Heat-pipe tectonics is a cooling mode of terrestrial planets and moons in which the main heat transport mechanism in the planet is volcanism through the outer hard shell, also called the lithosphere. [1] [2] Heat-pipe tectonics initiates when volcanism becomes the dominant surface heat transfer process. [1] Melted rocks and other more volatile planetary materials are transferred from the mantle to surface via localised vents. [1] Melts cool down and solidify forming layers of cool volcanic materials. [1] Newly erupted materials deposit on top of and bury older layers. [1] The accumulation of volcanic layers on the shell and the corresponding evacuation of materials at depth cause the downward transfer of superficial materials such that the shell materials continuously descend toward the planet's interior. [1]
Heat-pipe tectonics was first introduced based on the observations on Io, one of the moons of Jupiter. [1] [2] Io is a rocky body that is internally extremely hot; its heat is produced by tidal flexing associated with its eccentric orbit. [2] [3] [4] [5] It releases internal heat via frequent and extensive volcanic eruptions that transfer melts to the surface. [2] [6] [7] Its crust is a single thick, dense and cold outer shell made up of layers of volcanic materials, whose rigidity and strength supports the weight of high mountains. [3] [2] [8]
Observations suggest that similar processes occurred in the early history of other terrestrial planets in the Solar System, i.e. Venus, the Moon, Mars, Mercury and Earth, indicating they may preserve fossil heat-pipe evidence. [9] Every terrestrial body in our Solar System might have had heat-pipe tectonics at some point; heat-pipe tectonics may thus be a universal early cooling mode of terrestrial bodies. [9]
In heat-pipe tectonics, volcanism is the major heat transport mechanism in which melts of rock are transferred to the surface by localised vents. [1] [3] [9] Advection, referring to the transfer of mass and heat, occurs when a moving fluid carries substances or heat to or away from a source and through a surrounding solid along channels. [10] Melts are produced when mantle rocks bear temperatures between 1100 and 2400 °C at corresponding depths (pressure varies the melting temperature) with the presence of water. [11] [12] When melts reach surface via vertical vents, they cool down and solidify forming mafic or ultramafic rocks which are rich in iron and magnesium. [1] [9] A thicker lithosphere is formed when volcanic materials accumulate on the Earth’s surface via repetitive volcanic eruptions. [1] [9] The new materials at the top, with the corresponding void created in the planet interior, lead to the sinking of superficial deposits. [1] [9]
This vertical advection of volcanic materials causes compression of the lithosphere, because interior spherical shells of planets are progressively becoming smaller at increasing depths. [1] [9] The surface cools down and a cold, dense and strong lithosphere is developed. [1] [9] The thick lithosphere supports the mountains that result from the contraction of volcanic layers. [1] [9]
Cooling heat-pipe planets could enter the next stage of cooling history, either lid tectonics or plate tectonics, immediately from the heat-pipe stage after prolonged cooling. [1] [13]
Io, a moon of Jupiter, is a small terrestrial planet, its radius is 1821.6±0.5 km, with a size similar to the Moon's. [14] Yet, Io produces a much higher heat flow, 60~160 terawatts (TW), which is 40 times larger than that on Earth. [3] [2] [15] [16] Radioactive decay cannot generate this large amount of heat. Radioactive decay supplies heating on other terrestrial planets. [3] [2] Instead, tidal-generated heat is a better hypothesis as Io is under great tidal influence imposed by Jupiter and other large moons of Jupiter, similar to the Earth and the Moon. [3]
The first observation supporting this was the active volcanism found on Io. There are over 100 calderas with abundant and widely spread radiating lava flows. [2] [6] [7] And the composition of the lava is interpreted to be mainly sulfur and silicates from the high eruption temperature of at least 1200 K. [3]
In addition to extensive volcanism, mountain ranges are the second observation on Io's surface. Io has 100~150 mountains with mean height of 6 km and a maximum height of 17 km. [3] [2] Mountains found have no tectonic evidence of their origin. Neither are there volcanoes in the mountainous areas. [3] [2]
A hypothesis of the development of thick lithosphere is built on these observations. [2] [7] The old theory suggested any terrestrial planets have a thin lithosphere. However, a thin 5-km-thick lithosphere cannot withstand the large stress of 6 kbar exerted by a 10 km×10 km mountain. [2] [8] To compare, the maximum stress that the lithosphere of the Earth can withstand is 2 kbar. [2] Thus, Io requires a thicker lithosphere to bear the overwhelming stresses imposed by globally distributed mountains. [2]
Heat-pipe tectonics was then introduced to explain the situation on Io. The theory explains the globally distributed volcanic materials on the surface; the development of thick lithosphere; and the formation of contractional mountains. [3] [2]
Research in 2017 suggested that all terrestrial planets may possibly undergo volcanism to cool down in their early development when they were much hotter inside than at present. [1] [9] [13] In the Solar System, Mars, the Moon, Mercury, Venus and Earth show evidence of past heat pipe tectonics, while not undergoing it at present. [9]
Evidence | Explanation | |
Mercury | - Lobate scarps record limited lithospheric contraction. [9] [17] [18] - Large scale volcanism dominated the heat transfer mechanism until 4 billion years ago, smoothing out the surface. [9] - No volcano can be found, but evidence of volcanism covers an extensive area. [9] - Lava spilling out through the vent can flow easily over a large area, matching the mafic composition. [9] [19] [20] | - Little earlier structure and shape of the planet can be preserved under continuous volcanic resurfacing. [9] - Structures and special landscapes can only be preserved after heat-pipe tectonics terminates. This explains the limited contraction. [9] - Mafic volcanic materials and its formation match with the hypothesis of heat-pipe tectonics. [9] |
Moon | - The shape of the Moon is not a perfect circle, but a slightly flattened circle. [9] | - The changes in shape must be recorded and preserved, but only in a strong and thick lithosphere. Heat-pipe tectonics develops a strong and thick lithosphere quickly so that the shape could be preserved. [9] |
Mars | - The Martian dichotomy: great topographical contrast between the depression in the Martian northern hemisphere and the elevated southern hemisphere. [9] - Wide isotopic range of neodymium (Nd), i.e. four times of that on Earth. [9] [21] | - Heat-pipe tectonics produces a thick and strong lithosphere which could preserve the older shape and topographies. [9] - The first shell on Mars is formed by an immediate accumulation of the incompatible elements, such as neodymium. [9] [21] |
Venus | - Structures in Ovda Regio, a high plain, show vertical advection of superficial materials. [9] [22] | - Downward advection of surficial materials and the formation of the thick lithosphere in the high plain match with the heat-pipe tectonics. [9] |
A hypothesis has been introduced to the early Earth that the Earth followed heat-pipe tectonics theory and cooled down through volcanism. [1] From 4.5 billion years ago, the earth started cooling until 3.2 billion years ago when plate tectonics started. [1] [23] The age of plate tectonics initiation is verified by several pieces of evidence such as Wilson Cycle. [1] [23]
Two major existing theories explain early Earth tectonics, namely proto-plate tectonics and vertical tectonics. [1] [24]
Previous Theories | Motion | Example [1] [24] [25] |
Proto-plate tectonics | Horizontal | - Compressional - Extensional |
Vertical Tectonics | Vertical | - Sub/intra-lithospheric inverse-drip shaped intrusion |
New observations in Barberton, South Africa and Pilbara, Australia show no evidence of periods of non-diapiric deformation that lasted more than 300 million years. [1] Applying the existing theories to explain the deformation, upward inverse-drip shaped intrusion of melts is the solution. [1] [26] [27] In this case, horizontal motions must be involved. [1] Yet, no proof of horizontal motion could be found. [1] Based on this, some researchers applied heat-pipe tectonics to the early Earth.
Place | Observations | Heat-pipe Hypothesis |
Barberton and Pilbara | - Thick sequenced volcanic materials (rich in iron and magnesium), i.e. 12-km thick, in Pilbara. [1] [27] [28] [29] - Upward inverse-drip shaped intrusion metamorphose the volcanic layers into TTG (Tonalite - trondhjemite - granodiorite). [1] - Dome-shaped structures resulted from the intrusion. [1] - No deformational structure found until 3.2 billion years ago. [1] - Tectonism-induced structure found right after 3.2 billion years ago: | - Constant volcanic eruption through localised vents creates thick lithosphere (rich in iron and magnesium). [1] - No tectonism until 3.2 billion years ago. [1] - Intrusions occur at the base of the lithosphere. [1] |
Itsaq | - Most rocks older than 3.2 billion years ago are gneiss (4.03 billion years ago). [1] [31] [32] - Some deformations were found earlier than 3.2 billion years. [1] | - Planets cool down over time in heat-pipe tectonics. [1] - Subduction could explain the deformation, as planets should cool down after subduction. But the process is slow and progressive, which takes a long period to cool down after subduction events. [1] [33] - Yet, no tectonic evidence can prove the occurrence of subduction. [1] - Long-existing reverse faults with overlapping pattern (Duplex) is a better explanation. It does not involve any subduction and thus no cooling after any processes. [1] - A sharp decline in heat-pipe tectonics after 3.2 billion years ago. [1] |
As time goes by, terrestrial planets cool down as internal heat production reduces and the surface temperature becomes lower. [1] [13] On top of that, the major heat transfer process is changing towards conduction. [1] [13] Thus, an abrupt transition from heat-pipe tectonics to either plate tectonics or stagnant lid tectonics occurs when conduction heat is larger than the internal heat production. [1] [13] [34]
Stagnant lid refers to the relatively stable and immobile strong cold lithosphere with little horizontal movements, while plate tectonics, refers to the mobile lithosphere with many horizontal movements. [9]
In the plate tectonic stage, the plate starts breaking up when convective stresses driven by the mantle overcome lithospheric strength. [13] As volcanism is no longer the dominant heat transfer method, much less volcanic material would be deposited globally. [13] A thinner lithosphere is then developed with increasing lithospheric temperature gradient, i.e. 1500 degree Celsius at 100-km depth). [35]
In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase.
The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~80 and 200 km below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the rigid uppermost layer of the mantle. The crust and the rigid upper mantle layer together constitute oceanic lithosphere.
A volcanic arc is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc located further from the subducting plate than the trench. The oceanic plate is saturated with water, mostly in the form of hydrous minerals such as micas, amphiboles, and serpentines. As the oceanic plate is subducted, it is subjected to increasing pressure and temperature with increasing depth. The heat and pressure break down the hydrous minerals in the plate, releasing water into the overlying mantle. Volatiles such as water drastically lower the melting point of the mantle, causing some of the mantle to melt and form magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.
A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive and extrusive, arising when magma travels through the crust towards the surface. The formation of LIPs is variously attributed to mantle plumes or to processes associated with divergent plate tectonics. The formation of some of the LIPs in the past 500 million years coincide in time with mass extinctions and rapid climatic changes, which has led to numerous hypotheses about causal relationships. LIPs are fundamentally different from any other currently active volcanoes or volcanic systems.
The Iceland hotspot is a hotspot which is partly responsible for the high volcanic activity which has formed the Iceland Plateau and the island of Iceland.
Venus is a planet with striking geology. Of all the other planets in the Solar System, it is the one nearest to Earth and most like it in terms of mass, but has no magnetic field or recognizable plate tectonic system. Much of the ground surface is exposed volcanic bedrock, some with thin and patchy layers of soil covering, in marked contrast with Earth, the Moon, and Mars. Some impact craters are present, but Venus is similar to Earth in that there are fewer craters than on the other rocky planets that are largely covered by them. This is due in part to the thickness of the Venusian atmosphere disrupting small impactors before they strike the ground, but the paucity of large craters may be due to volcanic re-surfacing, possibly of a catastrophic nature. Volcanism appears to be the dominant agent of geological change on Venus. Some of the volcanic landforms appear to be unique to the planet. There are shield and composite volcanoes similar to those found on Earth. Given that Venus has approximately the same size, density, and composition as Earth, it is plausible that volcanism may be continuing on the planet today, as demonstrated by recent studies.
The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.
Mantle convection is the very slow creeping motion of Earth's solid silicate mantle as convection currents carry heat from the interior to the planet's surface.
Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.
In geology, the slab is a significant constituent of subduction zones.
Earth's internal heat budget is fundamental to the thermal history of the Earth. The flow of heat from Earth's interior to the surface is estimated at 47±2 terawatts (TW) and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of Earth.
NASA's Magellan spacecraft mission discovered that Venus has a geologically young surface with a relatively uniform age of 500±200 Ma. The age of Venus was revealed by the observation of over 900 impact craters on the surface of the planet. These impact craters are nearly uniformly distributed over the surface of Venus and less than 10% have been modified by plains of volcanism or deformation. These observations indicate that a catastrophic resurfacing event took place on Venus around 500 Ma, and was followed by a dramatic decline in resurfacing rate. The radar images from the Magellan missions revealed that the terrestrial style of plate tectonics is not active on Venus and the surface appears to be immobile at the present time. Despite these surface observations, there are numerous surface features that indicate an actively convecting interior. The Soviet Venera landings revealed that the surface of Venus is essentially basaltic in composition based on geochemical measurements and morphology of volcanic flows. The surface of Venus is dominated by patterns of basaltic volcanism, and by compressional and extensional tectonic deformation, such as the highly deformed tesserae terrain and the pancake like volcano-tectonic features known as coronae. The planet's surface can be broadly characterized by its low lying plains, which cover about 80% of the surface, 'continental' plateaus and volcanic swells. There is also an abundance of small and large shield volcanoes distributed over the planet's surface. Based on its surface features, it appears that Venus is tectonically and convectively alive but has a lithosphere that is static.
Lid tectonics, commonly thought of as stagnant lid tectonics or single lid tectonics, is the type of tectonics that is believed to exist on several silicate planets and moons in the Solar System, and possibly existed on Earth during the early part of its history. The lid is the equivalent of the lithosphere, formed of solid silicate minerals. The relative stability and immobility of the strong cooler lids leads to stagnant lid tectonics, which has greatly reduced amounts of horizontal tectonics compared with plate tectonics. The presence of a stagnant lid above a convecting mantle was recognised as a possible stable regime for convection on Earth, in contrast to the well-attested mobile plate tectonics of the current eon.
Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface.
The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:
Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.