Lithosphere
Earth’s thin outer shell is the lithosphere; a concept developed by American geologist Joseph Barrell. The lithosphere consists of the crust and uppermost mantle. Currently, 1/3rd of the lithosphere is continental, 2/3rds oceanic.
The crust is distinguished from the lithospheric mantle by different mineral composition. This boundary is the Moho discontinuity – named after Croatian seismologist Andrija Mohorovičić. The Moho discontinuity lies 5–10 km below the ocean floor, 20–90 km beneath the surface of a continent, at an average 35 km.
As mineral content differs, oceanic lithosphere is denser than the continental variety. Oceanic crust consists of mafic material: silicate rich in magnesium and iron.
The term mafic is a portmanteau of “magnesium” and “ferric” (referring to iron). Oceanic volcanoes tend to exude mafic magma, which has a lower viscosity and silica content than felsic lava.
Thicker-but-lighter continental crust is felsic: typically granite, but incorporating other silicate materials too, including quartz, muscovite, and feldspar. Felsic is a portmanteau of “feldspar” and “silica.”
Asthenosphere
The lithosphere extends to the depth where mantle rock becomes brittle and viscous. The lithosphere is strong compared to the layer below: the asthenosphere. At ~80–200 km, the asthenosphere is the top layer of the mantle and is relatively ductile, owing to temperature and pressure.
“Although comparatively thin, the asthenosphere has a remarkable impact on the mantle.” ~ American geophysicists Don Anderson & Scott King
The asthenosphere is a region of concentrated shear which distributes heat in the lithosphere according to geological patterns in both the asthenosphere and lithosphere. The asthenosphere is hotter than the mantle layer below it: so sizzling that it is close to, or even at, its melting point. How and why that is so is not fully understood.
The relatively low viscosity of the asthenosphere facilitates the movement of continents. It is impossible to imagine tectonics with an asthenosphere otherwise.
The low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics. ~ French geophysicist David Sifré et al
Convection – the flow of heat in mantle material – drives tectonics. The mantle transfers heat from the core.
“Heat from the base of the mantle contributes significantly to the strength of the flow of heat in the mantle and to the resultant plate tectonics.” ~ American geophysicist David Rowley
Tectonics
The development of plate tectonics and the differentiation of the lithosphere into oceanic and continental components that followed were key events in the evolution of the biosphere on Earth. ~ English geologists Chris Hawkesworth & Michael Brown
Earth’s crust rides on a patchwork of plates, in constant motion relative to each other, producing terrestrial effects: the lithosphere in motion. 20-some plates slide over the asthenosphere, propelled by volcanic heat.
In plate tectonics, the process by which Earth’s surface is constantly being reorganized and rebuilt, surface plates (the lithosphere) move over the underlying part of the mantle that is actively transporting heat by convection (the asthenosphere). ~ American geologist Rob Evans
“Plate tectonics is a relatively benign way for Earth to lose heat. You get what are catastrophic events in localized areas, in earthquakes and tsunamis. But the mechanism allows Earth to maintain a stabler and more benign environment overall.” ~ Australian geologist Peter Cawood
The intercourse at boundaries between tectonic plates is of 3 varieties: divergent, transform-fault, and convergent.
At a divergent boundary, plates move apart. A piece of lithosphere is spawned as plate area increases.
At a transform boundary, 2 plates rub, in the same or opposite directions. Plate area is unchanged. The sideswiping commonly creates earthquakes.
Plates come together at a convergent boundary. One plate is subducted: recycled back into the mantle. Plate area decreases.
Converging ocean plates produce deep-ocean trenches and island arcs. The Aleutian and Japanese islands are exemplary.
Converging plates, where one is oceanic and the other carries a continent, are marked with mountain ranges and ocean trenches. Their collision is celebrated with deep earthquakes and volcanoes. The Andes formed from such an event, when a subducting oceanic plate dragged South America westward, causing the continent to slam into the subduction zone.
Converging plates bearing continents create mountains, accompanied by intense deformation and earthquakes. The Himalayas are the penultimate example.
Continental Drift
Nobody has come up with a satisfying answer yet on how plate tectonics started. ~ American geochemist Kent Condie
Tectonic plates cannot subduct if the mantle is too hot, as plates break up from the heat when going down. The mantle of the early Earth was so hot that dynamic tectonic plate flow with subduction did not begin until ~3 BYA.
When Earth’s lithosphere was hotter, subduction did not drive continental drift like it did later. From 1.7–0.8 BYA, Earth’s crust was relatively stable, as the stasis of the supercontinent Rodinia 1.1–0.8 BYA illustrated.
Since then, the cooling mantle shifted the gyre of tectonics. The flow of subduction zones became increasing active. By 750 mya, with the breakup of the supercontinent Rodinia, the modern system of subduction has shaped the lithosphere.
Driven by the tunes of tectonic plates, continents dance a slow ballet: coming together to form larger landmasses or parting into pieces. This is continental drift. Shifting plates create an ongoing geological jigsaw.
Continental fragments, set adrift by the breakup of former supercontinents, can play a major role in the way new material is added to a growing continent. ~ Australian geophysicist Nicholas Rawlinson
The global mid-ocean ridge system is an interconnected network of volcanoes that produces the oceanic crust which covers 70% of Earth’s surface. In response to tectonic plate movements, lava forms at mid-ocean ridges.
Earth’s lithosphere is constantly reconfiguring and recycling itself, cooking new configurations by convection. A hot flow of magma rises to create an oceanic spreading ridge, thus forming new surface material. That pushes a plate; perhaps creating a zone of subduction, where one plate passes under another. The upper plate may deform to form mountains.
Plate motions at the surface cause earthquakes and volcanic eruption. The reason plates move on the surface is that slabs are heavy, and they pull the plates along as they subduct into Earth’s interior. So anything that affects the way a slab subducts is, up the line, going to affect earthquakes and volcanism. ~ Japanese American geologist Lowell Miyagi
Subduction zones are delineated at ocean trenches, where plates sink into the mantle as cold slabs. These slabs can break off from their surface plates, altering the driving forces of tectonics.
Slab detachment occurs if thick continental crust, or an oceanic plateau, is swept by plate motion into the subduction zone, thus plugging it up. Detachment can be accelerated by mineral grains in a slab getting smaller during deformation, causing the slab to weaken while being stretched.
This combination of a weakening slab and crustal plugs incites abrupt slab detachment (within a few million years, which is a short span of geological time). The result can be precipitous tectonic plate shifts and rapid continental uplift.
Subduction is abetted by lubrication in the asthenosphere. Water-caused defects in subsurface olivine and heat lower the viscosity of the boundary layer beneath plates. Subduction moves vast volumes of water into the Earth.
A viscous layer of melted rock 10-km thick or more may underlie a tectonic plate.
Subduction is typically slow. The average slab takes 300 million years to descend.
Some oceanic slabs, such as the Tonga plate off Japan, subduct speedily. These slabs stay relatively cold to great depth, which limits the exchange of mineral elements, and so helps keep such slabs intact and sliding down.
At sufficiently high pressure, minerals mix. The composition that afforded speedy subduction suddenly leads to a rapid rise in rock density.
Still, a slab is still cool compared to the mantle rock around it. The descent of a speedily submerging slab stagnates at around 650 kilometers down, as the slab becomes relatively buoyant compared to the ambient mantle.
The same temperature difference that afforded rapid descent above inhibits mineral transformation at depth, and so impedes penetration into the lower mantle.
Below 1,000 km, mantle viscosity abruptly increases. At 1,500 km there is a shallow lower mantle layer with impressive density and stiffness. Subducting slabs get stuck there.
Anything that would cause resistance to a slab could potentially cause it to buckle or break higher in the slab, causing a deep earthquake. ~ Lowell Miyagi
The oceanic lithosphere thickens via cooling as it ages and moves away from the mid-ocean ridge. The hot upsweep of the asthenosphere is converted into lithospheric mantle.
Recycled mantle and crust from subducted slabs eventually emerge as new seafloor via eruptions of volcanic vents along mid-ocean ridges. The magma in this new plate material is 1 of 2 types that come up. The other magma type, which emanates from island volcanoes like Hawaii, is disgorged from deeper hotspots.
Subducted plates create density differences in the mantle that affect gravitational pull in the crust. Slabs long buried have released water, reducing the density of the overlying rock. This result is lowered gravity, which can be accentuated if low-density rock is located above, near the surface.
Smaller hot spots of magma create movements and rifts within a plate, or collectively orchestrate continental drift.
Magma permeates portions of the mantle, shaping its structure and distributing materials that cool into different igneous rocks.
From below, the lithosphere is driven by magmatic convention currents. Upwelling plumes play a particularly powerful role in tectonics.
From deep within Earth’s mantle, large-scale upwelling occurs in 2 places: beneath Africa and the central Pacific Ocean. These major upwelling locations have been stable for at least 250 million years, despite dramatic reconfigurations of plates and continents on Earth’s surface.
Volcanoes have been instrumental in fashioning Earth’s crust. They reside where tectonics plates are converging or diverging, providing raw material for the dynamics of tectonics. Plumes from hot spots surge past the crust to the surface, creating active volcanoes that add new material to the crust. The Pacific Ring of Fire is a seismic belt of hot spots that runs from north of New Zealand up through Indonesia, Japan, and the Aleutian Islands, then down the west coast of the Americas, ending in Southern Chile.
The key to the origin of the continents and their continuing movements comes in relative density. Basalt sinks. Granite floats.
Magmas of basaltic and granitic composition separate into layers. Dense basalt sinks back into the mantle, leaving the granite floating like a cork, conserved on the surface.
Subduction shapes the land. Continental landmasses, which are less dense than crustal rock, ride passively atop tectonic plates.
A craton is the stable part of a continental plate, generally in the interior, built upon basement rock. Around a craton are elongated mountain belts – orogens – formed by later episodes of compressive deformation. The youngest mountain-building (orogenic) formations are along the active margins of continents, where plate movements deform relatively weak continental crust.
“You need plate tectonics to sustain life. If there wasn’t a way of recycling material between mantle and crust, all these elements that are crucial to life, like carbon, nitrogen, phosphorus, and oxygen, would get tied up in rocks and stay there.” ~ Scottish biogeochemist Aubrey Zerkle
Supercontinents
Tectonics has reconfigured the continents on Earth over the past 3+ billion years, to profound effect on life in the ocean and on land. Convergence to supercontinents has been a recurring cycle: roughly every 450 million years. A supercontinent is a landmass comprising multiple continental cores.
Supercontinents in Earth’s history include: Vaalbara (3.1–2.8 BYA), Kenorland (2.7–2.5 BYA), Nuna (1.9–1.5 BYA), Rodinia (1.1 BYA–750 MYA), and Pangea (300–200 MYA).
Periods of supercontinental cycles — when small continents smash together to make large supercontinents, and those supercontinents then rip apart into smaller continents again — could have put large pulses of nutrients into the biosphere and allowed organisms to really take off. ~ Aubrey Zerkle
The cycle from Rodinia to Pangea is illustrative. Rodinia formed 1.1 BYA by accretion via convergent collision of fragments from the previous supercontinent, Nuna (aka Columbia). Rodinia’s breakup was underway 750 MYA as plates diverged.
~600 MYA, Rodinia’s landmasses reconfigured into a short-lived supercontinent: Pannotia. So much landmass was around the South pole that there were more glaciers in the run-up to and during the time of Pannotia than in any other period in Earth’s history: Snowball Earth. Pannotia lasted 60 million years before breaking into 4 major land masses.
The plates continued their convolutions during the Palaeozoic era, concluding in the formation of Pangea 300 MYA. Pangea stayed a supercontinent over 100 million years.
Then the cycle began again, as Pangea began breaking up. The Tethys Ocean shrank as its ocean crust subducted, while the Atlantic Ocean grew at a divergent boundary, splitting Africa from a nascent North America.
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The supercontinent cycle follows a pattern. During breakup, rifting dominates; chasms appear as the lithosphere is pulled apart. This occurs at active margins which are at the leading edge of a continental plate, where subduction and uplifts occur. An active margin is a subduction zone or a strike-slip (aka transform) fault line, where plates rub against each other.
The Dead Sea Transform, which runs from the Sinai Peninsula up through southeast Turkey, is a continental strike-slip between the African plate to the west and Arabian plate to the east. Both plates are moving north-northeast, but the Arabian plate is moving faster.
Strike-slip structures also form within continental plates. The San Andreas fault in California is an intra-continental strike-slip.
Rampant rifting leads to passive margins, with the ocean growing via seafloor spreading. A passive margin is the transition region between oceanic and continental crust, absent an active plate margin.
Tectonic percussion follows. Island arcs and continents bump and grind as a prelude to collisions between continents. Continents come together again.
Thermal dynamics propel the supercontinent cycle. Plate tectonics and plume tectonics influence each other.
The shape of the Earth’s surface – its geoid – is highly variable. Plume and plate tectonics distort the geoid into highs and lows of both height and heat.
The supercontinent cycle is spurred by continents rifting toward geoid lows: cooler, lower areas of Earth’s surface. As continents converge into geoid lows, subduction zones concentrate around the margins of the emerging supercontinent.
Cold slabs of ocean crust subduct. As these slabs accumulate in the upper mantle, they form a self-organized criticality: eventuating into a massive collapse (avalanche) into the lower mantle, toward the mantle-core boundary. This provokes powerful plumes (superplumes) from below.
Superplumes rise through the mantle, mushrooming out below the lithosphere, causing the upper mantle to dome, rift, and melt, creating a geoid high. Massive quantities of basalt are produced, accelerating rifting and plate divergence.
Thusly material is recycled through the mantle, and heat convected from the core toward the lithosphere. A supercontinent starts to rift as it becomes a geoid high. The cycle restarts.
Earth is not done with supercontinents. Computer models suggest that a new supercontinent, dubbed Amasia, will form 250 million years from now, in the northern part of the globe.
Sea Level
The dynamics of ocean depth and planetary tectonics are intertwined, with ocean depth limited by the critical properties of water. Mid-ocean ridges are typically submerged to a depth at which seawater pressure is close to critical: the water is almost as dense as physically possible.
Ocean depth optimizes plate tectonics and the circulation of water by the plate system into subduction zones and out of andesite volcanoes. Intricate dynamics of plate ingassing and degassing balance ocean depth.
Little continental flooding occurred during the formation of the supercontinents Pannotia and Pangea. Conversely, the sea-level rise during the Cambrian is attributed to new ocean ridges formed from the breakup of Pannotia. Sea levels rose during the Cretaceous, in the wake of Pangea’s dispersal.
Sea level is generally low when continents converge and high when dispersed. This owes to the dynamics of the oceanic lithosphere, which controls the depth of ocean basins by conductive cooling and shrinking – decreasing the thickness and increasing the density of the oceanic crust, lowering the seafloor away from mid-ocean ridges. As the sea floor drops, the volume of the ocean basins increases.
The age of the sea floor reflects sea level. Hence, there is a relatively simple relationship between the supercontinent cycle and average sea floor age. With a supercontinent, much of the seafloor is old. Sea level is low.
Conversely, new seafloor is created at mid-ocean ridges during continental breakup, which characterizes the world today. Generating new sea floor lifts sea level.
Because continental shelves have a shallow slope, a small rise in sea level causes considerable continental flooding.
The area of global landmasses has varied considerably throughout Earth’s history, as continental flooding submerges crust from time to time.
Submerged Continents
Not all the continents are large landmasses. A few are submerged, including the Kerguelen Plateau and Zealandia.
The Kerguelen Plateau is in the southern Indian Ocean, 3,000 km southwest of Australia, about 3 times the size of Japan: extending for more than 2,200 km. The plateau was born from a hotspot that arose when Gondwana broke up 130 MYA. Some small islands sit above sea level.
New Zealand and New Caledonia are the lovely landmasses representing Zealandia, a largely (93%) submerged continental fragment that sank after breaking away from Australia 60–85 MYA, after a previous separation from Antarctica 85–130 MYA. 3.5 million km2, Zealandia is nearly half the size of Australia.
The Sunda Shelf extends from the continental shelf of Southeast Asia into the Gulf of Thailand to the Sunda Islands, notably Sumatra and Borneo.
During glacial periods, including the Last Glacial Maximum (26.5–19.5 thousands of years ago (TYA)), sea level dropped, exposing vast expanses as marshy plains. Sea level was at a minimum 22 TYA. Humans lived on the Sunda Shelf then.
In the post-glacial period, sea level rose; at first slowly, then moderately, peaking with a rapid rise of 16 meters in the 300 years between 14.6–14.3 TYA
On juicy rumor, Greek philosopher Plato wrote in 360 BCE that the legendary island of Atlantis was a conquering naval power ~9,600 BCE. According to Plato’s myth, after failing to invade Athens, Atlantis sank into the sea “in a single day and night of misfortune.” If there are grains of history in the legend of Atlantis, they rose and sank with Sundaland.
Evolutionary Effects
As a consequence of continental drift, biotas once together come apart or vice versa. This drives evolution, though the tectonic dynamic is on a much longer time scale than other factors more immediately affecting life: the landscape (including volcanic activity), atmosphere, and oceans – elements which influence climate as a demanding evolutionary impetus. Still, from an evolutionary perspective, tectonics and climate are 2 of most influential geophysical forces for the prospects of life on Earth. Though global climate may gyrate without land masses moving, tectonic changes have invariably caused major shifts of climate.
0.8 to 0.5 BYA, the planet experienced an extraordinary epoch, termed Snowball Earth, featuring 3 episodes of near-global glaciations. Thanks to the anomalous behavior of water, the planet was able to break its frozen grip.
Unlike most molecules, water’s solid phase is less dense than its liquid phase. If ice did not float, the oceans would have frozen from the bottom up, with the world long locked in ice.
Lacking sufficient mass of photosynthetic organisms to soak up all the atmospheric carbon dioxide from volcanic emissions, an energetic greenhouse effect developed, resulting in rapid melting. Volcanoes sated the seas with helpful chemicals, including phosphate. In the wake of Snowball Earth multicellular organisms proliferated.
Beyond tectonics (including volcanism) and climate, another major impact on life has been bolides. Though the age of dinosaurs was coming to a close on some continents due to tectonic shifts, the end was hastened by a strike from space.
Mammals began to thrive when biomes were particularly fragmented. Relative isolation from dinosaur dominion allowed a new evolutionary lineage to emerge.
Old and New World monkeys came from the same stock but followed independent paths as South America and Africa drifted apart some 50 MYA. The marsupial mammals of Australia evolved in isolation from placental mammals as the island continent drifted out to sea more than 60 MYA.
India crashed into Asia 45 MYA, creating the Himalayas, and inciting an exchange of life. Africa and Eurasia made contact 18 MYA. Primates joined other species making a south-to-north migration, while many antelope species moved the other way.
The Americas came together ~3 MYA at the Panamanian Isthmus, facilitating an exchange of species that had evolved separately for millions of years. This was the Great American Interchange.
Continental Stock Exchange
Complex evolutionary dynamics ensue after isolated continents unite. Many South American (Neotropic) mammals went extinct during the Great American Interchange. North American (Nearctic) carnivores rapidly occupied South American predatory niches.
Geography favored the Nearctic invaders. Any species reaching Panama had to be able to tolerate humid tropical conditions. Species heading south would then encounter climates in South America that were not markedly different. Conversely, heading north meant heading into the cooler and/or drier conditions found in the Sierra Nevada volcanic mountain range that runs across central-southern Mexico.
This climatic asymmetry was particularly hard on Neotropic species, which had specialized for the tropical rainforests. They had little prospect for getting beyond Central America. Central America currently has about 40 mammal species originating from South America compared to 3 Nearctic species.
There were exceptions to Nearctic takeover. A few Neotropic immigrants prospered. Porcupines and opossums became conspicuously successful northward migrants. The naturally armored armadillo fared fairly well too.
A broader historical perspective also factors in on the lopsidedness of the Great American Interchange. During the Cenozoic era (65 MYA–10 TYA), North America was periodically connected to Eurasia via Beringia: the Bering land bridge between Siberia and Alaska, which was at times 1,600 kilometers wide.
Beringia allowed multiple migrations back and forth. In turn, Eurasia was connected to Africa, adding further to the mix of species into North America.
On the other end, South America was connected to Antarctica and Australia, 2 much smaller continents, only during the earliest part of the Cenozoic. Further, this land connection carried little traffic; no mammals save some marsupials and a few monotremes (egg-laying mammals).
In sum, Nearctic species were descendants of a more competitive arena: an ideal setup to quicken evolutionary pace. Nearctic animals tended to be smarter and more efficient; generally able to outwit and outrun their Neotropical counterparts. Neotropic ungulates (hoofed animals) and their predators were replaced wholesale by Nearctic invaders.
Many of the Nearctic mammals speciated as they populated South America. This diversification dynamic is typical. Hominids speciated as they spread into Eurasia from Africa, as did antelopes as they diffused and thrived throughout Africa.
Sea levels and glaciation also define biotic isolation or connection. Beringia appeared at various times during the Pleistocene epoch ice ages, 2.6 MYA to 11 TYA. Humans migrated from Asia into the Americas over Beringia 26–20 TYA.
Continental Rift
Besides shuffling landmasses to profound effect, plate tectonics alter the ecology within a continent. Geological events in east Africa affected an epicenter of hominid evolution.
45 MYA, much of Africa was fairly flat, carpeted west to east in tropical forest in the equatorial region. A complex pattern of mantle circulation and plume development started 30–40 MYA in east Africa.
4 phases of movement in the African plate near Lake Victoria resulted in uplift and rift shoulders. The rifting was facilitated by the continental crust in that region being thin and so readily subject to deformation.
By 20 MYA, 2 major rifts had developed: an eastern branch, through Ethiopia and Kenya, and a western branch that formed a giant arc from Uganda to Malawi, interconnecting the rift lakes of eastern Africa.
This continental rifting altered vegetation patterns. Continuous forest fragmented into a patchwork of woodlands that came to include grassland savanna.
Habitat fragmentation and geological transformation encouraged allopatric speciation: evolution owing to isolating populations. Further, as the terrain became diverse, so too the local climates, ranging from hot, arid, lowland deserts to cool, moist highlands, along with various habitat types in-between these 2 extremes.
Every species is limited in its environmental tolerances, from temperature and terrain to water availability. Animals depend upon plants, so vegetation greatly defines a biome. While tolerances vary among species, with some species able to live in multiple niches, a topographically diverse area naturally begets biotic diversity.
In addition, topographic variability creates barriers to population migration. A species adapted to higher elevation may not migrate from one highland to another because the terrain in-between is inhospitable. Population isolation provides opportunity for localized adaptation.
The tectonic uplift and faults that formed the Great Rift Valley in east Africa created conditions conducive to adaptation by an ape into species capable of surviving in a broader range of biomes. The story of human descent had a complicated plot. That withstanding, climate by way of topology impacted hominid evolution, as it commonly does with life on land.
