North American Plate

Fessura di Silfra tra rifting tettonico basaltico e acque glaciali purissime.

Nel cuore della fossa tettonica di Thingvellir, la spaccatura sommersa di Silfra testimonia la deriva inarrestabile tra le zolle eurasiatica e nordamericana, aprendo una finestra trasparente su acque filtrate per decenni attraverso la roccia vulcanica.

La cicatrice geologica della dorsale medio atlantica.

Nel sud-ovest dell'Islanda, all'interno del Parco Nazionale di Thingvellir, dichiarato patrimonio dell'umanità dall'UNESCO, la superficie terrestre si squarcia scoprendo i meccanismi profondi della geodinamica globale.

In questo anfiteatro naturale, teatro nel 930 della nascita dell'Althing, il parlamento più antico del mondo, la dorsale oceanica medio atlantica emerge all'aria aperta.

Le due colossali placche tettoniche continentali, quella nordamericana a ovest e quella eurasiatica a est, si allontanano inesorabilmente a una velocità media costante di circa due centimetri all'anno.

Questa trazione divergente genera una depressione tettonica, un graben, caratterizzata da imponenti faglie parallele, con lassi vulcanici e profonde spaccature basaltiche.

La più celebre e spettacolare di queste fenditure è la fessura di Silfra, formatasi nel 1789 a seguito di un violento sciame sismico che provocò l'apertura e lo sprofondamento del fondo roccioso, aprendo una via d'acqua sotterranea direttamente collegata al grande lago Thingvallavatn.

wanting to go to the northeast.

And then you have the North American plate wanting to go to the southeast.

And the amount of power that can be released in a huge earthquake, it's hard to imagine.

So say you're standing right on the edge of the San Andreas Fault with your best friend.

These crusts are the mineralized footprints left behind by geothermal systems that boiled millions of years ago.

Because the North American Plate creeps steadily southwest while the mantle plume stays parked firmly in place, southern Idaho is effectively a 400-mile graveyard of extinct, cooled calderas.

If it were possible to peel back the heavy layers of younger, dark lava rock covering eastern Idaho, a drowned world of ancient basins would emerge, identical to the steaming valleys active today.

In some places, erosion has done the heavy lifting, exposing these buried thermal ruins to the open air.

Each peak appears to stand alone, separated by forests, rivers, and valleys, yet all belong to a single geological system that has been operating for millions of years.

Deep beneath the landscape, the Juan de Fuca Plate continues descending below the North American Plate.

As that slab sinks into the mantle, water released from the descending rock lowers the melting point of the surrounding mantle, generating magma that slowly rises toward the surface.

The process has not ended, nor does it show signs of slowing.

The valleys, forests, rivers, and mountains of the Pacific Northwest appear settled and enduring, yet each owes its existence to countless volcanic events spread across millions of years.

The eruption of May 18, 1980, was one chapter in that continuing history, dramatic within human memory, yet entirely consistent with the geological processes that created the region itself.

The Cascade Range exists because the Juan de Fuca Plate continues descending beneath the North American Plate along the Pacific Northwest Coast.

As that oceanic crust sinks into the mantle, portions of it release water that encourages surrounding rock to melt.

The Alaska Range extends in both directions from Denali, a crescent of granite and ice stretching roughly four hundred miles from the Alaska Peninsula to the northeast toward the Canadian border.

It is a young range, geologically speaking, still being pushed upward by the collision of the Pacific Plate diving beneath the North American Plate along the great arc of the Aleutian Trench.

The mountains are still growing.

They gain a fraction of an inch per year, though erosion and glaciation claw back most of that gain, grinding the peaks down almost as fast as the tectonic forces push them up.

The mountains hold all interpretations simultaneously because the mountains do not interpret.

They simply are.

The range has stood for roughly sixty million years since the tectonic forces that drive the Pacific Plate beneath the North American Plate began pushing the sedimentary and volcanic rock of the coastal margin upward into the peaks that now define the southern skyline of interior Alaska.

In those sixty million years, the range has been glaciated and deglaciated multiple times.

Their outer shell is broken into enormous sections called tectonic plates, and that is where we're going to move this conversation away from, because I could get right now into a conversation about these tectonic plates and blah, blah, blah.

They have names for them, Pacific plate, North American plate, African plate, Eurasian plate, until our heads are spinning off, but I think that would only add to our confusion, because The measurement they're using, this MW thing, not the Richter scale, was developed during the 1970s.

Now it's the international standard.

It measures fault size, amount of movement, strength of the rocks, total energy release.

The Alaska Range extends in both directions from Denali, a crescent of granite and ice stretching roughly 400 miles from the Alaska Peninsula to the northeast toward the Canadian border.

It is a young range, geologically speaking, still being pushed upward by the collision of the Pacific Plate diving beneath the North American Plate along the great arc of the Aleutian Trench.

The mountains are still growing.

They gain a fraction of an inch per year, though erosion and glaciation claw back most of that gain, grinding the peaks down almost as fast as the tectonic forces push them up.

The mountains hold all interpretations simultaneously because the mountains do not interpret.

They simply are.

The range has stood for roughly sixty million years, since the tectonic forces that drive the Pacific Plate beneath the North American Plate began pushing the sedimentary and volcanic rock of the coastal margin upward into the peaks that now define the southern skyline of interior Alaska.

In those sixty million years, the range has been glaciated and de-glaciated multiple times.

This, uh, shows the instabilities.

They're called the Juan de Fuca instabilities, and this is the Juan de Fuca Plate, and this is the North American Plate over here, and these are velocities measured by GPS.

Just to calibrate you, the length of that arrow is one inch per year.

[coughs]

And if we compare this gentle giant to the height of explosive Mount Rainier, which is southeast of Seattle, Washington in the Cascade Range of volcanoes in the US, the difference is striking and hints at the fact that they actually have different tectonic origins.

Unlike Mauna Loa over a hotspot, Mount Rainier was created when the lighter continental crust of the North American Plate collided with and subducted the denser oceanic crust of the Juan de Fuca Plate.

Because hundreds of meters of sediments are often carried along when a plate is subducted and melted into magma, it produces a felsic lava, which is 50 to 75% light minerals like silica and aluminum, and creates continental crust.

So the magma from Mount Rainier is thicker than magma from volcanoes like Mauna Loa.