Semi-circle of large lakes on Canada map (APOD 19 Sep 2004)

[Chris]

I noticed, (on the map of Canada on which you showed the magnetic North Pole), that beginning with the great lakes and going in a semi-circle North-West are a number of very large lakes. I wondered if this marks an intersection of two plates which are slowly pulling apart?

In other words, at some (very remote) time, will this continent divide in two?

Chris

[Sputnick]

Living in Canada I've often wondered if Hudson Bay was a huge impact crater.

[neufer]

Living in Canada I've often wondered if Hudson Bay was a huge impact crater.

GRAVITY ANOMALY SOLVED - Flora Lichtman
NEWS BRIEF - POSTED MONDAY, MAY 14, 2007
http://www.sciencefriday.com/news/05140 ... 14071.html
-------------------------------------


grav map Gravity map.The University of Texas Center for Space Research
-------------------------------------
<<About 1000 miles north of the Great Lakes, near Hudson Bay, Canada, the pull of gravity is a little weaker. A new study in the journal Science explains why.

Differences in gravity are not unusual on Earth: “If the Earth was a perfect sphere, the gravitational field would be the same everywhere,” says Mark Tamisiea, a geodetic geophysicist at the Proudman Oceanographic Laboratory in the United Kingdom and lead author on the study.

But our planet’s bumpy surface, the water flow on the planet, and the movement of plates below Earth's crust can all change the pull of gravity. “Gravity changes on many different time scales, from a matter of weeks to centuries,” says Tamisiea, adding that the variation in gravity can explain “quite a bit about what’s going on in the Earth.”

Differences in gravity can be mapped by tracking the movement of two orbiting satellites, Tamisiea says. As the Earth’s gravitational pull increases and decreases, the position of the satellites also changes.

For example, if two satellites were flying above a large mountain, Tamisiea explains, the first satellite would be pulled closer to the mountain. The mountain has more mass, so there is a stronger gravitational pull. As the first satellite is pulled toward the mountain, it moves farther away from the second satellite—the distance between the satellites increases. As the second satellite approaches the mountain it is pulled in and the distance between the satellites decreases again. The changes in distance between the satellites can be translated into a map of the Earth's gravitational field.

Using data like this, Tamisiea and his team mapped gravity, and how gravitational pull is changing over time, which allowed them to understand what is causing the Hudson Bay gravity anomaly.

The researchers created models that predicted 25 – 45 percent of the Hudson Bay gravitational signal was due to the last Ice Age—20,000 years ago, when a 2-mile-thick Laurentide ice sheet covered Canada, the land sank under the ice. “Although that ice sheet has long disappeared that ice sheet is still recovering,” Tamisiea says. The depression means less mass, which means less pull.

The scientists also discovered that the changes in gravity were larger in two spots, suggesting the Laurentide ice sheet was made up of two domes of ice—not one.

The primary driver of the lower gravity? The data suggests that plate tectonics and mantle convention is responsible. Mantle convection can cause plates inside the Earth to move vertically—in this case, Tamisiea says that convection probably dragged the crust down, which decreased the mass under Hudson Bay, decreasing the gravitational pull.>>
---------------------------------------
How can parts of Canada be "missing" gravity? by Jacob Silverman
http://science.howstuffworks.com/missing-gravity.htm
.
<<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.>>
------------------------------------------------
http://earthobservatory.nasa.gov/Newsro ... g_id=13768
http://earthobservatory.nasa.gov/Newsro ... g_id=17919
http://earthobservatory.nasa.gov/Newsro ... g_id=17972
http://earthobservatory.nasa.gov/Newsro ... g_id=17954

http://earthobservatory.nasa.gov/Newsro ... g_id=11268
http://earthobservatory.nasa.gov/Newsro ... g_id=17960
http://earthobservatory.nasa.gov/Newsro ... g_id=17994
http://earthobservatory.nasa.gov/Newsro ... g_id=17988
http://earthobservatory.nasa.gov/Newsro ... g_id=17979
http://earthobservatory.nasa.gov/Newsro ... g_id=17208
http://earthobservatory.nasa.gov/Newsro ... g_id=17965
http://earthobservatory.nasa.gov/Newsro ... g_id=15317

[Sputnick]

Art you certainly have a talent for research - with less gravity at Hudson's Bay all teenage girls have to do to lose weight is to move up there.

But - with Hudson Bay's nearly circular shape, I still wonder if a huge impact occured there long, long, long ago.

[Qev]

Art you certainly have a talent for research - with less gravity at Hudson's Bay all teenage girls have to do to lose weight is to move up there.

But - with Hudson Bay's nearly circular shape, I still wonder if a huge impact occured there long, long, long ago.

According to the Geological Survey of Canada, it's almost certainly not an impact crater.

http://www.unb.ca/passc/ImpactDatabase/faq.html#2

[Sputnick]

Qev,
You say the Geological Survey says 'almost certainly not' an impact. I thought geology in this age would be more exact. Okay - I've read your reference now and the Survey sounds certain that it is not an impact. Nice round shape though.

[BMAONE23]

Qev,
You say the Geological Survey says 'almost certainly not' an impact. I thought geology in this age would be more exact. Okay - I've read your reference now and the Survey sounds certain that it is not an impact. Nice round shape though.

I thought the same thing about Hudson Bay myself. In my opinion though the Gulf of Saint Lawrence is another good candidate wth Prince Edward Island being a raised shockwave form.

[Sputnick]

I guess we shouldn't get too excited about circular landforms as the semi-circle of the coast of northwestern Africa shows that the simple breaking apart of land masses can create large curves instead of long, straight cracks .. and then we have the circle of Antarctica.

Not being a geologist, but having lived there, my first impression of Prince Edward Island is a sedimentary island built up from rock ground into sand by the glaciers of the ice age and washed down that St. Lawrence channel.