Gravity spreading is a phenomenon in which a geological body laterally extends and vertically contracts to reduce its

gravitational potential energy Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum Work (physics), mechanical work t ...

.Schultz-Ela, D. D. (2001). Excursus on gravity gliding and gravity spreading. Journal of Structural Geology 23, p. 725-731. It has been observed on many different scales, and at numerous locations on Earth, from

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

flows to

passive margin A passive margin is the transition between Lithosphere#Oceanic lithosphere, oceanic and Lithosphere#Continental lithosphere, continental lithosphere that is not an active plate continental margin, margin. A passive margin forms by sedimentatio ...

s.Smith, J. V., Houston, E. C. (1994). Folds produced by gravity spreading of a banded rhyolite lava flow. Journal of Volcanology and Geothermal Research 63, p. 89-94.Rowan, M. G., Peel, F. J., Vendeville, B. C. (2004). Gravity-driven Fold Belts on Passive Margins. AAPG Memoir 82, p. 157-182 Additionally, gravity spreading is likely to have occurred on both

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

and

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

.Montgomery, D. R., Som, S. M., et al. (2009). Continental-scale salt tectonics on Mars and the origin of Valles Marineris and associated outflow channels. GSA Bulletin 121, p. 117-133Kozak, R. C., Schaber, G. G. (1986). Gravity-spreading origin of the Venusian Tesserae. Lunar and Planetary Institute 17, p. 444-445

Distinction from Gravity Gliding

Historically, geologists have used the terms "gravity spreading" and "gravity gliding" interchangeably, or with little distinction. This article follows the convention of "Excursus on gravity gliding and gravity spreading" by D.D. Schultz-Ela, which defines gravity spreading as a lateral extension and vertical contraction, which thus must be applied to a non-rigid body. Gravity gliding, however, is applied to a block that is not being deformed, and is therefore less common to observe. However, it can be difficult to distinguish between the two in real world scenarios, and often both occur simultaneously.

Mechanism

For gravity spreading to occur, a rock mass must be driven to deformation by gravity. As long as the center of gravity of the system descends, portions of the system may rise. Of course, a material normally resists such deformation. For gravity spreading to occur, the differential stress must be greater than that rock body's

yield strength In materials science and engineering, the yield point is the point on a stress–strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and w ...

. Gravity spreading can be thought of as a mound of molasses that spreads out, and gravity gliding can be imagined as a wooden block sliding down a slope.

Examples

Earth

Mountains

Heart Mountain in Wyoming, United States, has been extensively studied, because Ordovician age carbonates ( Madison Limestone), sit on top of a much younger (~50Ma) sedimentary formation ( Willwood Formation).NASA Earth Observatory
Heart Mountain, Wyoming.
/ref> It is now largely accepted that this juxtaposition of old rocks on top of young is the result of gravity spreading and gliding. Field observations, such as slight internal deformations of the older formation, indicate the gravity gliding and spreading of the Madison Limestones.Hauge, T. A. (1985). Gravity Spreading origin of the Heart Mountain allochton, northwestern Wyoming. GSA Bulletin 96, p. 1440-1456. The specific details of the gravity spreading event are unclear, but it is thought that it was induced by the

Laramide Orogeny The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 80 to 70 million years ago, and ended 55 to 35 million years ago. The exact duration and ages of beginning and end of the o ...

, approximately 50 Ma. This caused the Madison Limestones to slide into the nearby

Bighorn Basin The Bighorn Basin is a plateau region and intermontane basin, approximately 100 miles (160 km) wide, in north-central Wyoming in the United States. It is bounded by the Absaroka Range on the west, the Pryor Mountains on the north, the Bigho ...

, where it came to rest on top of the Willwood Formation. The cause for block motion is debated, with numerous models to explain how such a large block could have moved tens of kilometers down a slope of less than 2°.Beutner, E. C., Gerbi, G. P. (2005). Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA. GSA Bulletin 117, p. 724-725. Models have ranged from lubrication by hydrothermal circulation, movement initiation from

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

genic seismicity, to frictional heating dissociating CO₂ from the carbonates, resulting in dramatically reduced friction. The last of these theories is among the most recent, and by far the most spectacular. The authors envisage initially slow sliding, likely as the result of a volcanic eruption, until frictional heating of the carbonate rocks creates a supercritical CO₂ layer, decreasing the friction tremendously. From this point, the sliding would occur rapidly, perhaps as high as 150 km/h.

Passive Margins

Gravity spreading in passive margins occurs when gravitational forces are strong enough to overcome the overburdens resistance to motion along its basal surface, and the internal strength. The gravitational forces are a function of the dip of the slope and the dip of the

décollement Décollement () is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are a Deformation (geology), deformational structure, resulting in independent styles of deformation in the rocks above and below t ...

layer.

Lava Flows

Rhyolite lava flows in northeastern

New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...

, Australia show recumbent folds that record a history of vertical shortening and lateral extension during deposition, consistent with what one would expect from gravity spreading. This is the result of lava being displaced by new lava extruding from the vent.

Mars

Satellite images of

have shown that the

Thaumasia Plateau The Thaumasia Plateau is a vast sloping volcanic plain in the western hemisphere of Mars, and is the most extensive component of the Tharsis Rise by area. Syria Planum, Solis Planum, Sinai Planum, and Thaumasia Planum are the constituent sector ...

has large amounts of thrust faults, normal faults, and ridges. This rifting has resulted in canyons, and compression at the front of the "mega-slide" has caused the ridges and thrust faults observed at low end of the region. To explain these faults and ridges, a four-stage model involving gravity spreading is used: # A thick salt layer is deposited. This is possible in either wet or dry conditions. #

Tharsis Tharsis () is a vast volcanic plateau centered near the equator in the western hemisphere of Mars. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and A ...

, a volcanic plateau, was emplaced. This increased both the heat flux of the area, as well as the topographic slope. Volcanism associated with Tharsis also deposited ash and lava flows. # The layers of salt and ice beneath the volcanics provided detachment points for the initiation of gravity spreading to the southeast. # Fractures from the basal detachment plane opened an aquifer, resulting in the release of water and incision of outflow channels.

Venus

It has been suggested that the "blocky" surface of Venus is the result of gravity spreading. This is thought because of flow-like structures correlated with topography, and as well as potential regions of thermal uplift, and has been reinforced by terrestrial analogues.

References

{{Reflist Geological processes