eruption occurs which empties the magma chamber, the unsupported rock that forms the roof of the magma chamber then collapses to form a large depression or bowel-shaped feature on the surface with steep walls and within which occurs a vent. Crater Lake National Park and many other calderas on Earth are thought to have formed by this process.

2. There is no surface material from outside the crater are occurring around or inside the caldera crater, but a strong evidence of lava flows or pyroclastic materials usually occurs either along the outer rim and sides of the crater depression (Plate 1, Figures A-J, Plate 2, Figures A-J), around the central vent beside the walls of the crater depression (Plate 3, Figures A-J) at a central-peripheral vent (Plate 4, Figures A-J) at a peripheral vent in the bottom floor of the collapsed caldera (Plate 5, Figures A-J) or around the parasitic volcanic landforms which may re-erupt at the bottom floor of the pre-existing caldera (Plate 3, Figures G-J). No water is found in the depression of the craters but the sides or outer rim of many depressions are covered by fine dust arranged in streaks formed by water vapor and hematite which accompanied the eruption of lava.

3. Many calderas on Mars exhibit a lava tube (open cave) which is a natural conduit formed by flowing lava which moves beneath the collapsed roof of the caldera. (Plate 6, Figures A-E). Draining of the liquid lava from these tubes will leave open caves.

4. A younger steam explosion (water vapor and hematite) took place after complete cessation of volcanic activity through a natural conduit (a vent or lava tube) on the collapsed roof of the lava tube of different calderas on Mars (Plate 7, Figures A-J). Very probably this steam explosion continued in calderas for a long time.

5. Many calderas belonging to different types have a zonal structure (Plate 8, Figures A-J) resulting from a series of gradual collapse of the roof of magmatic chamber rather than a single event (Plate 9, Figures A-J).

6. The so called “complex crater” which exhibits “central peak” uplift in the center of the crater and which is usually used to identify the impact craters is simply a large caldera crater with a younger post-caldera phase of lava reeruption leading to the formation of cinder cone (Wizard Island), composite volcano or volcanic domes over the central vent at the bottom floor of the pre-existing caldera (Plate 10, Figures A-J). This type of caldera is commonly of a large to huge size. In few cases the central elevated mass at the bottom floor of small-sized caldera results from upheaval of the caldera floor due to compression of the remaining magma beneath the collapsed roof of the chamber, not refilling of the magma chamber and re-eruption of new lava flow on the bottom floor of the collapsed caldera (see later). This phenomenon is almost confined to calderas which are lying near or at the southern ice-cap pole.

7. The calderas on Mars are not uniform in age, general morphology of the crater depression (flat with steep walls, broadly rounded, strongly concave), diameter of crater ( from 7.7 km to 162 km), total width of volcano (from 11.2 km to 243 km ), subsurface depth of the depression below ground surface (from 626 m to 2900 m), height of outer rim above ground surface (from 12.5 m to 1038 m), topographic level, geographic location, presence or absence of parasitic landforms (cinder cones, composite volcanoes, volcanic domes) formed by refilling of the underlying magma chamber and re-eruption of lava at the bottom floor type of the pre-existing caldera. All these morphologic variations suggest that these craters were formed by volcano caldera collapsing, rather than from an impact. In craters of equal size caused by a meteorite collision, it is assumed that these craters represent meteorites of the same size, same weight, same speed, same vertical collision force, and accordingly the depressions arising from these craters become consequently of equal depth and equal degree of slope since the surface rock is lithologically uniform. But the reality on Mars is that many of the caldera craters of equal sizes are different in the depth of depression, in the degree of its slope below the surface and in the surface topography of the bottom surface despite their adjacent occurrence in the same place (Plate 11, Figures A-F). There are also large caldera craters of little depth, while there are quite next to them smaller

8. Beside the calderas there are an innumerable number of volcanic pipes varying in diameter from less than 1 km to 10.25 km and reaching up 22.5 in width. These pipes are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. They rise onto surface of Mars through a series of deep fissures in the crust where magma rises through the crust by exploiting a vertical or inclined path of weakness on the surface ground where they penetrate the surrounding rock and create a bowel-shaped, craterlike depression. Unlike other kinds of volcanic eruption, the magma which eject as volcanic pipes does not collect in a subsurface reservoir prior to the eruption. The volcanic pipes are structurally controlled; those which are inclined on the surface possess a carrotshaped profile and showing different trends, following the trends of surface fractures (Plate 12, Figures A-J; Plate 13, Figures A-B) The volcanic pipes which are perpendicular to the surface are deeper and expose a vertical funnel-shaped profile (Plate 13, Figures C-J).The outer rims and the sides of depressions of both inclined and vertical types of volcanic pipes are covered by fine dust arranged in streaks formed by water vapor which accompanied the eruption of magma (Plate 14, Figures A-E; Plate 15, Figures A-E), sometimes showing intense steam explosion (water vapor and hematite) associated with massive eruption (Plate 12, Figure J). It is thus apparent that the volcanic pipes have encountered rock layers containing groundwater during the ascent of the magma through a series of fissures. The water is vaporized together with other volatile materials (carbon dioxide) producing an explosive event at the surface. The impact craters which caused by perpendicular collision of meteorites are never occurring inclined to the ground surface and never becoming structurally controlled (i.e. following fissures or fault planes) in the crust as the volcanic pipes do on the surface of Mars.