La côte Est des États-Unis s'enfonce
In many parts of the U.S. East Coast, rising seas driven by melting ice and the thermal expansion of warming water is only part of what threatens coastal areas. The land is also sinking. This geologic two-step is happening rapidly enough to threaten infrastructure, farmland, and wetlands that tens of millions of people along the coast rely upon, according to a NASA-funded team of scientists at Virginia Tech’s Earth Observation and Innovation (EOI) Lab.
The researchers analyzed satellite data and ground-based GPS sensors to map the vertical and horizontal motion of coastal land from New England to Florida. In a study published in PNAS Nexus, the team reported that more than half of infrastructure in major cities such as New York, Baltimore, and Norfolk is built on land that sank, or subsided, by 1 to 2 millimeters per year between 2007 and 2020. Land in several counties in Delaware, Maryland, South Carolina, and Georgia sank at double or triple that rate. At least 867,000 properties and critical infrastructure including several highways, railways, airports, dams, and levees were all subsiding, the researchers found.
The findings follow a previous study from the EOI Lab, published in Nature Communications, that used the same data to show that most East Coast marshes and wetlands—critical for protecting many cities from storm surge during hurricanes—were sinking by rates exceeding 3 millimeters per year. They found that at least 8 percent of coastal forests had been displaced due to subsidence and saltwater intrusion, leading to a proliferation of “ghost forests.”
(...) Part of the reason that the Mid-Atlantic is sinking more rapidly than the northeastern U.S. is because the edge of the massive Laurentide ice sheet, which covered much of northern North America during the height of the most recent Ice Age, ran through northern Pennsylvania and New Jersey. Ice-free lands to the south of that line, especially in the Mid-Atlantic, bulged upward while ice-covered lands to north were pushed downward by the weight of the ice, Shirzaei explained. When the ice sheet started retreating 12,000 years ago, the Mid-Atlantic region began sinking gradually downward—and continues to do so today—while the northeastern U.S. and Canada began rising as part of a rebalancing process called glacial isostatic adjustment.
While the edge of the Laurentide ice sheet never got close to northern Florida, that region has relatively high rates of uplift due to another geologic process—the gradual dissolution and lightening of karst landscapes due to the infiltration of groundwater.
La gravité ‘'manquante'' du Canada expliquée
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Les hommes et les femmes qui habitent dans le nord du Canada, autour de la Baie d’Hudson, ressentent un peu moins que leurS congénères l’attraction terrestre. Concentrant l’attention des satellites jumeaux de la mission GRACE sur cette zone, des chercheurs présentent désormais un schéma et une explication détaillés de cette anomalie du champ gravitationnel de la Terre.
La gravité étant étroitement liée à la répartition des masses à la surface de la Terre, le champ gravitationnel de notre planète varie légèrement d’un endroit à un autre. Les premières cartographies établies dans les années 60 ont révélé qu’au nord du Canada la gravité était anormalement faible. La Laurentide, l’ancienne calotte glaciaire qui recouvrait le nord de l’Amérique il y a 20.000 ans, et qui mesurait jusqu’à trois kilomètres d‘épaisseur, a été rapidement rendue responsable.
Les travaux de l’équipe de Mark Tamisiea (Harvard-Smithsonian Center for Astrophysics, USA), publiés dans la revue Science du 10 mai, confirment le rôle du rebond post-glaciaire. Comprimée pendant l‘Age de glace, la croûte terrestre a commencé à remonter lorsque la glace a fondu. Cependant, ce rebond est très lent et même si la gravité augmente progressivement les chercheurs estiment qu’il faudra encore 300.000 ans avant que les effets de la période glaciaire disparaissent.
Les données des satellites GRACE révèlent que l’ancienne calotte Laurentide comptait deux énormes dômes de glaces, un de chaque côté de l’actuel baie d’Hudson, qui ont provoqué une élévation du niveau des eaux de 60 mètres lorsqu’ils ont fondu.
Cependant, d’après Tamisiea et ses collègues, le rebond post-glaciaire n’explique qu’une partie de l’anomalie du champ gravitationnel. Le reste serait dû aux mouvements de brassage au sein du manteau terrestre.
Se déplaçant en formation à 500 km au-dessus de nos têtes, les deux satellites de la mission GRACE fournissent une cartographie très précise du champ de gravitation de la Terre. Les variations de la gravité modifient la vitesse de déplacement des satellites. Or la distance qui les sépare est constamment mesurée par des instruments qui détectent les écarts au micron près. Les satellites ont été lancés en mars 2002 pour une mission de cinq ans.
(...) That’s right: Canada actually has less gravity than it’s supposed to. The reasons for the shortage have puzzled scientists for decades.
Gravity isn’t uniform all over the Earth’s surface. It’s a result of mass, which means the varying density of the Earth at different locations can affect how much you weigh there.
(...) Satellite data collected by GRACE—the Gravity Recovery and Climate Experiment—has recently solved this mystery. During the last ice age, Canada was covered by a vast glacier called the Laurentide Ice Sheet. This sheet was two miles thick over northern Quebec and stretched as far south as modern-day New York and Chicago.
Ice is heavy, so five million square miles of it pushed down on the rock underneath, squishing it like a Nerf ball. When the ice began to melt, about 21,000 years ago, the Earth began to spring back, but, like a Nerf ball, it takes a while. To this day, the Earth in the Hudson Bay region is still deformed, with lots of rock-mass having been pushed outward by all the ice. Less mass means less gravity.
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For more than 40 years, scientists have tried to figure out what's causing large parts of Canada, particularly the Hudson Bay region, to be "missing" gravity. In other words, gravity in the Hudson Bay area and surrounding regions is lower than it is in other parts of the world, a phenomenon first identified in the 1960s when the Earth's global gravity fields were being charted.
Two theories have been proposed to account for this anomaly. But before we go over them, it's important to first consider what creates gravity. At a basic level, gravity is proportional to mass. So when the mass of an area is somehow made smaller, gravity is made smaller. Gravity can vary on different parts of the Earth. Although we usually think of it as a ball, the Earth actually bulges at the Equator and gets flatter at the poles due to its rotation. The Earth's mass is not spread out proportionally, and it can shift position over time. So scientists proposed two theories to explain how the mass of the Hudson Bay area had decreased and contributed to the area's lower gravity.
One theory centers on a process known as convection occurring in the Earth's mantle. The mantle is a layer of molten rock called magma and exists between 60 and 124 miles (100 to 200 km) below the surface of the Earth . Magma is extremely hot and constantly whirling and shifting, rising and falling, to create convection currents. Convection drags the Earth's continental plates down, which decreases the mass in that area and decreases the gravity.
A new theory to account for the Hudson Bay area's missing gravity concerns the Laurentide Ice Sheet, which covered much of present-day Canada and the northern United States. This ice sheet was almost 2 miles (3.2 km) thick in most sections, and in two areas of Hudson Bay, it was 2.3 miles (3.7 km) thick. It was also very heavy and weighed down the Earth. Over a period of 10,000 years, the Laurentide Ice Sheet melted, finally disappearing 10,000 years ago. It left a deep indentation in the Earth.
To get a better idea of what happened, think about what happens when you lightly press your finger into the surface of a cake or a piece of really springy bread. Some of it moves to the sides and there's an indentation. But when you remove your finger, it bounces back to normal. A similar thing happened with the Laurentide Ice Sheet, the theory proposes -- except the Earth isn't so much "bouncing" back as it is rebounding very slowly (less than half an inch per year). In the meantime, the area around Hudson Bay has less mass because some of the Earth has been pushed to the sides by the ice sheet. Less mass means less gravity.
So which theory is correct? It turns out that both of them are. Convection and the ice sheet's rebound effect are both causing some of the decrease in gravity around Hudson Bay. First, we'll consider the ice sheet theory.
To calculate the impact of the Laurentide Ice Sheet, scientists at the Harvard-Smithsonian Center for Astrophysics used data gathered by the Gravity Recovery and Climate Experiment (GRACE) satellites between April 2002 and April 2006. The GRACE satellites are highly sophisticated machines, orbiting about 310 miles (500 km) above the Earth and 137 miles (220 km) apart. The satellites can measure distances down to a micron, so they can detect minor gravitational variations. When the lead satellite flies over the Hudson Bay area, the decrease in gravity causes the satellite to move slightly away from the Earth and from its sister satellite. This shift in distance is detected by the satellites and used to calculate the change in gravity. Any shifts detected can also be used to create maps of gravitational fields.
The GRACE data allowed scientists to create topographical maps approximating what Hudson Bay looked like during the last ice age, when it was covered by the Laurentide Ice Sheet. These maps revealed some interesting features about the area, including two bulging areas on the western and eastern sides of Hudson Bay where the ice was much thicker than the rest of the sheet. Gravity is now lower there than in other parts of the gravity-depleted bay.
Another important finding came from the GRACE data: It turns out that the ice sheet theory only accounts for 25 percent to 45 percent of the gravitational variation around Hudson Bay and the surrounding area. Subtracting the "rebound effect" from the area's gravitational signal, scientists have determined that the remaining 55 percent to 75 percent of gravitational variation is likely due to convection.
The Hudson Bay area is going to have less gravity for a long time. It's estimated that the Earth has to rebound more than 650 feet to get back to its original position, which should take about 5,000 years. But the rebound effect is still visible. Although sea levels are rising around the world, the sea level along Hudson Bay's coast is dropping as the land continues to recover from the weight of the Laurentide Ice Sheet.
While the mystery surrounding Canada's gravitational anomalies has been put to rest, the study has wider implications. Scientists involved in the Harvard-Smithsonian Center study were amazed that they were able to see how the Earth looked 20,000 years ago. And by isolating the influence of the ice sheet's rebound effect, researchers better understand how convection affects gravity and how continents change over time. Finally, the GRACE satellites have provided scientists with data on many ice sheets and glaciers. By examining climate change that took place thousands of years ago, scientists may gain a better understanding of how global warming and rising sea levels are affecting our planet today and what impact they will have on our future.
L'étude de Tamisiea, Mitrovica et Davis:
The free-air gravity trend over Canada, derived from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, robustly isolates the gravity signal associated with glacial isostatic adjustment (GIA) from the longer–time scale mantle convection process. This trend proves that the ancient Laurentian ice complex was composed of two large domes to the west and east of Hudson Bay, in accord with one of two classes of earlier reconstructions. Moreover, GIA models that reconcile the peak rates contribute ∼25 to ∼45% to the observed static gravity field, which represents an important boundary condition on the buoyancy of the continental tectosphere.
Possible découverte d’un astéroïde minganois sur Google Maps
Une équipe de scientifiques français étudie un possible cratère d’astéroïde sur la Côte-Nord, jamais identifié auparavant. À quelque 100 kilomètres au nord du village de Magpie, les premiers indices de son existence ont été découverts par un curieux internaute en quête d’aventure.
Joël Lapointe promenait candidement son curseur dans Google Maps sur des chemins forestiers afin de planifier ses vacances en camping lorsqu’un détail lui a accroché l'œil.
"Je vois quelque chose de rond et d’un peu effacé, comme une espèce de fosse d’une quinzaine de kilomètres de diamètre", décrit-il. Quelque chose dans la courbe lui semble suspect. À l’intérieur de cette fosse-là, un autre anneau de petites montagnes d’à peu près huit kilomètres de diamètre, et puis au centre : le lac Marsal.
"Ça, c’est bien trop concentrique pour que ça soit naturel!", s'est-il dit.
M. Lapointe s’est donc tourné vers des chercheurs professionnels. Il a envoyé des courriels à des centres d’ici et d’ailleurs pour enfin attirer l'attention de la Meteorotical Society. C’est à travers cette société vouée à l’étude des matériaux extraterrestres fondée en 1933 que le dossier du lac Marsal tombe sur le bureau du géophysicien Pierre Rochette.
Quelques mois plus tard, M. Rochette reçoit des échantillons envoyés spécialement par le Service géologique du Canada : "Les roches que j’ai récupérées sont des roches qui ont été fondues", commente-t-il. Son équipe et lui sont à la recherche de preuves précises, qui prouveraient qu’un météorite est entré en collision avec la Terre.
"Cela pourrait être des roches volcaniques, mais notre hypothèse, c'est qu'elles ont été fondues par l'impact. Alors, pour vraiment apporter la preuve que ça a été fondu pour un impact, il faut trouver les preuves de très haute pression", indique le chercheur.
Les analyses sont encourageantes, assure M. Rochette, mais elles ne sont pas encore conclusives. "On a reçu dix échantillons et l’un d’entre eux contient un minéral qu'on appelle le zircon, qui est très souvent utilisé pour prouver les impacts", explique-t-il. "Le zircon, c’est un minéral très résistant qui, sous l’effet d’un impact, se transforme."
La découverte d'un impact d'une telle puissance serait majeure, affirme Pierre Rochette. "Pour creuser un cratère de 15 kilomètres de diamètre, en gros, il faut un astéroïde de deux kilomètres de diamètre." Le dernier astéroïde découvert de cette dimension l’a été en 2013, selon le scientifique. "Ça fait un bail."
Si les analyses d’échantillons confirment qu'un astéroïde est bel et bien tombé en Minganie, Pierre Rochette et son équipe prévoient un voyage dans la région pour en tirer de plus amples leçons. "L'idée, c'est d'aller en hydravion sur le lac, qui est vraiment au centre du cratère", prévoit-il. "Et il se trouve qu’en suivant différents plans d'eau, on peut naviguer sur une dizaine de kilomètres à l'intérieur du cratère."
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