A plethora of spectacular surface images have accrued from the Mariner 9 and Viking missions. Below, we show only a few, to whet your intellectual appetite, but for the curious, consult these two references for many more pictures: The Geology of Mars, T.A. Mutch et al., 1976, Princeton University Press and Viking Orbiter Views of Mars, C. R. Spitzer, Ed., 1980, NASA SP-441.
As has been alluded to before in this subsection, Mars seems to consist of two dominant terrains - the northern half is mostly a plains made up by volcanic rocks with some conspicuous volcanic structures, but is only moderately cratered; the southern part of Mars also consists of an igneous (probably volcanic) rock crust but that is much more heavily cratered and has not experienced major surface reworking or lava paving in the last billion years or so. Tectonic features - almost entirely faulting (probably tensional) - are found over the entire planet.
No firm evidence of large scale folding in the Mars rocks, which in many places are layered, has been found to date, although inclined strata have been noted in crater walls. This suggests that compressional force activity is very uncommon on Mars, i.e., plate tectonics as acts on Earth has not taken place. Faults are frequent, however, indicating some extensional forces have pulled the martian crust apart in places. Directions of tension have changed over time. The first image on this page shows three such tensional faults or grabens:
This scene clearly indicates the "Law of Cross-cutting" included in the basic review of Geology in Section 2. The law indicates relative ages. The oldest faulting is indicated by the structure starting in the upper left that slants towards the lower right. It is cut by the fault running upward to the right. The youngest fault, cutting this second fault, appears to the right of the first and appears the freshest (note the sand dunes within it).
Linear faults on Mars are fairly commmon. This image shows a set of parallel faults; along one is a chain of pit craters (surface material sinks into the fractures). Some have interpreted this alignment and association with fresh-looking normal faults as a sign of recent movement in the martian crust, causing "marsquakes" as a consequence:
One type of fracture, called "deformation band", is important as a channel for ancient groundwater (a surmise based on analogs found on Earth):
In February, 2007 an image made by the Mars Reconnaissance Observer was released, largely to support a claim that water-related alteration of rocks had been detected in the scene. The image also reveals the best evidence yet of faulting with complications.
In the upper half of this image, there are curved bands which may be strata. A long line separates this block from that in the lower half, where brown lines trend towards the upper left. These relationships may be a sign of a fault, whose plane intersets along the line, and movement (horizontal? vertical? oblique?) of the upper block with its stratalike bands as an offset that brings it against once separate rocks showing a very different assemblage of beds (these are interpreted by others as joints along which alteration is visible).
Structural control by faulting may account for the location of some first-order topographic features on Mars. Many of the scenes depicted below that illustrate the topics listed in the page heading occur in and east of the Tharsis region of Mars. To place these features in their physiographic context, here is a MOLA derived topographic map that includes some of the landforms we will visit:
We turn now to the greatest trench system in the ground ever discovered in the Solar System. This is Valles Marineris and some subsidiary trenches. It is a challenge to find an image that shows the entire system but the view made by Viking fits the requirement:
To get a sense of just how big the trench system is look at this Mariner 9 mosaic, extracted from the near hemisphere view, here centered on this most conspicuous feature on Mars. Valles Marineris extends nearly 4,000 km (2,486 mi), can be up to 200 km wide, and attains depths between 2 and 7 km (1.25-4.35 mi). When the outline of the 48 contiguous United States is overlain on mosaic, the eastern edge of Valles Marineris touches the Outer Banks of North Carolina and its western edge reaches to Central California.
The main features of the Valles Marineris system are labeled in this view:
Part of the Valles Marineris around Candor Chasma is shown in color in this Viking image:
This is what you would see if flying over the Valles (canyon). The image is made by combining a Viking view with MOLA data; there is no vertical exaggeration.
Even more impressive is this low angle view looking at the main canyon, made from Mars Express and MOLA data:
The canyon is actually a series of structural troughs, produced by faulting, radial to the Tharsis bulge to the northwest, which rises some 11 km (6.8 mi) above the surrounding plains, on which are the three dark-shield volcanoes , named on the preceding page. These volcanoes reach about 10 km (6.2 mi) above the bulge. A look inside the canyon wall, along a segment called South Candor Chasma, conveys the sense of steep slopes, perhaps furrowed by water erosion, and basal landslide deposits.
Another landslide into Valles Marineris appears here; below it is hummocky terrain often found in the deposits at the slide's foot.
A close-up of a landslide in Valles Marineris gives details of the massive debris pile-up, as material pulled away from the steep canyon wall.
Some of the cliffs in the valleys leading into Valles Marineris are dauntingly high, as for example this 1000 meter scarp face in Echus Chasma:
Along some edges of Valles Marineris are what appear at first to be tributary valleys. But they don't enter at levels equivalent to the floor base. They seem to criss-cross in a pattern that suggests tectonic control. One proposed explanation has them as due to subsurface water sapping.
Other tensional grabens are found in various parts of Mars, especially in the newer terrains. These can occur in intersecting networks, such as below which portrays Noctis Labyrinthus in the northern Tharsis region. This tecto-morphological feature is also called fractured terrain.
Sets of subparallel fractures cut across the terrain on the flank of the Tharsis region (Tharsis is a huge upbulge of martian crust more than 4000 km across and 10 km higher than surrounding lowlands at its top; Olympus Mons and the Tharsis volcanoes attest to it volcanic nature). In overall pattern, the sets are radial to the Tharsis apex. Here is one such set which cuts across older craters (but several younger craters superpose on the fractures).
As is often true for volcanic terrains (such as the East African Rift), sets of close-spaced parallel grabens (fault-bounded downdrop blocks of crust) related to tension induced by loss of support after lava withdrawal have been found also on Mars:
At the other extreme, short fractures may appear as isolated gashes, as seen here. These may be incipient or early stage breaks in a surface undergoing only moderate tensional stress.
Radiating fractures associated with small domes or swells produce a feature on Martian volcanic lava terrain known colloquially as "spiders". This is a good example:
Next, we switch our review to volcanism on Mars. In the Tharsis bulge region, some 4000 km across and 10 km above the mean martian elevation, are four of the biggest volcanoes in the Solar System. The huge structure alone in the western end of the Tharsis region is known as Olympus Mons, which is a broad shield volcano (now dormant), many times the area and volume of the big island of Hawaii, which consists of basaltic outflows from several major vents. Olympus Mons has a median diameter of 625 km (388 mi) and a height of 25 km (16 mi). We show first the famous discovery image from Mariner 9 (top), then a color version from Viking (center).
Olympus Mons is the largest volcanic structure known on any of the planets. The major volcanoes of the Tharsis region are all huge by Earth standards. This is self-evident when the next two illustrations are examined. The first plots Olympus Mons and its three Tharsis companions on a map of the United States. Only Mauna Loa on the Island of Hawaii can compete with the three. When plotted as cross-sections the size of Olympus Mons is even more awesome.
Olympus Mons' elliptical central caldera is 80 km (50 miles) in major axis. Here is a color view looking down from the Mars Express spacecraft:
A steep cliff up to 6 km (3.7 miles) high surrounds Olympus Mons, and stands out in the perspective view (below), derived by combining a Mars Express image with topographic data obtained from laser altimeter data. Scientists still debate the origin of this cliff, but some of them cite it as evidence of an escarpment resulting from wave erosion by an ancient (now vanished) ocean that may have covered at least part of Mars.
Mars Express imagery has also been manipulated to produce a view of the scarp (cliff) as though seen from the sloping plains beyond it:
Along the scarp these flows have spilled over into a moat-like shallow depression. This indicates that volcanism continued on Olympus Mons after scarp formation.
Once above the fringing escarpment, the slope of lower Olympus Mons is gentle - 1 to 3°. However, this incline allows lava flows to move downslope, as shown in this example from Mars Orbiter:
There are three volcanoes lined up in the Tharsis Montes area east of Olympus Mons; from south to north they are Arsia Mons, Pavonis Mons, and Ascraeus Mons. Each is a large shield volcano with a well developed central caldera. Typical is Arsia Mons, about 300 km (200 miles) wide at its base:
A side view made by combining a Viking image with MOLA elevation data gives this impression of Arsia Mons:
One of the best formed volcanoes in Bilbis Patera, which lies between Olympus Mons and the Tharsis group. As seen here by Viking Orbiter 1, the base of the volcano is about 100 km (62 miles) in diameter. The large caldera is like some volcanoes in the Galapagos Islands off the Ecuador coast.
Domes are also found on Mars. Here is an example:
The largest volcanic complex on Mars is the great bulge in the northern Tharsis region known as Alba Patera, only a few kilometers high but 1600 km (1000 miles) in longest dimension. As such, it rates as the largest shield volcano complex in the Solar System. It is broken by a series of concentric fractures and a set of elongate, subparallel fracture grabens, as seen in this Mariner mosaic.
At Alba Patera's top are an older summit caldera (left) and a smaller, more recent one. Note the lava flows descending its gentle slopes.
Alba Patera's flanks show numerous overlapping flows, indicating multiple periods of lavas extruded from tube outlets both at the caldera and along the slopes. In this Viking Orbiter image, the flows are flat-topped but steep-sided; volcanic ridges appear in the lower left.
Some lava flows are regular, smooth-to-rough surfaced, and with definite steep fronts, similar to those observed on the island of Hawaii. Here are two examples:
Lava flows issue from volcanoes, vents, and fissures. A narrow fissure can be filled with lava that hardens. As erosion removes its surroundings, the lava mass stands above the surface as a dike, as shown here:
Similar to a dike that has been brought out by erosion is this lava tube which served as a feeder for surrounding flows:
Smaller, more conventional volcanoes on Mars are known as tholii - an example is Ceraunius. Small ones are equivalent to large volcanic cones found on Earth:
Resembling the central caldera of a martian volcano but much smaller is the collapse pit. Here is an example - a string of pits - on the flank of Ascraeus Mons: