Slab breakoff returns sunken continental crust from the mantle
Granite-composition continental crust off western Norway should have risen buoyantly, but the subducting oceanic slab below held it as an anchor and dragged it into the mantle; only after the slab fractured did buoyancy reverse and the crust follow the same path back to the surface.
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Continental crust can sink 100 kilometers into the mantle and return
Gray gneiss off the coast of western Norway contains scattered dark eclogite bodies, some with microscopic inclusions invisible to the naked eye that can only form ~100 kilometers deep in the mantle. This shouldn't happen: continental crust is inherently buoyant—it's why continents rise and ocean floors sink—and would rebound if pushed down or shouldn't sink at all. Yet this rock did reach mantle depth, stayed long enough for minerals to recrystallize, then returned to the surface.
Coesite remained hidden inside common minerals until 1984
Coesite and diamond, ultrahigh-pressure phases, exist only as microscopic inclusions within common eclogite minerals like garnet and omphacite and require careful examination of individual minerals to detect—this is why coesite in Norway and the Alps wasn't confirmed until 1984. Complicating matters, coesite often reverts to ordinary quartz as rock depressurizes, and the volume expansion cracks the surrounding minerals, creating radial fractures—researchers often first suspect coesite was present from these fractures, but confirming coesite or diamond requires Raman spectroscopy to measure atomic spacing within the mineral.
— Torger AndersenFast exhumation is why ultrahigh-pressure minerals survive instead of reverting
These ultrahigh-pressure minerals are stable only under extreme pressure, so they should revert when pressure drops, but reversion requires time. If rock ascends fast enough, the minerals don't have time to fully revert. Andersen calls this ‘isothermal decompression’: during rapid exhumation, eclogite barely cools and may even warm slightly approaching the surface. Fast exhumation is what allows coesite and diamond to survive.
— Torger AndersenGranite's buoyancy saves eclogite from sinking when they're interlayered
Mantle density is ~3.1-3.2 g/cm³, while eclogite reaches 3.6 g/cm³—eclogite alone should sink into the mantle. It survives because it's embedded as bodies within far more buoyant granitic gneiss: granite always has positive buoyancy relative to mantle and refuses to sink, becoming a ‘life raft’ for eclogite bodies. But then, how did positively buoyant continental crust get pulled down in the first place? Because it's attached to subducting oceanic lithosphere that has already fully transformed to eclogite—denser material acting as an ‘anchor,’ dragging the whole continent into the depths.
— Torger AndersenSlab fracture reverses buoyancy once continental crust descends far enough
After subducting some distance, continental crust remains positively buoyant relative to surrounding mantle, so the slab below lacks sufficient strength to pull it deeper—the slab fractures and breaks away from the continent. Once separated, the dragged continental crust immediately becomes positively buoyant relative to mantle again, reversing its motion: it begins rising along the same subduction interface, and as it heats, it transitions from vertical extension to vertical compression. Exhumation results from this upward motion combined with vertical shortening.
— Torger AndersenPapua New Guinea achieves exhumation speeds 10 times faster than typical
The youngest known coesite-bearing eclogite globally occurs in the Woodlark Rift system and D'Entrecasteaux Islands north of Australia, only 5-6 million years old, with exhumation rates of centimeters per year rather than the typical millimeters. This extreme speed results from eclogite-metamorphosed lower crust beginning to melt and rising diapirically, combined with large-scale extensional rifting. Andersen notes no other examples of diapiric-style exhumation are known.
— Torger AndersenMylonite strain patterns record over 100 kilometers of vertical displacement
Western Norway's exposure of these deep rocks owes mainly to a massive extensional detachment—a normal-fault-style shear zone where the upper plate slid downward relative to the lower, with total displacement reaching 100-150 kilometers. This 3-5 kilometer-thick band of intense deformation consists entirely of mylonite, with structural features from outcrop to microscopic scales indicating the upper plate moved westward. Strain is quantifiable: each meter of mylonite corresponds to up to 20 meters of westward displacement, accumulating to the 100+ kilometers of exhumation distance required to bring these deep rocks to the surface.
— Torger AndersenSealed rock bodies can sustain pressure independent of depth
Pressure is often used as a proxy for depth, but solid rock differs from fluid—strong rock can maintain overpressure in sealed pockets like a pressure cooker, exceeding the weight of overlying rock, sourced from tectonic compression or volume expansion from reactions or melting. Many geologists reject overpressure, arguing rock strength is insufficient to sustain it, yet others use numerical models and physical parameters to argue overpressure is plausible. Andersen himself accepts overpressure might exist and has coauthored papers invoking it to explain an isolated diamond occurrence in western Norway that doesn't fit the regional metamorphic gradient (eastern 10 kbar and 500°C, western 30 kbar and 650-700°C). He emphasizes overpressure doesn't change the overall subduction-exhumation narrative, but affects interpreting isolated outcrops.
— Torger AndersenIn their own words · checked verbatim
And yet here is a piece of it that went to mantle depths, stayed long enough for its minerals to recrystallise, and then came back up to the surface.
Oliver Strumpel1:04
Coesite forms only at pressures above about 27,000 atmospheres, equivalent to a depth of 90 to 100 kilometres.
Oliver Strumpel2:05
So they need a life belt in order not to sink. And the life belt is the granitic rocks in which they are sitting, that don't want to go into the deep mantle, because they will always have a buoyancy with respect to the mantle.
Torger Andersen8:28
They are five, six, maybe a bit more million years old. And they return from some depth that is capable of producing cosite to the surface. So that means that the exhumation velocity, we're talking centimeters per year rather than millimeters per year, that is a more common velocity for this exhumation.
Torger Andersen12:35
For every meter or thickness of mylonite, you can have up to 20 meters of down-to-the-west movement.
Torger Andersen17:43
As they say, rocks are not strong enough to sustain overpressure, whereas others have used numerical models and physical parameters to argue that overpressures can exist.
Torger Andersen23:05
Figures
| Coesite stability pressure | ~27,000 atmospheres | 2:05 |
| Coesite depth equivalent | 90-100 kilometers | 3:06 |
| Coesite first identified | 1984 | 8:28 |
| Mantle density | 3.1-3.2 g/cm³; eclogite density up to 3.6 g/cm³ | 16:42 |
| Exhumation zone displacement | 100-150 kilometers | 17:43 |
| Mylonite strain ratio | 1 meter of mylonite = up to 20 meters displacement | 12:35 |
| Caledonian orogeny | ~400 million years ago | 18:47 |
Glossary
- eclogite
- Basaltic rock compressed under high pressure, containing garnet and omphacite (green clinopyroxene) in place of lower-pressure minerals.
- coesite
- High-density polymorph of quartz stable only at mantle pressures (~27,000 atmospheres), marking burial to extreme depth.
- mylonite
- Rock deformed intensely in shear zones whose internal strain patterns record the magnitude of displacement.
- diapir
- Lower-density material rising through overlying rock driven by buoyancy, like bubbles in liquid.
- isothermal decompression
- Rapid uplift of rock with minimal cooling, necessary to preserve ultrahigh-pressure minerals that would otherwise revert.
- slab break-off
- Fracture and separation of a subducting oceanic slab when it can no longer pull down buoyant continental crust.
How to listen
Geologists and Earth-science readers interested in continental collision zones, subduction-exhumation mechanisms, and ultrahigh-pressure metamorphic rocks.
The closing discussion of Devonian basin reconstruction—the speaker says it is not yet available for publication, offers limited information, and can be skipped.