This is Geology Bites with Oliver Strimpel.
About 80% of Greenland is covered by ice.
While we've been able to map the geology of the 20% that is
exposed, our existing geological maps of the glaciated interior
have relied on extrapolation from the exposed regions and
educated guesswork. But in April 2024, Joe McGregor,
a research physical scientist at the NASA Goddard Space Flight
Centre, and his colleagues published a new geological map
of Greenland. The map incorporates new data
about the geology of the rocks below the ice, derived from
seismic, gravity, magnetic and topographic surveys.
The map features 3 hitherto unknown provinces that lie
beneath the central part of the ice cap north of about 70°, as
well as an extensive network of long subglacial valleys.
How does the new data enable us to learn about the rocks buried
by up to three kilometers of ice?
And what is the significance of the three new provinces and the
network of valleys? Joe McGregor, welcome to Geology
Bytes. Thank you for having me.
Oliver. What was the best map we had of
Greenland's geology before yours appeared?
And how was that one compiled? The best one that.
Existed before ours was. Compiled by Peter.
Dawes, a British geologist. Working for the.
Geological Survey. Of Denmark and Greenland, and he
published it in 2009. It represented.
The best? Understanding that we had at the
time of how the various provinces that had been mapped
around the periphery of Greenland might connect within
the interior. He described.
It as conjectural and he. Left it at that.
And so we have attempted to revise that map across the 80%
of the island of Greenland. That is covered by ice, for
which we have. Almost no direct geological
information. I said in the introduction that
your map uses data from seismic, gravity, magnetic, and
topographic surveys. Let's talk about how we captured
this data and what kind of information each data type
provides. Taking seismic data first, did
you use natural sources, IE earthquakes or artificial
sources of seismic waves? And where were the seismometers
placed? We used natural sources of
seismic signals, so those include earthquakes and other
seismic. Sources for example.
Calving of icebergs around the periphery of Greenland and those
seismometers. Are mostly located.
On land around the periphery of Greenland, there are a handful
that have been placed on the. Ice typically located.
Next to major stations, but we're dealing with a very small
number of seismometers compared to.
The set that is. Distributed across the United
States and the observations that they make of these earthquakes
and other seismic sources. Allow us.
To generate tomography. Of the subsurface.
In Greenland and around the Arctic, roughly how many
seismometers are we talking about here to cover the whole of
Greenland? So currently it's on the order.
Of two dozen, it's relatively few and it gets sparse.
As you go farther north in Greenland, understandable given
the population centers and so that leads.
To a degree of. Bias in how we're able to
interpret what the seismic structure is, particularly in
what we think is the more complicated northern region
because you rely on the ray paths between.
Wherever those sources were. And those stations to infer what
the velocity might have been between them.
So the more stations you have, the more ray paths you're going
to have and the wider variety thereof to infer that seismic
structure. So that really goes to the
resolution of the tomographic images you can generate, I
suppose. Yes, so the.
Deeper you want to sense. Typically the courser the.
Resolution will be we're. Interested in the near surface
geology, so the. Tomographic velocity anomaly
pattern in. The upper.
Five to 10 kilometers. Of crust that's relatively hard
to resolve. Given the nature of the sources.
That exist in the rate paths that they travel, but yeah.
That's the challenge we're faced with.
So we're getting an. Estimate that's of the.
Order. Of 50 kilometers square across
an island, that's roughly in rectangular terms 2000
kilometers NS and 1000 kilometers.
East West to be absolutely clear, although.
Some of my co-authors have generated these seismic
tomography maps. We didn't generate a new one for
this. Study we took what?
We thought was one of the best existing ones.
When you try and interpret one of these tomographic images, do
you interpret the seismic velocity as reflective of
variations in temperature of the crust or mantle, or is it a
combination of temperature and composition?
So it's really a combination of the two.
The density, the temperature, and the lithology can all factor
in to what generates an anomaly relative to some reference.
Field that we. Attempt to interpret in this
context and before we start talking about the other data
types, were there any areas just looking at the seismic that
appeared to be significantly different or hotter than other
parts of Greenland? There are.
Places in Greenland. Particularly as you head towards
the East Coast, the Blossville Coast.
Where there are known basalt outcrops where it does in fact.
Appear. That the seismic velocities are
lower and that would be consistent with warmer rock,
potentially a shallower mantle depending on how you.
View that structure. OK, let's talk about the gravity
surveys. How were those captured and what
does a map of the gravity variations or gravity anomalies
tell us about the bedrock? So those were captured in two
ways. The first is.
From satellite derived gravity measurements and here we use the
Gravity and Ocean Climate Experiment mission that was run
by. ESA.
And that allows us. To resolve large scale.
Gravity anomalies. That's also been combined.
With Aero. Gravity measurements and that is
one of the things that I've. Specialized in which is.
Having Gravimeter's onboard aircraft and we travel across
Greenland mapping the gravity anomaly as we.
Cross the ice. That's a challenging measurement
to make, because gravity and its variations are fundamentally
about measuring the vertical acceleration and the minute
variations. Associated with.
Changing rock composition beneath, and when you're doing
that measurement from the air, you've got the added
complication of the turbulence. That the aircraft is.
Experiencing and so that can knock you all.
Sorts of different ways. And you have to.
Record how? The aircraft is moving.
In space, Very. Precisely with GNSS antennas so
that they can then remove that noise.
To get at the. Gravity signal beneath.
Where you see higher gravity, that's indicative.
Of denser rock or depending on the scale that you're looking at
potentially. A deeper.
Crust, mantle boundary and. That allows you to resolve the
large scale structure of the rocks.
That you're flying over. But unlike the seismic
tomographic images, presumably if you're actually flying pretty
low in an aircraft, you can get better spatial resolution.
Yes, that depends on a number of.
Factors how fast your. Platform is traveling, how high
it's traveling and then of. Course the quality of your
gravimeter. And in the ideal scenario, if
you want to cover a lot of ground, you've got a low and
slow aircraft. And in some of these
measurements we have. Had that, so the field that we
worked with was. The combination of these
satellite gravity measurements and some of the aero gravity
measurements that have been made by not just NASA over.
The past couple decades. In Greenland, but also various
Danish institutions who of course interested in Greenland
and in some cases German institutions as well.
So just looking at the gravity data alone for a moment, where
there any regions that stuck out in the sense of having
anomalously high gravity or low gravity?
Yes, in the vicinity. Of the.
Caledonian origin. We do.
See. Often.
A lower gravity so. That's in northeast Greenland
and that signal seems. To propagate a little.
Inland toward the center. Of the island.
Conversely, in the northwest of Greenland and in the far north
of Greenland. We see positive gravity.
Anomalies those. Appear to be associated.
With different type of. Origin perhaps a different.
Rock composition and in one case, apparently a basin.
Or a past basin. And how are the magnetic surveys
performed? The magnetic map.
That we used. Was generated in a similar way
to the gravity map, but of course with different sensors.
There are magnetometers on board satellites that can give you
large scale magnetic anomalies. We're talking order 100
kilometers and then those have been combined with aeromagnetic
measurements. Those were also made by NASA in
the. Past 2 decades.
And other institutions. In the case of magnetics, it's
not so much a concern how fast you're flying, but it is.
Helpful to get. Low if you're trying to sense
signals. Closer to the ice.
Rock interface. In this case, we used a data set
that was generated by the US NOAA, the National Oceanic and
Atmospheric Administration. In 2017.
We also flew an aircraft with NASA.
That was particularly well suited to.
The challenge of making airborne magnetic measurements.
As you might imagine, you would like your magnetometer, which is
very sensitive to magnetic. Fields to be as.
Far away from your metallic plane as possible, and to have
that plane be well characterized.
NASA used AP3. Orion that it had inherited.
From the US Navy. And that aircraft was originally
designed as a submarine hunter by flying very low and having a
magnetometer boom that allowed them to detect any rapidly
changing magnetic signals that might be indicative of
submarine. Just to give roughly an idea of
the extent of the survey and how big a project it was, roughly
how many traverses were made across the Greenland continent?
To compile the data across Greenland, NASA flew somewhere
north of 800,000 kilometers since 1993.
That's mostly in reference to ice thickness, so not all.
Of those surveys had. Magnetometry.
But there are other predecessor surveys collected.
In the 80s and early 90s. That did.
Collect that kind of information.
So there's a long history here again, not just the.
US. Of going out and collecting
these sorts of airborne measurements to unravel the
mysteries of Greenland. Can you give me an idea of the
resolution of these magnetic surveys?
Sure. So that's a function of the.
Altitude that the aircraft. Flies from that ice rock
boundary. Ice is very kindly non magnetic,
which makes it easier to interpret the signals.
And you can think of us in most cases as gaining exceptional
information along the flight line, getting an independent
data point on the order of 500 meters.
And then we have to combine interpolate all of those
crisscrossing flight lines. That held all sorts of.
Competing scientific priorities into.
A larger grid. That is typically interpreted.
On the scale of. Order 10 kilometers and is it
fairly even coverage or do you have spots that are very well
covered and then big gaps in other places?
There are still. Gaps in the Greenland ice sheet.
Which is roughly. 2,000,000 square kilometers in size.
There are gaps of the order of. 10,000 square kilometers in
size. There are several.
Of those but. It's increasingly good,
especially as compared to Antarctica which is 7 or 8
times. Larger the ice sheet.
A more challenging beast. To survey.
And finally, what about the topographic surface?
How were these performed and did they capture the topography of
the rock surface below the ice or the topography of the ice
surface? Great question.
So those are collected. Using radar Sounders Radar
Sounders are essentially an evolution of what some of your
listeners may be more familiar with, which are radar altimeters
in the. 50s or 60s? Pilots were flying aircraft over
parts of the Greenland ice sheet and discovered that sometimes
their radar altimeters didn't work.
Very well relative to what they. Could see and they seemed to be
returning. Much larger values in.
Terms of the distance to the surface than what was actually
apparent and what someone eventually.
Realized is that that is because ice is relatively.
Transparent to radio waves. Polar ice is typically.
Quite dry and quite cold. All conditions that favor low
attenuation of those radio wave signals, so that's been
subsequently exploited. Throughout the world over.
Ice To measure ice thickness relatively easily, the.
Velocity through the ice. At radio frequencies is pretty
well constrained, and so that allows us to make widespread ice
thickness measurements more or less whenever we're flying an.
Aircraft over the ice. For a scientific survey, when
you say you can use the radar to tell you what the thickness of
the ice is, are you basically comparing the reflection from
the top of the ice with the reflection from the bottom of
the ice and then taking the difference?
Exactly, yes. And you mentioned are we using
radar to observe? The top of the ice.
We are, but we have another tool to measure that surface.
Elevation very precisely, which is a laser.
Altimeter and NASA has developed a.
Series of those. Over the years, and that
motivated most of our scientific surveys of Greenland and
Antarctica. Which is to collect.
Observations of changing ice thickness or ice elevation to
interpret in terms. Of the broader.
Response of these ice. Sheets to.
Ongoing anthropogenic climate change.
This began in the early 90s and has continued more or less ever
since and has resulted in now 2. Satellites that are laser.
Altimeters. One was.
Called isat that orbited. From 2003 to 2009 and collected
laser altimetry data in a way that we've never had before.
And the second is. Called isat 2 which?
Launched in 2018 and is continuing to produce excellent
quality laser altimetry data across both ice sheets and allow
us to have. A very.
Precise understanding of their total mass change.
When we spoke earlier, you mentioned stereo photogrammetry.
Is that relevant to the topographic survey?
Absolutely. We mentioned that we.
Collect all these Criss. Crossing flight lines, We get
incredible data along the track of the flight line, but we
remain relatively ignorant as to what is going on between the
flight lines. So that's where stereo
photogrammetry. Can come in.
It's not as accurate. As laser altimetry but it.
Fills in the gaps extremely well, and so when those two are
combined, we get a. Happy medium of the accuracy of
laser altimetry. And the wide coverage of stereo
photogrammetry. When you measure the topography
of the rocks below the ice and then the topography of the
surface itself, does the topography of what's happening
below project itself onto the surface of the ice, even if it's
kilometers above it? It does indeed.
It's been known for a long time that when ice flows over a bump
or a valley, even if it's a kilometer thick, 2 kilometers
thick. It will leave a surface.
Impression of that bump. Or valley at the.
Surface Now that valley might be 100.
Meters deep. And the resulting bump might
only be a meter in amplitude at the surface, but therein lies
the benefits of. Stereo photogrammetry and its
ability to resolve. Relative surface elevation
change and. Also these ice sheets.
Acting as superb low. Pass filters of these.
Signals. As our ability.
To resolve relative changes. In surface.
Elevation has improved. We now.
Through this study have the ability.
To see how the ice sheet is responding to all the bumps
across Greenland and directly compare it.
To places where we. Have ice thickness data and to.
See that those. Line up and that allows us to
look at a map of the surface. Of the ice sheet which at its.
Thick is just three kilometers thick and then be able to infer
where. All the valleys are.
Even if we haven't measured them all.
Directly what we learned. From that for example.
Is that these fjords that have beautiful glaciers?
That calve off into the ocean. That we see at the edge of the
ice sheet. In some cases.
Those fjords. More or less.
Propagate. Back into the ice.
Sheet sub glacially. In a straight line for up to 300
kilometers. So as I said in the
introduction, your map shows 3 new geological provinces under
the ice. How do you define a province?
We describe them generically as regions.
And that they may yet be associated.
With some of the known provinces that have been mapped at the
periphery of Greenland, how we define a province is not
intended to be any different from how.
Geologists describe it elsewhere.
Which is a common. Set of geologic features that
are. Contemporaneous with each other
that are reconcilable at. Large scale so for.
Example Large igneous provinces We mapped part of the North
Atlantic igneous province in the east of Greenland.
Large basins such as the the independence of Fjord basin.
In the north origins such as Engelfeld origin.
In the Northwest, the Rinkian origin.
Farther South. And cratons and.
Other large scale features. That have common types.
Of rock. Whether it's igneous rock.
Associated with igneous provinces.
Or sedimentary successions. Associated with basins.
But when you decide to put a boundary between one region and
another, are you looking at specific features and saying,
OK, I think here we have a combination of data sets that
indicate we're dealing with an igneous rock, say, and in
another one here we're dealing with sedimentary rock.
Or is it more that you just put all your data together and just
look for the greatest amount of contrast considering all the
data as a whole, without taking a view as to what the data is
actually telling you about what's underneath?
So it's really a combination of the two.
In the case of igneous provinces, those are typically
going to be associated. With large positive.
Magnetic anomalies and relatively slow seismic speeds,
and a little more nebulous with the gravity anomaly and the.
Topography could be rough. That's the simplest case.
In most other cases. In Greenland.
Typically with very old. Rock and provinces that were
formed 1 billion. Years ago plus.
Except in the case of the very youngest ones and.
We took our geophysical. Fields and looked at those
fields island wide, including. At the periphery where?
We have a much better understanding, obviously.
Of what the geology is. And we looked at the pattern
that we saw. There the.
Magnitude of anomalies if they had a.
Particular texture. A common wavelength or azimuth,
and then looked at how that propagated inland and searched
for contrasts in those fields, and we did that independently.
For each of those fields. So what we ended up with was a
mishmash of lines in some places, and relatively coherent,
consistent boundaries between most fields in other places.
More often than not, the. Coherent.
Boundaries were in southern Greenland, and they were much
more complex in northern Greenland, which appears to be
an amalgamation of several relatively ancient provinces.
Can you describe a little bit more about what characterizes
each of these regions? You mentioned the simplest case
that would indicate an igneous rock in terms of what you
actually found in that northern region that you chose to divide
into three regions. Would you be able to give them a
qualitative description and then maybe even go so far as to say
something about how they were assembled and what their origin
is? We purposefully stayed away.
From that latter question as to how they were assembled and
instead focus on what distinguished.
Them geophysically so that we have a.
Baseline for future interpretation, recall from.
The DAWS, 2000. Nine map, our reference map.
They essentially used. No geophysical data to.
Constrain what was happening across. 80% of the island.
So here we're trying to. Take that next step where we
look at different. Regions say OK, it seems like in
the West we often. Have a shallower Moho.
Boundary we seem to have. Faster shear wave.
Velocities in the southern 2/3 of Greenland and the gravity
anomalies and the magnetic anomalies, which typically a
finer resolution, that's where. You really start to see a lot.
Of the strange contrasts across northern Greenland.
That give you a clue that this is a.
Complicated region subglacially in terms of the geology, not
just, as is apparent from the. Periphery of the ice sheet.
When you looked at it after you generated the map and you
defined what you think were the boundaries of the regions that
created the greatest contrast, was there any particular data
type that in retrospect seemed to map most closely into each of
the different regions that you could say characterize those
regions best if you were just to pick one?
If I had to pick two data types. And revealing my bias.
As a geophysicist here, I would pick the.
Gravity and magnetics. Those give us.
The best combination of resolution of what is going on
in the subsurface. Beneath the ice.
And insight into really what the nature.
Of those rocks are topography, the bedrock topography.
Both observed and inferred. That's an indirect indicator at
best. And the depth to Moho and the.
Shallow shear wave velocities. Those are helpful context, but
they're. Relatively coarse resolution.
We believe that the hotspot that is currently contributing to all
the volcanism taking place in Iceland previously traced the
path below Greenland as it drifted to the West as part of
the North American plate. Were you able to identify a
channel of basaltic rock marking the hotspot in your data?
Not. Directly, it's widely accepted
that. 50 to 60 million years ago, the Icelandic hotspot
passed through Greenland. But where exactly it?
Passed through Greenland on this northwest to east traverse
remains. An issue of debate.
And still not. Clearly resolved there are.
Hints. That perhaps the hotspot didn't
breakthrough the crust, but it left a.
Thermal signature Lateral. To the path of the hotspot.
There are other studies that have suggested that perhaps
there's. Elevated geothermal activity in
particular regions. We did notice that there are Co.
Located gravity and seismic and in particular magnetic anomalies
in this region that we called Central Region B that is in East
Central Greenland, it's subglacial.
It does not appear to have any known outcrop and it is an area.
That three decades ago. Someone.
Described as. Based on some preliminary
magnetic and gravity data, the Central Greenland Magmatic
province and we went back and forth between our group as to
whether or not. We would give these.
Specific genetic indicators to these regions, but ultimately.
We decided to leave. Them as generic so as to not
lead the witness too much. But.
This is an interesting region. Whether it has high.
Geothermal activity remains to be seen.
Because it just so. Happens to be next to a.
Glaciological feature, that is. Unique in Greenland and indeed
relative to Antarctica. As well.
That feature is called the. Northeast Greenland ice stream
and it starts almost at a point close to the middle.
Of the ice sheet. And then expands.
As it slowly heads. Downstream for several 100
kilometers until. It reaches the ocean.
And becomes this very. Crevassed and beautiful glacier.
In the far northeast, Greenland called Zechariah Isstrom.
A glacier that's actually. Changing very rapidly.
Today that we're quite concerned.
About in terms of future mass change of the ice sheet?
So what starts? The Northeast Greenland Ice St.
There's nothing like it anywhere else.
There are hints that there may have been similar features
elsewhere in northeast Greenland, but it's the only one
we. See today.
And it's a quite interesting coincidence that it happens to
be located right next to this anomalous.
Subglacial geologic area. One of the things that you and
your team are interested in was to learn about the flow of ice
on Greenland and particular to see if there's a relationship
between the bedrock and the flow of overlying ice.
And you've just mentioned a specific correlation that you
think strongly suggests there might very well be.
You developed a new visualization technique to
facilitate this, which you called Flow Aware Hill Shade.
Can you tell us about that? Sure.
So this is a hill shade. Of a digital elevation model.
Which is a stereo. Photogrammetric model.
That we talked about earlier. Instead of lighting the DEM from
a single azimuth, it takes into account the direction of the.
Flow of ice so that instead of. Being limited to a single
azimuth, it can vary. Relative to the direction.
Of ice flow. In this case, we found that
orthogonal to ice flow seemed to work best, and that.
Allows you to visualize. These bumps as the ice is
flowing over features relatively cleanly.
And what did you find by means of this visualization technique?
What we found was clear evidence.
That there are large. Networks of very long, sometimes
hundreds of kilometers. Subparallel valleys across the
ice sheet. And they're often quite straight
and they. Have a.
Predominant southwest, northeast azimuth, although not exclusive.
They often reveal features. That you can see at the margin
of the ice sheet that come out as, say, a river or a fjord.
So you can look at this flow over a hill shade.
And then see. Where you get towards the very
edge of the ice sheet if you're. Following a single trend line,
then oh. There's a river.
And So what that tells. Us is that.
We're dealing with something that's glaciologically useful.
But it's also. Revealing a long term erosion
pattern that is. Present across the ice.
Sheet in this systematic way, so you were able to see the very
long valley best by examining the ice flow on the surface
rather than looking at the radar signal for example.
So what the radar does is it allows you to get the depth of
that valley precisely, however. We don't have blanket coverage
of the entire. Ice sheet.
So what happens is the radar. Only occasionally Criss crosses.
Many of these valleys. Particularly in the deep
interior and in some. Cases.
Some of these valleys, not really the longest ones, but
some of the more prominent ones. Curiously, we've never flown
over. But it's clear that they're
there. And they have an effect on the
flow of ice above them. Kilometers above, they leave an
imprint. Of the flow of ice kilometers.
Above. I wouldn't say that they have a.
Strong influence on how the ice is flowing, but they could
potentially in the future because what valleys?
Are are places where any water. That is generated.
Beneath the ice. Is going to tend to end up and
then. Flow toward the periphery.
And that could ultimately influence how fast that ice is
flowing because. Water, we know, is very
effective. At lubricating the base of the
ice and ultimately leading to sliding, do we think that these
long valleys predated the ice and really have nothing to do
with the ice? That.
Is our working hypothesis the. Fact that there are.
Long. Networks.
They are subparallel to each other and straight.
And also the radar observations that tend to observe more V
shaped. Valleys than U-shaped valleys.
That suggests that these valleys probably predate the ice sheet.
And they may be indicative. Of some sort of.
Tectonic imprint on the rock in which the valleys formed that
led to preferential paths of erosion.
For these valleys. Yeah, I'm wondering, do these
represent sutures between different terrains or major
strike slip faults that were weaker and so somehow rather
gouged out by the ice? We did consider a hypothesis.
Like that initially, but our conversations with
geodynamicists suggest that the. Stresses that would be.
Required to generate. Patterns like that are likely
far too. Large to explain them directly,
but it is very remarkable that you have these parallel valleys,
thousands of kilometers long and roughly parallel to each other
is not really something we see anywhere else in the world, is
it? Yeah, it's not very common, and
that's why we felt that it was very much worth noting.
Do you have a new mission to Greenland in the works?
We do. Actually, I'm part of a new
mission called Snow for Flow. We will be.
Measuring snow thickness and ice thickness in key.
Glaciers that are relatively. Understudied both in
southeastern Greenland but also elsewhere.
In the Arctic. So that we can.
Better project how these? Regions are going to change in
the context of ongoing anthropogenic climate change.
Joe McGregor, thank you very much.
Thank you, Oliver. It was really nice to join you
and talk about this. Study.
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