Stories of Other Worlds
How to Build a Landscape: Unusual Landscapes--Text
Earth Science Essentials
by Russ Colson
Introduction
There are far too many amazing landscapes for us to be able to talk about all of them, or even most of them, or even to cover all the different types of amazing landscapes. So, I've picked a tiny fraction to look at and think about how they formed. Later in this unit, you'll have a chance to pick one of your favorite amazing landscapes and think about how it might have formed.
Dunes and Yardangs:
Sand dunes are dynamic features, shifting as the winds shift and as sediment is carried into a region or carried away. Presence of dunes tells us about the typical wind conditions that transport and deposit the sediment. At Sand Dunes National Monument in Colorado, the presence of the mountains to the east (downwind) slows the wind and causes sand carried by the wind to be deposited.
Other kinds of dunes might give us different information. For example, Barchan dunes might indicate not only wind direction but the abundance of sediment in the region.
Mars Reconnaissance Orbiter--Dec 30 2013--dunes in crater near Mawrth Vallis—Mars. Barchan dunes like these often form in sediment-poor regions and are a complex result of amount of sediment, wind speed, and steadiness of the wind direction.
Mars Reconnaisance orbiter Jan 2015--Arsinoes Chaos region
The picture of yardangs below comes from the lecture--how do the yardangs form? Which direction would the wind blow typically?
Mars Reconnaisance orbiter—Yardangs in the Arsinoes Chaos region
Wind-blown features can include unexpected types of sediment. Snow crystals are a dominant kind of sediment on many worlds, including on Northern Earth in the wintertime!
Winter Landscape NW MN.
In MN in winter, and on many planets, ice crystals (snow) are the primary sediment of the landscape. The sediment can be deposited or eroded by wind, making snow landscapes similar to wind-formed landscapes made of silicate sediment. For example, in the picture above from northwest Minnesota, we can see crescent-shaped barchan dunes similar in character to the much bigger versions we saw on Mars. Barchan dunes form when there is a limited supply of sediment. In the case above, the limited sediment is because most of the snow has partly melted and then re-froze so cannot be it is not easily remobilized by the wind. The barchan dunes also reveal the wind direction, with the pointy ends of the crescents pointing downwind. In the foreground of the picture above are small-scale yardang-like features, with the ridges running parallel to the wind direction.
In the picture below, an ancient former dune sand has been buried by other sediment, become rock (sandstone), and then been exposed by erosion. The swirly appearance is from the crossbedding of the sand that formed when this rock was a sand dune. Crossbedding forms when the wind lays down layers of sand at different angles and in directions along the surface of the dune.
Ancient petrified sand dune, central Utah.
Spires, pillars and columns:
Split Rock State Park, Minnesota
This imposing cliff in northeast Minnesota is the result of many geological processes, beginning when a large block of anorthosite (a low-density igneous rock) was rafted upward within a body of magma that rose up toward the surface when this region was volcanic long ago. The magma cooled to form a sill with the large block of anorthosite still in it. Eventually, erosion exposed the sill and the anorthosite. Glaciers carved into the rock, and then later the waves along the shoreline eroded it further. The anorthosite was more resistant to weathering than the surrounding rock, producing this cliff along the shore.
Multiple processes have produced the arches of Arches National Park, like that seen in the picture below. Flexing and gentle folding of the rock by tectonic forces while it was still deep within the crust and buried under other rock produced joints and fractures in the rock. As erosion exposed the rock at the surface, the joints and fractures allowed in water that weathered the fractured areas more quickly. These areas washed away, leaving narrow walls of rock called fins. If the fins are undercut by more erosion at the base, they can form arches
Rock fin at Arches National Park, Utah.
Monument Valley in Arizona in the picture below formed as a wide plain eroded down to a new base level. Resistant sandstone made a caprock that protected remnants of the original landscape in a series of isolated buttes and mesas.
Monument Valley in Arizona.
We talked about pillar formation in the lecture. Below is a picture of a pillar in a badlands region. Pillars like this form when resistant caprock overlies more easily eroded rock (typically shale forms the more easily eroded rock in a badlands landscape. Once the caprock is breached by erosion, falling rain and running water quickly erode down into the underlying soft rock, first producing gullies, and sometimes making pillars if part of caprock protects some of the soft rock from falling rain.
Badlands pillar in western US.
Bryce canyon, seen in the picture below, is made of many pillars and ravines formed where landscape erodes down from a high area to a lower base level (similar to the situation we examined at Theodore Roosevelt National Park in a previous lecture. In this picture, the story of the rapid erosion downward is preserved by the exposed roots of a pine tree.
Bryce Canyon NP.
Weathering of a resistant sandstone caprock has produced pillars at the edge of a butte in the picture from Monument Valley, USA, below.
Three Sisters, Monument Valley Arizona.
There are non-erosional processes that can produce pillars as well. The very unusual formation in the picture below is formed not by erosion, but by lava. Lava flowed around a tree. Water in the tree trunk cooled the lava quickly, forming a crust around the trunk. The remainder of lava, still fluid, flowed away, leaving the frozen tree trunk behind. This feature is not only an unusual type of pillar, but a fossil!
Hawaii, main island.
From the lecture—How might the landform shown below (found on Callisto, moon of Jupiter) form? Note that the pillars are brighter than surrounding landscape.
There is an intermediate apron around base of the pillars—darker than the pillar but lighter than valleys. The pillars are 100s of meters tall (This can be measured accurately using traingulation based on the angle of the sunlight and the length of the shadows).
Pillars of Callisto, NASA, Galileo, 2001
Archways and Bridges:
We talked about how fins form above. Once a fin forms, we can get an arch if the lower part of the fin weathers more quickly for some reason. For example, it's often wetter lower down which causes more weathering as the water can dissolve the cement holding the sand grains together. Windblown sand might have a bigger effect lower down. Sometimes the upper part of the wall is made of more resistant rock. These and other processes can cause a hole to weather through the wall which eventually becomes an arch.
Arches National Park, Utah.
Arches can form not only in desert landscapes, like Utah, but along beaches. Weak places in the limestone combined with wave refraction can undermine the rock wall at an erosional shoreline, sometimes producing an arch, like that shown below.
Etretat, France.
Arches can also form when the tops of columns collapse in such a way as to produce boulders perched against each other, as seen below.
Baraboo region, WI
Bryce Canyon NP.
This window through a fin in Bryce Canyon is the result of both water and wind weathering.
Nevada Near Las Vegas.
This 'holey' or honeycombed weathering pattern is typical of wind weathering in sandstone in arid regions. Wind carries sand grains that abrade the sandstone face, and areas of the sandstone that are less well cemented (the sand grains aren't stuck together as well) will blow out, leaving holes.
Ayres Bridge, Wyoming
Some natural bridges form when water dissolves the underlying rock and are common in some karst terranes. However, the bridge above is made of sandstone, which does not dissolve in water and so did not form this way. Ayres Bridge formed when a cut off of a meander in a river undercut the sandstone bluff from both sides, as shown in the map view below.
Cave and Karst features:
Caves and overhangs, like this case in the bluff along the Dordogne River in France shown below, form when slightly acidic water dissolves passageways in underground rock. Which areas get dissolved and which left behind depend on variations in the solubility of the rock and the location of the water table at the time of cave formation. Along the Dordogne, the passageways were later exposed by erosion of the Dordogne valley, producing overhangs and caves where people lived. The Dordogne valley is famous for ancient cave paintings and these caves were occupied by people up into the mid 20th century.
Dordogne Valley, France.
Non-limestone caves usually form by processes other than dissolution since sandstone if not very soluble in water. At Mesa Verde below, the sandstone overhang forms a resistant caprock while lower portions of the sandstone may stay wetter (and thus be more easily weathered by chemical and mechanical processes) or be made of more easily-eroded rock. The ancestral Pueblo culture used these overhangs to build their as well-fortified villages, abandoned about 700 years ago.
Mesa Verde NP, Colorado
Like sandstone, shale also does not dissolve in water. Caves in shale like that seen below are typically formed when water from the surface leaks down into a crack in the rock. The running water washes out more mud, widening the crack until a wash-out cave forms. This type of cave is typical in badlands landscapes and is often not safe to explore as it can easily collapse.
Mikoshika SP Montana—shale.
The Messenger mission discovered mysterious pits on Mercury that were not circular (so are not impact craters) and had no lip or rim (unlike most volcanic features).
These features, shown in the picture below, are thought to form when the brutally hot sun evaporates sulfur rich deposits with the rock, causing the landscape to collapse (like sinkholes) into the area where the vaporized rock used to be. Because of its similarity to karst terrane which forms when underlying rock dissolves, this terrane is called a pyrothermokarst.
Instead of dissolving in water. the rock vaporizes into gas.
Hollows on Mercury-- messenger--NASA--Raditladi hollows on mercury
The circular hole in the picture below formed by pressure solution in the ceiling of a cave room —As water filled an underground chamber, pressure built, which caused water to force its way out through tiny holes in the rock. The pressurized water dissolved the limestone, widening the initially-small hole into the larger hole seen here.
Mystery Cave, MN—pressure solution hole in ceiling, picture courtesy of Laurie Eli.
Canyons and Valleys
Hadley Rille from the ground with Jim Irwin--Apollo 15
On Titan, moon of Saturn, frozen water is the primary bedrock due to the very cold conditions, 180°C below zero. The sea to the right in the picture below (Ligeia Mare) and the branching drainage system, most likely reflect a hydrocarbon cycle similar to the 'water cycle' on Earth.
Hydrocarbons evaporate from the seas, are moved by winds to new locations where they condense out as rain, fall on the landscape, and then flow in rivers back to the sea. The 400km length of this river makes this one of longest river systems known in our solar system.
Titan is the only world other than Earth that is known to have flowing rivers today.
NASA/ESA/ASI--Cassini—Sept 26, 2012, Dendritic drainage valley system—Titan, moon of Saturn
Miranda, moon of Uranus, Voyager 2
The details of the origin of Miranda's strangely jumbled terrain, immense canyons, and towering cliffs like Verona Rupes seen at bottom right, are uncertain. However, Verona Rupes is a fault-formed cliff, and the highest in the solar system. Because of both the height of the cliff and the low gravity on the moon, a stone would take many minutes to fall from the top to bottom of the cliff.
Valles Marineris on Mars— Flight into Mariner Valley, Phil Christenson, Arizona State University.
The straightness of the canyon is one clue to its tectonic origin. The branching pattern of the side canyons suggest flowing liquid has been involved in their formation. From the lecture, some scientists believe that this flowing liquid
Other odd features:
Giant Boulders in an otherwise empty plain
Big boulders of rock that don't match the rock beneath the surface are common in many areas of northern Europe and North America. You might be thinking, meteorite debris! On other planets, boulders lying around in a plain are often ejecta from impact craters. On Earth, other processes have covered up or weathered away most impact ejecta. Here, boulders that don't match the rock type of the region are most commonly material brought in by glaciers and left behind when the ice melted. These abandoned boulders are called glacial erratics.
Faces in Rock
The iconic outcrop in the picture below was formed by a combination of glacial erosion and weathering by rain. Erosional features are ephemeral, existing for only a geological moment. One evidence for the ephemeral nature of erosional features is that this feature no longer exists. The Old Man of the Mountain collapsed in 2003. I was there to see it in 1988.
Old Man of the Mountain or Great Stone Face, New Hampshire
Strange patterns in rock
This picture of the Hawaii coast shows a weathering pattern common along shores of saltwater seas, especially in basalt. As water evaporates, salt crystals form, wedging open cracks in the basalt. The salt water also chemically attacks the rock. Combined mechanical and chemical processes produce this odd pattern.
North Shore Lake Superior
The odd geometrical pattern in the picture above is the result of two different process. Long ago, basaltic lava flowed over this region. As the lava cooled, in shrank, causing the rock to break into polygonal columns, kind of like the mud cracks that form as mud dries and shrinks. Much, much later, glaciers moved across the ends of the exposed columns, polishing them off. You can see the scratch marks left by the bits of sand and gravel embedded in the glacial ice.
Curiosity Rover. Sandstone at Kimberley Waypoint.
The layers in the picture above are tilted from left to right. The sand was deposited in a delta as water flowed downhill from the high Gale Crater rim to the left down into the lake below the crater rim.
Slumps on hillside in Northcentral Kansas
Even on worlds without running water, sediment moves downhill under the force of gravity. One type of downhill movement is called a slump, like the slumps seen on the hillside in the picture above. The slumps below are on Mars.
Vallis Marineris on Mars—Flight into Mariner Valley, Phil Christenson, Arizona State University.
The slumping along the sides of the cliffs produces the scalloped edges of the canyon.
Last updated October 14, 2015. All text and pictures are the property of Russ Colson, except as noted.
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