New map reveals ships buried below San Francisco

Cartographers are still putting the finishing touches on the new map, which will appear in the visitors’ center at the San Francisco Maritime National Historical Park.

This detail from a new map of buried ships in San Francisco shows the original shoreline extending inland to the current location: the iconic Transamerica Pyramid building (top center).

 

From National Geographic by Greg Miller

Dozens of vessels that brought gold-crazed prospectors to the city in the 19th century still lie beneath the streets.

Every day thousands of passengers on underground streetcars in San Francisco pass through the hull of a 19th-century ship without knowing it.
Likewise, thousands of pedestrians walk unawares over dozens of old ships buried beneath the streets of the city’s financial district.
The vessels brought eager prospectors to San Francisco during the California Gold Rush, only to be mostly abandoned and later covered up by landfill as the city grew like crazy in the late 1800s. Now, the San Francisco Maritime National Historical Park has created a new map of these buried ships, adding several fascinating discoveries made by archaeologists since the first buried-ships map was issued, in 1963. It’s hard to imagine now, but the area at the foot of Market Street, on the city’s eastern flank, was once a shallow body of water called Yerba Buena Cove, says Richard Everett, the park’s curator of exhibits.
The shoreline extended inland to where the iconic Transamerica Pyramid now rises skyward.

In 1848, when news of the Gold Rush began spreading, people were so desperate to get to California that all sorts of dubious vessels were pressed into service, Everett says.
On arrival, ship captains found no waiting cargo or passengers to justify a return journey—and besides, they and their crew were eager to try their own luck in the gold fields.

This is one of five panels in a panoramic daguerreotype taken by William Shew in or around 1852. Rincon Point, the southern end of the cove, appears in the foreground. 

The ships weren’t necessarily abandoned—often a keeper was hired to keep an eye on them, Everett says—but they languished and began to deteriorate.
The daguerreotype above, part of a remarkable panorama taken in 1852, shows what historians have described as a “forest of masts” in Yerba Buena Cove.
Sometimes the ships were put to other uses.
The most famous example is the whaling ship Niantic, which was intentionally run aground in 1849 and used as a warehouse, saloon, and hotel before it burned down in a huge fire in 1851 that claimed many other ships in the cove.
A hotel was later built atop the remnants of the Niantic at the corner of Clay and Sansome streets, about six blocks from the current shoreline (see map at top of post).

Localization with the GeoGarage platform (NOAA chart on Google Maps)

A few ships were sunk intentionally.
Then as now, real estate was a hot commodity in San Francisco, but the laws at the time had a few more loopholes.
“You could sink a ship and claim the land under it,” Everett says.
You could even pay someone to tow your ship into position and sink it for you.
Then, as landfill covered the cove, you’d eventually end up with a piece of prime real estate.
All this maneuvering and the competition for space led to a few skirmishes and gunfights.

One of these intentionally scuttled ships was the Rome, which was rediscovered in the 1990s when the city dug a tunnel to extend a streetcar line (the N-Judah) south of Market Street.
Today the line (along with two others, the T and the K) passes through the forward hull of the ship.
Eventually Yerba Buena Cove was filled in.
People built piers out into it to reach ships moored in deeper water, Everett says.
“The wharves are constantly growing like fingers out from the shore.”
Then people began dumping debris and sand into the cove, which was only a few feet deep in many places to begin with.
“By having guys with carts and horses dump sand off your pier,” Everett says, “you could create land that you could own.”
It was a land-grab strategy with lasting ramifications—as evidenced by the ongoing controversy over a sinking, tilting skyscraper built on landfill near what was once the southern edge of Yerba Buena Cove.
Three archaeologists—James Allan, James Delgado, and Allen Pastron—consulted on the making of the new shipwreck map, and discoveries by them and their colleagues have added several fascinating details that weren’t on the original buried-ships map created by the San Francisco Maritime National Historical Park in 1963 (see below).
Red circles on the new map indicate sites that have been studied by archaeologists.

This detail from the original 1963 buried-ships map shows “Sydney Town,” where Australians congregated in Gold Rush days.

There was a Chilean enclave just inland from here, and fights sometimes broke out between the two groups.
Map courtesy San Francisco Maritime National Historical Park 

One of the most interesting additions to the new map is a ship-breaking yard at Rincon Point at the southern end of Yerba Buena Cove, near the current anchorage point for the Bay Bridge.
A man named Charles Hare ran a lucrative salvage operation here, employing at least 100 Chinese laborers to take old ships apart.
Hare sold off brass and bronze fixtures for use in new ships and buildings.
Scrap wood was also a valuable commodity in those days, Everett says.
The 1851 fire ended Hare’s business.
Archaeologists have found the remnants of six ships at the site that were presumably in the process of being salvaged at the time of the fire. One—the Candace—was another whaling vessel pressed into service to bring gold-crazed prospectors to San Francisco.
A lighter, small, flat-bottomed boat that was used to shuttle goods from moored ships to shore has also been found.A development project near Broadway and Front streets, which began in 2006, turned up bones that archaeologists suspect came from Galapagos tortoises (the site is marked by an asterisk in the map at the top of this post).
After passing around Cape Horn, many ships stopped in the Galapagos Islands and threw a few turtles in the hold—a source of fresh meat for the long voyage north to California.
“They got to San Francisco, and lo and behold: They had more turtle than they could eat,” Everett says.
Menus from the era show that turtle soup was a common offering at restaurants and lodging houses around the cove.

Illustrator and designer Michael Warner says his inspirations for the new map included the “Maps of Discovery” from a mural painted by N.C. Wyeth in 1928 for the headquarters of the National Geographic Society.
Wyeth’s imaginative painting evokes the romance of the Age of Discovery, and Warner says it inspired him to go beyond just showing the details of the buried ships and historic wharves.

“My hope is that I have not only enhanced the image for the history enthusiast,” he says, “but created something that might even make people learn by accident!”

The team is still ironing out some final details, such as how to most accurately represent the boundaries of Charles Hare’s ship-breaking yard.
They hope to have posters of the new map available for purchase early next year and plan to eventually put it on display in the visitors’ center.

Links :

• Upout : This maps shows ships buried underneath San Francisco

Yacht race starts from Plymouth (1964)

The boats with full sails head out to the bay

for the start of the second edition of the single handed Transatlantic yacht race (OSTAR)

(May 23, 1964)

Thirteen competitors started this race in 1964, 
which by now was firmly established on the racing scene.
All of the five original competitors entered, and all five improved their original times (Gipsy Moth III with Sir Francis Chichester the winner of the first edition in 1960 making ready);

but the show was stolen by French naval officer Éric Tabarly, who entered a custom-built 44-foot (13 m) plywood ketch, Pen Duick II.
The days of racers sailing the family boat were numbered following Tabarly's performance, for which he was awarded the Légion d'honneur by president Charles de Gaulle.
It is also noteworthy that Tabarly and Jean Lacombe were the only French entrants in this race; Tabarly's success was instrumental in popularising the sport in France, the country which in future years would come to dominate it.

This was to be the year in which several future trends were established.

Multihulls made their first appearance — sailing in the same class as the other boats; and the race featured the use of radio, for the first time, by several competitors who gave daily progress reports to their sponsors.

Links :

• YouTube : Jean Lacombe : un rebelle à travers l'Atlantique

Mini tsunami on European coast

A rare tidal wave caught on camera early in the morning.
Location: Zandvoort Noord-Holland, Netherlands.
Date: 29 May 2017 06:21

Radac Cie checked the measurements of  wave radar at Prinses Amalia Wind Park (23km in front of Ijmuiden) : Heave measurements, 29-05-2017

The picture (see zoom) shows a nice peak just before high tide, an increase of +/- 80 cm.

Prinses Amalia Wind Park in the GeoGarage platform (NLHO chart)

The mini tsunami was caused by a fast approaching high pressure front with some decent thunderstorms. 
(see Nos)

The quest to save coral reefs

Dr. David Vaughan is working to combat the crisis in the world’s coral reefs—that is, that humans have lost 25 to 40 percent of the world’s corals in recent decades due largely to seawater temperature rise and ocean acidification.

Vaughan has developed a game-changing technique called “microfragmenting” that allows corals to grow more than 25 times faster than normal, which could rapidly restore the dwindling population of healthy coral reefs.

The Atlantic visited Dr. Vaughan in the Florida Keys to uncover how the process works and understand how much hope there is to revitalize our reefs. 

From The Atlantic by Meehan Crist

With some of the world’s richest ecosystems hanging in the balance, scientists are turning to technologies like 3-D printing.

A coral is an animal that demands imagination.
Look closely through a dive mask (or a Google image search) and you’ll see that a coral reef’s rocky undulations are coated in an astonishing skin of tiny creatures that look like upside-down jellyfish, bells rooted in place, mouths open and ringed with tentacles waving to the sea.
These are coral polyps.
And right now, around the planet, they are dying with breathtaking speed.
It’s uncertain how many will survive into the near future, and unclear what we can do to make sure they survive.

Often mistaken for plants, corals are cousins to jellyfish and sea anemones, with whom they share a phylum and a distinctive physique.
Each coral polyp is shaped like a tube, with a mouth, a simple stomach, and a base where it secretes a cup-shaped exoskeleton of calcium carbonate that roots it in place and protects it from predators.
Many corals reproduce asexually, when polyps clone themselves.
As new polyps form, they build their cup-shaped skeletons on top of the empty shells of previous generations, creating limestone reefs as they go.
A single coral is often an animal composed of hundreds or thousands of interconnected polyps, a colony of genetic clones that share a single set of DNA, clinging to the skeletal remains of its own past dead.
Most corals are also hermaphroditic spawners, which means that in addition to cloning, they produce both eggs and sperm.

One night a year, in a wildly improbable mass-spawning event, all the coral of a single species will release eggs and sperm bundled together into tiny translucent globes that cloud the water and rise to the ocean’s surface.
Here, the globes break apart, sperm and eggs intermingle, and baby coral larvae are born.
Researchers used to think a larva would float along helplessly, tossed by ocean currents, until it happened on a place to land.

But in recent years researchers have discovered that a baby coral polyp can sense light, temperature, pH levels, and even sound in the ocean through which it navigates, waving its tiny cilia and swimming in search of a future home.
Once a larva lands and attaches, it stays put for life.

3D-printed reefs, a new addition to the growing assortment of artificial reefs being dropped into ailing oceans worldwide, are designed with these polyps in mind, their nubbly surfaces grooved and inviting, intended to offer safety and succor.
The world’s first 3D printed reef was sunk in the Persian Gulf in 2012.
Made of pale sandstone with nubbly branches designed to look like actual coral, it was just one artificial unit among 2,620 (the others made of molded concrete) dropped off the coast of Bahrain in a massive effort to replenish dwindling fish stocks.

The area’s coral reefs had been ravaged by pollution and overfishing, leaving “once complex marine habitats now reduced to rubble.”
Artificial reefs can provide shelter for a limited species of fish and sea creatures in the short-term, but can they help us keep vibrant coral ecosystems alive long-term?
Bolstering fish stocks is a worthy project, but no artificial reef is a replacement for living coral, an animal that has evolved for millions of years to interact in equilibrium with its environment.
Coral-reef ecosystems cover only a tiny sliver of planetary real estate, just 0.0025 percent of the world’s ocean floor, but they are home to fully 25 percent of all marine species—by some estimates, reefs beat even rain forests for biodiversity.

The value of this biodiversity to humans is staggering.
By one estimate, coral reefs account for over $6.7 trillion of the annual global economy, more than four times the U.K.’s share.

Coral reefs also filter and clean polluted ocean water, and serve as protective barriers against increasingly violent storms.

Perhaps most critically, coral-reef ecosystems provide half of the earth’s oxygen and absorb 30 percent of the carbon dioxide emitted from burning fossil fuels.
Without reefs, this warming planet will get hotter, faster.
We need coral, even if it needs us like the proverbial hole in the head.
And yet, across the globe coral is dying at unprecedented rates.
Across the Caribbean and Florida Keys, two key coral species—staghorn and elkhorn—have declined by an astonishing 98 percent since the 1970s.
Worldwide, coral has already declined by roughly 40 percent.

Just last October, The National Oceanic and Aquatic Administration (NOAA), made the devastating announcement that with the return of El Niño, we are seeing the third worldwide coral bleaching event in recorded human history.
“Bleaching” occurs when ocean temperatures stay too warm for too long—sometimes just a degree or two warmer than usual—and corals react to the stress by kicking out their symbiotic zooxanthellae, the tiny algae that live in their tissues, giving corals their vibrant colors and providing them with energy through photosynthesis.
Without their colorful symbiotic partners, the coral turns an eerie, skeletal white.
And without its main source of energy, it starts to starve.
When coral bleaches, reef creatures flee or die in droves.

In a matter of days, what was once a vibrant underwater ecosystem becomes a barren field of bone fingers reaching into an empty ocean.
As I write this, a massive band of unusually warm water is spreading around the middle of the planet.
Corals have already bleached across the Caribbean, Southeast Asia, and the Florida Keys.
Just two weeks ago, coral started bleaching in Fiji.
Thousands of blue and turquoise and pink reef fish washed up dead along the beaches of the Coral Coast.

Victor Bonito, marine biologist and director of Reef Explorer Fiji, told New Zealand Radio that nearly a third of inshore corals have bleached and he has already witnessed “decades of damage.”
The first time a global coral bleaching event happened, when El Niño hit in 1997-98, 16 percent of the world’s coral was severely damaged.
In the Maldives, it was as high as ninety percent.
This time around, the bleaching is predicted to be even worse and is expected to stretch well into 2017.

As Mark Eakin of NOAA put it in a statement released just a few days ago, “We are currently experiencing the longest global coral bleaching event ever observed.” Right now, the band of warm water is heading west from Fiji toward Australia’s Great Barrier Reef.
Cooler weather could mitigate the damage, but already there are reports of up to 80 percent bleaching in sites along the northern edge of the World Heritage site.

The Guardian reports that “authorities are praying for clouds and rain.”
There is no doubt that a profound shift is underway in today’s ocean, and coral reefs are the canaries in the coal mine of our carbon-obsessed planet.
As a result of human activity, particularly the burning of fossil fuels, our ocean is not only warmer, on average, but also more acidic, because CO2 emitted from burning fossil fuels gets trapped in the ocean, and turns into acid.

A landmark study published in Nature last month offers the first evidence that rising CO2 levels and acidification are severely stunting coral growth.
To say that the ocean we have known in our lifetimes is already gone is not doomsaying or pessimism.
It’s a realistic assessment of where we stand, now.

On Feb 19, the UN World Meteorological Organization (WMO) announced that for the first time in recorded history the world passed the threshold of 1 degree Celsius above pre-industrial temperatures, halfway to the Paris treaty’s controversial 2 degree Celsius threshold, a point at which, once it becomes the average, a recent paper in Nature Geoscience reports all the world’s coral reefs will already be gone.
Some estimates have us on track to speed past that 2 degree Celsius threshold in the next 20 years, but just a few days ago the planet briefly heated all the way up to the dreaded 2 degree Celsius, leaving climate scientists reeling.

Given the scope of devastation under way in our ocean, it’s hard to know whether new technologies like 3D printed reefs can make a difference.
A bit like aquatic birdhouses, artificial reefs are often designed with a certain species in mind (red snapper in Bahrain), but they provide shelter for myriad species, including algae, anemones, octopus, and crab.
If molded concrete units are the Soviet-era apartment blocks of the sea, the 3D printed unit off Bahrain is an aquatic Craftsman, the buff surface carefully grooved and pitted to attract free-floating baby coral polyps—the hope being that one day those artificial limbs might be carpeted in living coral.
Similarly, a new system of 3D printed reef soon to be unveiled by Reef Design Labs, co-founded by Reef Arabia founder Dave Lennon, features interlocking units with a porcelain coating that boasts “dimples” and “a chemical makeup similar to coral” that may attract baby coral polyps.
While promising as a substrate for baby coral polyps, the materials these reefs are built of are guaranteed to last just sixty years.

Most are not large or heavy enough to withstand being tossed around by a major weather event, and there is very little scientific data on what happens when you actually put them in the ocean.
In a maddening catch-22, 3D printed reefs lack the imprimatur of data from scientific testing, which means it’s hard to secure funding to put them in the ocean, where data could be collected.
So far, the unit off Bahrain is the only 3D printed reef in any ocean in the world, though plans are underway to sink six 3D printed reefs—designed to help corals recuperate from damage—off the coast of Monaco later this year. (Lennon is an advisor to the project.)

Meanwhile, coral around the world is struggling to survive in warmer, more acidic waters.
For millions of years, corals have lived in changeable environments, pummeled by storms and the vicissitudes of climate, and they have evolved to be inherently dynamic and resilient systems.
“Resilience” is a word overused to the point of nonsense in recent years, but the concept is meaningful in the context of coral-reef ecology.
After the first bleaching event in 1998, 16 percent of the world’s coral in fifty countries bleached.
Forty percent of that coral died, but that means 60 percent of it lived.
“They can bounce back from disruption.
They can bounce back from mortality,” says Gabriel Grimsditch, the senior project officer at the International Union for the Conservation of Nature, currently helping to develop coral-reef management plans.

Battered corals can recover from catastrophic events like bleaching or cyclones, but they need time.
Corals grow slowly, averaging between .02 to 8 inches per year (a rate stunted by rising acidity), so even a fast recovery takes years.
“You can’t stop a bleaching event,” says Grimsditch, “but you can manage for recovery.
We can reduce local stressors like pollution and overfishing.
We can design measures that might help give aquatic life a fighting chance.”
Grimsditch is focused on managing reefs longterm.

Less local pressure from overfishing, land-based pollution, and destructive coastal development means healthier coral before global events, which means greater resilience afterward and the possibility of healthier reefs in the future.
Of course, some species of coral will undoubtedly fare better than others, which will fundamentally alter the makeup of the world’s coral reef ecosystems.
Recently, researchers have make the remarkable discovery that some genetically younger corals are able to live in hotter and more acidic waters than their forebears.
There may already be corals that have adapted to live in our future ocean.
“Let’s focus on the factors we can manage and help reefs be more resilient,” said Grimsditch.
“If 3D printing helps, that’s great.”

This perspective—short-term pessimism, long-term optimism, a willingness to try—is increasingly prevalent among those concerned about the future of our ocean.
In Florida, marine biologist David Vaughn is using new aquaculture techniques to speed the growth and resettlement of centuries-old coral.
Ruth Gates, a researcher at the Hawai’i Institute of Marine Biology, is breeding coral in an attempt to speed evolution of a new “super-coral” that can thrive in warmer and more acidic water.
In Curaçao, marine biologist Kristen Marhaver is using 3D printed discs to study coral larvae, and has found the species she studies prefer to settle on discs that are pink or white—the color of a healthy coral reef.

One could imagine a super-coral farmed to healthy adulthood on 3D printed reefs.
Artificial reefs may help some corals survive the global transition from fossil fuels, or they may be all that’s left, underwater birdhouses of concrete and porcelain built for species that have adapted to survive without coral—the dark green algae and the glittering handfuls of homeless fish searching for a place to hide.
This future ocean may not be ideal, but it too, is worth fighting for.

The oldest marine organism on the planet is a deep-water black coral, Leiopathes, living off the coast of Hawaii and carbon-dated to 4,265 years old.
Down where Leiopathes live, temperatures are less dependent on fluctuating weather patterns at the surface, so this coral might have better odds at surviving the epic changes underway.

The future remains uncertain, but we know that change is inevitable (the calcerous part of the Alps, known as the Northern Limestone Alps, used to be coral reefs) and we know we can’t reverse the effects of climate change on our oceans.
The real hope is that some corals survive long enough for human civilization to wean itself from a carbon-based economy.
In the meantime, we can ventilate the coal mine until we no longer need coal, and we can breed heartier canaries.
Letting go is not the same as giving up.

Links :

• GeoGarage blog : Coral odyssea / Great Barrier Reef at 'terminal stage': scientists despair at ... / Coral colors /

Seafloor speading

Earth's newest crust is created at sites of seafloor spreading—red sites on this map.
(NOAA)

From National Geographic

Seafloor spreading is a geologic process in which tectonic plates—large slabs of Earth's lithosphere—split apart from each other.

Seafloor spreading and other tectonic activity processes are the result of mantle convection.
Mantle convection is the slow, churning motion of Earth’s mantle.
Convection currents carry heat from the lower mantle and core to the lithosphere.
Convection currents also “recycle” lithospheric materials back to the mantle.

3D fly-thru of bathymetry from the Rodriguez Triple Junction in the Indian Ocean, 

with the flight going from east to west and diving down the Southwest indian Ridge (SWIR).

Seafloor spreading and rift valleys are common features at “triple junctions.”
Triple junctions are the intersection of three divergent plate boundaries.
The triple junction is the central point where three cracks (boundaries) split off at about 120° angles from each other.

Seafloor spreading occurs at divergent plate boundaries.
As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense.
The less-dense material rises, often forming a mountain or elevated area of the seafloor.

Eventually, the crust cracks.
Hot magma fueled by mantle convection bubbles up to fill these fractures and spills onto the crust. This bubbled-up magma is cooled by frigid seawater to form igneous rock.
This rock (basalt) becomes a new part of Earth’s crust.

Mid-Ocean Ridges

Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor.
The Mid-Atlantic Ridge, for instance, separates the North American plate from the Eurasian plate, and the South American plate from the African plate.
The East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate.
The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate.

Seafloor spreading is not consistent at all mid-ocean ridges.
Slowly spreading ridges are the sites of tall, narrow underwater cliffs and mountains.
Rapidly spreading ridges have a much more gentle slopes.

The Mid-Atlantic Ridge, for instance, is a slow spreading center.
It spreads 2-5 centimeters (.8-2 inches) every year and forms an ocean trench about the size of the Grand Canyon.
The East Pacific Rise, on the other hand, is a fast spreading center.
It spreads about 6-16 centimeters (3-6 inches) every year.
There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop!

The newest, thinnest crust on Earth is located near the center of mid-ocean ridge—the actual site of seafloor spreading.
The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge.

 

Geomagnetic Reversals

The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century.
Basalt, the once-molten rock that makes up most new oceanic crust, is a fairly magnetic substance, and scientists began using magnetometers to measure the magnetism of the ocean floor in the 1950s. What they discovered was that the magnetism of the ocean floor around mid-ocean ridges was divided into matching “stripes” on either side of the ridge.
The specific magnetism of basalt rock is determined by the Earth’s magnetic field when the magma is cooling.

Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge.
The continual process of seafloor spreading separated the stripes in an orderly pattern.

Geographic Features

Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading.
As it moves, it becomes cooler, more dense, and more thick.
Eventually, older oceanic crust encounters a tectonic boundary with continental crust.

In some cases, oceanic crust encounters an active plate margin.
An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other.
Active plate margins are often the site of earthquakes and volcanoes.
Oceanic crust created by seafloor spreading in the East Pacific Rise, for instance, may become part of the Ring of Fire, the horseshoe-shaped pattern of volcanoes and earthquake zones around the Pacific ocean basin.

In other cases, oceanic crust encounters a passive plate margin.
Passive margins are not plate boundaries, but areas where a single tectonic plate transitions from oceanic lithosphere to continental lithosphere.
Passive margins are not sites of faults or subduction zones.
Thick layers of sediment overlay the transitional crust of a passive margin.
The oceanic crust of the Mid-Atlantic Ridge, for instance, will either become part of the passive margin on the North American plate (on the east coast of North America) or the Eurasian plate (on the west coast of Europe).

New geographic features can be created through seafloor spreading.
The Red Sea, for example, was created as the African plate and the Arabian plate tore away from each other. Today, only the Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict, seafloor spreading will completely separate the two continents—and join the Red and Mediterranean Seas.

Mid-ocean ridges and seafloor spreading can also influence sea levels.
As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes more dense.
This increases the volume of the ocean basin and decreases the sea level.
For instance, a mid-ocean ridge system in Panthalassa—an ancient ocean that surrounded the supercontinent Pangaea—contributed to shallower oceans and higher sea levels in the Paleozoic era. Panthalassa was an early form of the Pacific Ocean, which today experiences less seafloor spreading and has a much less extensive mid-ocean ridge system.
This helps explain why sea levels have fallen dramatically over the past 80 million years.

Seafloor spreading disproves an early part of the theory of continental drift.
Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans.
Seafloor spreading proves that the ocean itself is a site of tectonic activity.

 Mid-ocean ridges extend for 70,000km around the globe.

They are the Earth's tectonic plate boundaries and creators of its crust.

Yet theyremain almost completely unexplored.

In this short film, Frank Pope, a marine biologist and ocean, gives a
glimpse of what we do know about these fiery sea monsters.

Keeping Earth in Shape

Seafloor spreading is just one part of plate tectonics.
Subduction is another.
Subduction happens where tectonic plates crash into each other instead of spreading apart.
At subduction zones, the edge of the denser plate subducts, or slides, beneath the less-dense one.
The denser lithospheric material then melts back into the Earth's mantle.

Seafloor spreading creates new crust.
Subduction destroys old crust.
The two forces roughly balance each other, so the shape and diameter of the Earth remain constant.

Links :

• Universal-Sci : What is the Mid-Atlantic Ridge
• The Times : The great explosive mountain range under the sea