Glacial Erratics

[otseng]

This thread is an offshoot of Drumlins, Ribbed Moraines, and Giant Ripples. Though these are all related, I'd like to narrow down on erratics in this thread.

A glacial erratic is a piece of rock carried by glacial ice some distance from the rock outcrop from which it came. Erratics can range in size from pebbles to massive pieces such as the Okotoks (16,500 tons) and Airdrie erratics found in Alberta, Canada. They can be found miles away from their original location.

http://en.wikipedia.org/wiki/Glacial_erratic

For debate:
How did these rocks get there?

[Goat]

This thread is an offshoot of Drumlins, Ribbed Moraines, and Giant Ripples. Though these are all related, I'd like to narrow down on erratics in this thread.

A glacial erratic is a piece of rock carried by glacial ice some distance from the rock outcrop from which it came. Erratics can range in size from pebbles to massive pieces such as the Okotoks (16,500 tons) and Airdrie erratics found in Alberta, Canada. They can be found miles away from their original location.

http://en.wikipedia.org/wiki/Glacial_erratic

For debate:
How did these rocks get there?

To answer that question, we can use an example of the Mendenhall glaciers, which is currently in reteat (as are all of the worlds glaciers right now).

From The FAQ of the vistor center web site

What evidence do glaciers leave behind?
The base of Mendenhall Glacier works like a giant piece of sandpaper. As the ice flows towards Mendenhall Lake, the glacier plucks rocks that become imbedded in the ice from the valley floor. The glacier scrapes these rocks across the bedrock creating grooves and striations. The glacier's erosive power changes the landscape and scrapes much of the soil and rock from the valley walls. Rocks scoured from the surrounding valley walls create dark debris lines called moraines on the edges and down the center of the glacier. As the glacier continues its path towards Mendenhall Lake, it grinds rock to a fine powder call rock flour that escapes with glacial melt water and creates the lake's murky color. Mendenhall Glacier's retreat exposes its trimline, slightly sloping changes in vegetation on the valley walls that indicate the glacier's height at its point of maximum advance. As the glacial ice melts or calves icebergs, the glacier drops geologically misfit rocks called erratics that its ice either quarried further up the valley or that fell onto the ice from rock walls above the glacier. These granite boulders can be seen lying on the metamorphic rock around the visitor center.

We can see this process happening in current glaciers.

[otseng]

As the glacial ice melts or calves icebergs, the glacier drops geologically misfit rocks called erratics that its ice either quarried further up the valley or that fell onto the ice from rock walls above the glacier. These granite boulders can be seen lying on the metamorphic rock around the visitor center.

So, a rock fell off from someplace and landed on a glacier. The glacier advanced for several miles. Then as the glacier melted, it placed the erratic on the ground.

Before I start making comments on this, are there any other methods for glaciers to move erratics?

[Jose]

I imagine that glaciers would move large rocks simply by breaking them off and carrying them along, then melting away and leaving them behind.

Now, we might ask if other methods could move large rocks. Could a flood do so? Sure, but we would expect to see the characteristics of floods in that case--the rocks would tend to be at the bottoms of gullies/canyons/runs/barrancas or whatever you'd like to call them; water tends to flow downhill. We'd expect signs of turbulence, which would be necessary to keep the boulders from settling out (i.e. to move them). Once the boulders do settle out, we'd expect smaller sediments to pile up on and around them--unless they are in watercourses where these sediments can be washed away periodically. We would also expect them in places where rapid floods are likely, whether or not they are in regions that show other characteristics of glacial activity.


For instance, there are lots of alluvial fans in the Southern California deserts, such as this one in Death Valley. They are from flash floods carrying debris out of the mountains. I've hiked over lots of these things, but can't recall any boulders that are the sizes of good glacial erratics. Water seems to move smaller things shorter distances, while glaciers can move really large things really large distances.

Just because they're so pretty, here's another in aerial view (Ivanpah, NV)

[otseng]

I imagine that glaciers would move large rocks simply by breaking them off and carrying them along, then melting away and leaving them behind.

So, is the mechanism then that the rocks would have to be "carried" rather than "pushed"?

If this is so, how did the glacier break off the rock originally? If it fell on the glacier, then the glacier could not have had contact with the rock to break it off.

Suppose that it was a rock slide that occurred. A rock slide generally has more than a single boulder that falls off. So, wouldn't there then be several pieces of erratics close together?

Suppose the erratics were placed there by glaciers pushing them instead. And as the glacier advanced, it broke off some rocks and kept pushing them until the glacier retreated.

But, if this is true, then all the erratics would be in a line where the glacier stopped advancing. But, erratics are usually randomly spread out.

Now, we might ask if other methods could move large rocks. Could a flood do so? Sure, but we would expect to see the characteristics of floods in that case--the rocks would tend to be at the bottoms of gullies/canyons/runs/barrancas or whatever you'd like to call them; water tends to flow downhill.

We can see that in the Channeled Scablands, it was a flood that placed the erratics there. So, I think a flood is certainly a possibility.

[Jose]

I imagine that glaciers would move large rocks simply by breaking them off and carrying them along, then melting away and leaving them behind.

So, is the mechanism then that the rocks would have to be "carried" rather than "pushed"?

I think "carried" is the more appropriate term. At some point, I imagine, the glaciers reach a sort of equilibrium for the climatic conditions. Snow is added at one end, and chunks break off at the warm end. I use "cold end" loosely, since we're really talking about snow piling up during the winter over the entire glacier, but not melting entirely during the summer. Eventually it gets thick enough to start pushing outward (or downward if there are mountains). At the steady state, most of the rocks are carried along and left at the melting end--to form the terminal moraines. Frankly, I don't know the size range of stuff in the moraines. Maybe some of it is as large as the erratics. It also includes lots of stuff that got ground up.

If this is so, how did the glacier break off the rock originally? If it fell on the glacier, then the glacier could not have had contact with the rock to break it off.

From the reading I've done, it seems that freezing and thawing would be a good way to do it. As ice wedges form in cracks, and the ice expands, it will eventually break off chunks of rock. This happens today, of course, so it's a verifiable process. I think we're more likely to see this happening with mountain glaciers, where there are peaks sticking out above the glacier, than in the great lakes area. It would be interesting to see a map of glacial erratics, coded both for location and size.

Also, as glaciers move over hills, they tend to smooth the front side that they are pushing up over, and they tend to break off large chunks of the down-ward side.

I don't know if it's reasonable to imagine them picking up boulders from ground they're moving over. Maybe they can, if there are freeze/thaw cracks under the glacier. This might be a reasonable idea for warm periods, when meltwater would flow through cracks in the glacier and then flow underneath it. The channels where such water flows tends to leave debris piles, which we call eskers--but the few of these that I've seen have all been small pieces of rock and earth.

Suppose that it was a rock slide that occurred. A rock slide generally has more than a single boulder that falls off. So, wouldn't there then be several pieces of erratics close together?

Makes sense to me. However, most of these would end up being carried to the terminal moraine (or lateral moraines). What with the slow progress of the ice, and the fact that it snows every winter, we'd expect that most rocks would be covered and eventually become contained entirely within the glacier. There are probably pretty strong, if slow, forces there that could move things around, and break up larger rocks into smaller stuff. Again, though, most of it would end up in the moraines.

I think that erratics would pretty much have to be rocks in/on the glacier that are set down when the glacier finally melts. It seems reasonable that there would be several chunks near each other, from rockfalls, but I can't say whether this is seen often or not.

Suppose the erratics were placed there by glaciers pushing them instead. And as the glacier advanced, it broke off some rocks and kept pushing them until the glacier retreated.

But, if this is true, then all the erratics would be in a line where the glacier stopped advancing. But, erratics are usually randomly spread out.

I agree. As a glacier advances, it seems that it has to push stuff ahead of it. But I bet that's a small fraction of the total material that is eventually scraped up as the glacier moves over the same ground for thousands of years. Again, the stuff would end up in a line where the glacier ended-and would be called a moraine rather than an erratic.

Now, we might ask if other methods could move large rocks. Could a flood do so? Sure, but we would expect to see the characteristics of floods in that case--the rocks would tend to be at the bottoms of gullies/canyons/runs/barrancas or whatever you'd like to call them; water tends to flow downhill.

We can see that in the Channeled Scablands, it was a flood that placed the erratics there. So, I think a flood is certainly a possibility.

OK--there's a good example of flood-moved boulders. There are probably others as well. This raises the interesting question of what criteria do geologists use to determine whether an out-of-place rock was moved by flood, by glacier, or by United Van Lines? I think we have to agree that the presence of an erratic boulder is, by itself, not enough to tell us how it got there.

With glacial erratics, the additional evidence is of the sort that has been raised earlier: moraines "downstream" (downhill or toward the equator), drumlins, kettle holes, scrape marks, etc. In mountains, we'd add the shape of the valley(s)--V-shaped for water-carved valleys, U-shaped for glacial valleys. For flood erratics, I think we'd have to require an uphill source of water that could move vigorously enough to carry such large boulders. We ought to see flood erratics pretty much in or at the mouths of mountain canyons. But...I don't have data to tell us one way or the other.

[otseng]

OK, now that the holidays are over, I hope I can get back into actively debating again.

At some point, I imagine, the glaciers reach a sort of equilibrium for the climatic conditions. Snow is added at one end, and chunks break off at the warm end. I use "cold end" loosely, since we're really talking about snow piling up during the winter over the entire glacier, but not melting entirely during the summer. Eventually it gets thick enough to start pushing outward (or downward if there are mountains). At the steady state, most of the rocks are carried along and left at the melting end--to form the terminal moraines. Frankly, I don't know the size range of stuff in the moraines.

This brings up a question I've always wondered. Glaciers can only move downhill. What causes them to move is gravity making the ice go down.

Yet, supposedly during the ice age, glaciers were able to move from northern Canada to the northern US. Yet, the terrain is not downhill for that entire area. As a matter of fact, it is mostly uphill. How were glaciers able to move on that large of a scale?


http://atlas.nrcan.gc.ca/site/english/m ... image_view

I think "carried" is the more appropriate term.

There is one characteristic of erratics that challenges the mechanism of glaciers carrying the rocks. Most erratics are rounded and smoothed. This indicates to me that they have been tumbled during its journey.

Most glacial erratics appear worn and rounded, and sometimes include beveled or faceted surfaces. During the course of their journey, the rocks were jostled against other erratics or scraped against the underlying bedrock, rounding off corners and planing smooth surfaces, eventually producing their characteristic appearance. Glacial transport also caused some boulders to fracture, producing fresh angular edges. Rocks carried by rivers also undergo abrasion and become rounded in the process. In fact, most of the igneous and metamorphic rocks in Iowa's river valleys were originally transported into the general area by glaciers, then eroded from the glacial deposits and moved some additional distance by a river.

http://www.igsb.uiowa.edu/Browse/boulders/boulders.htm

Most glacial erratics are rounded and worn, but some of them have beveled or faceted surfaces.

http://www.state.nd.us/ndgs/Erratics/Gl ... ratics.htm

These loose, rounded rocks were moved here by glacial ice between 24,000 and 16,000 years ago. Many of these rounded rocks, called glacial erratics, are also Monson Gneiss, as the ice usually didn't move rocks very far.

http://dep.state.ct.us/stateparks/geolo ... eldgeo.htm

Glacial erratics are rounded boulders that were left by a melting glacier and are often seen perched precariously on exposed rock faces after glacial retreat.

http://en.wikipedia.org/wiki/Glacier


http://www.turnstone.ca/gabbro.htm


http://www.fettes.com/Orkney/Westray%20glaciation.htm

So, because most erratics are rounded and smoothed, water action would be the best explanation to account for this. Even if a glacier had a part in carrying it, water also must have a part in it if it is rounded.

I think we have to agree that the presence of an erratic boulder is, by itself, not enough to tell us how it got there.

And since most erratics are rounded, this would support that most erratics are moved by flood action.

It would be interesting to see a map of glacial erratics, coded both for location and size.

It would be. Perhaps some ambitious geology grad student out there would like to pursue this?

[Goat]

OK, now that the holidays are over, I hope I can get back into actively debating again.

At some point, I imagine, the glaciers reach a sort of equilibrium for the climatic conditions. Snow is added at one end, and chunks break off at the warm end. I use "cold end" loosely, since we're really talking about snow piling up during the winter over the entire glacier, but not melting entirely during the summer. Eventually it gets thick enough to start pushing outward (or downward if there are mountains). At the steady state, most of the rocks are carried along and left at the melting end--to form the terminal moraines. Frankly, I don't know the size range of stuff in the moraines.

This brings up a question I've always wondered. Glaciers can only move downhill. What causes them to move is gravity making the ice go down.

Yet, supposedly during the ice age, glaciers were able to move from northern Canada to the northern US. Yet, the terrain is not downhill for that entire area. As a matter of fact, it is mostly uphill. How were glaciers able to move on that large of a scale?


http://atlas.nrcan.gc.ca/site/english/m ... image_view

I think "carried" is the more appropriate term.

There is one characteristic of erratics that challenges the mechanism of glaciers carrying the rocks. Most erratics are rounded and smoothed. This indicates to me that they have been tumbled during its journey.

Most glacial erratics appear worn and rounded, and sometimes include beveled or faceted surfaces. During the course of their journey, the rocks were jostled against other erratics or scraped against the underlying bedrock, rounding off corners and planing smooth surfaces, eventually producing their characteristic appearance. Glacial transport also caused some boulders to fracture, producing fresh angular edges. Rocks carried by rivers also undergo abrasion and become rounded in the process. In fact, most of the igneous and metamorphic rocks in Iowa's river valleys were originally transported into the general area by glaciers, then eroded from the glacial deposits and moved some additional distance by a river.

http://www.igsb.uiowa.edu/Browse/boulders/boulders.htm

Most glacial erratics are rounded and worn, but some of them have beveled or faceted surfaces.

http://www.state.nd.us/ndgs/Erratics/Gl ... ratics.htm

These loose, rounded rocks were moved here by glacial ice between 24,000 and 16,000 years ago. Many of these rounded rocks, called glacial erratics, are also Monson Gneiss, as the ice usually didn't move rocks very far.

http://dep.state.ct.us/stateparks/geolo ... eldgeo.htm

Glacial erratics are rounded boulders that were left by a melting glacier and are often seen perched precariously on exposed rock faces after glacial retreat.

http://en.wikipedia.org/wiki/Glacier


http://www.turnstone.ca/gabbro.htm


http://www.fettes.com/Orkney/Westray%20glaciation.htm

So, because most erratics are rounded and smoothed, water action would be the best explanation to account for this. Even if a glacier had a part in carrying it, water also must have a part in it if it is rounded.

I think we have to agree that the presence of an erratic boulder is, by itself, not enough to tell us how it got there.

And since most erratics are rounded, this would support that most erratics are moved by flood action.

It would be interesting to see a map of glacial erratics, coded both for location and size.

It would be. Perhaps some ambitious geology grad student out there would like to pursue this?

Then woudl TEND to go down hill. But, if there is more pressure pushing them forward from higher elevations, I would not be surprised if there are some local
areas that they would go uphill for a bit. And some erratics have sharper edges than others. Also, anytime you have grinding followed by turning, (as a rock that is being pushed along the bottom of a glacier), you will have a rounding effect.

[WelshBoy]

So, because most erratics are rounded and smoothed, water action would be the best explanation to account for this. Even if a glacier had a part in carrying it, water also must have a part in it if it is rounded.

Rain?

[otseng]

Then woudl TEND to go down hill. But, if there is more pressure pushing them forward from higher elevations, I would not be surprised if there are some local areas that they would go uphill for a bit. And some erratics have sharper edges than others. Also, anytime you have grinding followed by turning, (as a rock that is being pushed along the bottom of a glacier), you will have a rounding effect.

If you look at the map of Canada, this could not happen. If the glacier moved from north to south, it would be predominantly uphill. There would not exist any pressure from higher elevations. Also, it would not just be a local uphill movement, but over large distances.

So, because most erratics are rounded and smoothed, water action would be the best explanation to account for this. Even if a glacier had a part in carrying it, water also must have a part in it if it is rounded.

Rain?

Yes ... a very large amount of rain.