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This is Geology Bites with 
Oliver Strimpel. 

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When we compare the average 
composition of the Earth's 

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continental crust with the 
composition of magmas generated 

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by melting of the mantle, we 
find they don't match. 

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Continental crust is estimated 
to be made-up of 60% silica, but

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the magmas that come from the 
mantle are basaltic in 

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composition, with only about 52%
silica. 

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Continental crust also has much 
less magnesium than basaltic 

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rocks. 
Since continental crust is 

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believed to be derived by 
melting of the mantle, this 

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presents a puzzle. 
About 30 years ago, Roberto 

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Rudnik was among the first to 
recognize what has come to be 

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known as the crustal composition
paradox. 

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Since then, she has developed 
new approaches to improve our 

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estimates of the bulk 
composition of continental 

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crusts and has used various 
geochemical and isotopic 

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measurements to help distinguish
among the various theories that 

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purport to resolve the paradox. 
But to this day, there is no 

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consensus among the theories. 
Roberto Rodnik is a 

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distinguished professor in the 
Department of Earth Science at 

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the University of California, 
Santa Barbara. 

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Roberto Rudnick, welcome to 
Geology Bytes. 

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Thank you very much, Oliver. 
It's a pleasure to be here. 

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Let's start by talking about 
both sides of the paradox, as it

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were, the continental crust on 
the one hand and the basaltic 

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magma on the other. 
To start with, the continents 

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are very heterogeneous both from
place to place laterally and 

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from top to bottom, IE from the 
upper crust near the surface to 

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the lower crust. 
So how do we go about 

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determining the overall average 
composition of continental 

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crust? 
And can we be confident enough 

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in our measurements to know that
the composition of continental 

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crust really is actually 
different from that of basalt? 

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There's a number of approaches 
that have been taken to trying 

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to estimate crust composition 
through time. 

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People looking at upper 
continental crust, which is the 

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most accessible to us either do 
a go out and sample everything 

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approach or people have looked 
at terrigenous sediments or 

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sedimentary rocks. 
Things like shales lurse, which 

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is a windblown dust. 
And the idea is that these 

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sediments derive from the 
exposed crust that is exposed to

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chemical weathering and erosion 
and therefore they should 

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provide us with a very robust 
grand average of what's at the 

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surface. 
And that works pretty well for 

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elements that are not affected 
by chemical weathering that are 

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not removed because they're 
soluble during chemical 

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weathering, things like the rare
earth elements, zirconium and 

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hafnium, etcetera. 
So the people looking at 

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sedimentary rocks have 
approached the composition of 

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the upper continental crust and 
pretty much gotten the same 

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result for all the different 
studies that have been done, 

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which gives us confidence that 
we're honing in on the right 

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answer. 
So you said that the shales and 

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the Lus, because they sample a 
lot of different rocks on the 

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surface. 
I didn't quite understand the 

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point about chemical weathering 
in. 

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There well, so shales in 
particular are a product of 

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chemical weathering. 
So they're formed when, let's 

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say, igneous rock is exposed to 
the surface and it's subjected 

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to chemical weathering. 
And some elements just dissolve 

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into water and get washed into 
the oceans. 

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Things like magnesium and 
sodium, they're highly soluble 

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elements. 
Calcium is highly soluble, but 

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other elements don't dissolve 
easily into water and so they 

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remain behind in the weathering 
product. 

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And shales are fundamentally 
sampling the weathering product,

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the clays that are produced 
during chemical weathering. 

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Is that what we actually want 
then? 

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To measure the residue as it 
were, or So what we're. 

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After is measuring the rocks 
before the weathering. 

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Nevertheless for elements that 
are insoluble, the shales are 

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really good proxy for the 
exposed continental crust, upper

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continental crust. 
But you couldn't go out and take

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a shale and look at its 
magnesium concentration or its 

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sodium concentration etcetera to
try to get those elements. 

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Or strontium is a trace element 
that's very, very soluble. 

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So shales are great, but only 
for insoluble elements and Lurs 

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also suffers a little bit from 
chemical weathering. 

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And so it's has the same 
problems. 

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The only way then that we can 
pin down for the upper 

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continental crust those soluble 
elements is through the brute 

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force method of going out and 
analyzing a whole bunch of 

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exposed rocks and taking the 
grand average of those. 

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Now that's the upper continental
crust. 

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If we go deep into the crust, it
becomes more problematic because

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it's less accessible. 
So average continental crust is 

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let's say 35 kilometers thick. 
So if we go down 15 kilometers, 

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what's at the surface may not be
representative of what's at 15 

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kilometers. 
And certainly if we go down to 

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near the base of the crust, 30 
kilometers, 35 kilometers, it's 

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quite likely that the bulk 
composition down there is 

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different. 
And the reason I say that is 

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because we can look at things 
like seismic velocities, how 

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fast seismic waves travel 
through rocks, and we can see 

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that seismic velocities increase
with depth in the crust 

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everywhere you go worldwide. 
Now, could that be just due to 

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taking the rocks sort of the 
surface and metamorphosing them,

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changing their assemblage and 
thereby changing their seismic 

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velocity? 
And yes, this could and would 

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increase velocity, but the 
observation is that the increase

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in velocity is actually too 
large to be attributed solely to

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metamorphism of the rocks, and 
that requires probably a change 

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in the bulk composition. 
So how do you get to the lower 

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cross? 
Well, we do have lower crystal 

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samples. 
We have rocks that are 

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metamorphosed to add high 
pressure and temperatures. 

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They're called granulites, and 
we have some of those granulites

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exposed at the Earth's surface 
because they've been brought up 

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along fault zones. 
We have granulites that we can 

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access that are brought up as 
rock fragments in volcanoes. 

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That's what I've worked on a lot
through my career. 

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They're called xenoliths, which 
literally means foreign 

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xenoliths rock, foreign rock. 
And there's certain types of 

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volcanoes, certain alkali 
basalts, and also kimberlites. 

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Kimberlites are the type of rock
that brings up diamonds, for 

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example. 
So these rocks, these magmatic 

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rocks, have a whole bunch of 
volatiles in them, particularly 

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CO2. 
And as they decompress, they 

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erupt explosively through the 
solid rock that's the Earth's 

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outer lithosphere. 
And they bring up fragments of 

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that rock. 
And so we can go out to these 

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volcanoes and collect those 
fragments, those zenoliths, and 

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study them and get an idea of 
what's down there. 

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Now, the fundamental problem, 
though, is we don't know how 

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representative the zenoliths are
of what is in the lower crust. 

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And we don't know how 
representative that the 

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granulites that are sitting at 
the surface might be of what's 

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presently in the lower crust. 
And that's why I and others have

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turned to geophysical methods to
try to get in grand average of 

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the seismic velocity of the deep
crust and use that to try to 

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infer the bulk composition of 
the deep crust. 

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OK, so let's turn to the other 
side of the paradox, the 

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basaltic magnetism. 
As I mentioned in the 

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introduction, basaltic magma is 
thought to be the raw material 

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of continental crust formation. 
What are the various mechanisms 

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that form new continental crust?
I always like to think of 

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continent cross formation as 
being a mass transfer from the 

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mantle to the crust and that's 
where the basalt comes in 

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because mantle we know is made 
of a rock called peridotite. 

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And when we melt peridotite, we 
make basalt, and that's what we 

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see. 
For example, to generate the 

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oceanic crust, which is 
basaltic, the same processes 

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must somehow be involved in 
generating the continental 

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crust. 
And yet, as you mentioned 

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earlier, the continental crust 
is not basaltic. 

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So where does continental crust 
form? 

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What tectonic settings? 
It has a very strong chemical 

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signature of subduction zones, 
which is where the oceanic plate

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dives below overriding plate and
produces volcanoes, arc 

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volcanoes. 
And that's where we see basalts.

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But we also see a rock called 
anbesite, which is more 

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intermediate composition, 
igneous rock. 

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And it turns out that if we 
frame the continental crust 

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composition in terms of an 
igneous rock, we would call it 

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an anbesite. 
So there's that connection. 

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And there's also that we look at
the trace element composition of

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the bulk continental crust. 
We see that it has this very 

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strong fingerprint, chemical 
fingerprint of magmas formed in 

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subduction zone settings. 
So probably subduction zone 

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settings are the main location 
of crust formation. 

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There could be some contribution
from interplay. 

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Things like Hawaii or 
Yellowstone could contribute, 

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but their contributions are 
probably minor, relatively 

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speaking. 
But in these settings, don't we 

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get differentiation of the 
basaltic magma that rises from 

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the mantle into more silica rich
or Felzik components and more 

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magnesium rich or matic 
components, both via partial 

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melting and sometimes fractional
crystallization? 

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That's absolutely correct. 
The fundamental problem though 

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is that if the magma that has 
risen out of the mantle is 

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basaltic, even if we 
differentiate it, we still have 

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those maphic to ultra maphic 
cumulates at the base and the 

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more differentiated rock at the 
top. 

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And as long as those cumulates 
remain in the crust and we 

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reintegrate the whole crust 
composition, we end up with a 

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basalt. 
Ah, OK. 

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And just to be clear, the 
basaltic composition of the 

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magma derived from the mantle is
itself already different from 

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the bulk composition of the 
mantle, which consists of 

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ultramafic peridotite with less 
than 45% silica. 

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So although I said there is no 
consensus on how the paradox 

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came about, and we'll talk about
the different ideas now, there 

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is consensus that the paradox 
actually exists, namely that we 

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do know the average composition 
of the continental crust well 

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enough to know it is antisitic 
when on the face of it, it 

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should be basalting. 
That is absolutely correct, 

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yeah. 
So let's talk about the theories

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then that have been proposed to 
resolve the paradox. 

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And maybe if you could just give
us the high level summary and 

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then we'll go into each class if
you like. 

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There are basically three 
different types of hypothesis 

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that could explain why the crust
started perhaps as assault and 

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now is anthesite. 
One is the idea of loss of 

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mafic, ultra mafic cumulus that 
are complementary to what's 

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currently in the crust. 
That could happen through 

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density foundry and it could 
happen through a process called 

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re lamination. 
It could happen because perhaps 

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we know that the earth was 
hotter in the Archaean, and if 

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there was subduction happening 
in the Archaean, then the hotter

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Archaean earth could have 
generated more prevalent melts 

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of that subducting basaltic 
crust and created granites that 

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go up to create the crust and 
the residue then gets subducted 

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away. 
And even chemical weathering has

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been proposed as a possible 
process by which we could change

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the bulk composition of the 
continental crust. 

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OK, so let's take these one at a
time and start with the theories

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that invoke the loss of some 
mafic material, presumably from 

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the base of the crust into the 
mantle. 

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So it's popularly called 
delamination, although that may 

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not be the best name for it, 
more like foundering because 

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it's dense material that sinks. 
That's one version of this and 

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the other one I mentioned is 
relamination. 

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And the idea there is that you 
could subduct differentiated 

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crust like continental crust and
have the buoyant material rise 

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and the dense material sink. 
So you're actually subducting 

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continental crust? 
I thought that was almost 

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impossible to do. 
Well, this is very interesting. 

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20-30 years ago, you read any 
introductory text, they tell you

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can't subduct continental crust,
it's too buoyant. 

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But now we know it happens. 
And the reason we know it 

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happens is because there are 
metamorphic rocks that have been

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metamorphosed and mantle 
conditions, they're called ultra

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high pressure metamorphic rocks.
We know continental crust is 

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subducting in some cases. 
And so if you could cause the 

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buoyant material of the 
subducted crust to rise and the 

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denser material to sink, that's 
another way of differentiating 

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the crust. 
So how would the buoyant 

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material actually get back up 
into the crust according to the 

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re lamination theory? 
If you go back to the original 

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paper, they have many different 
scenarios, but the one that most

231
00:14:01,600 --> 00:14:06,600
people latch onto, which was the
really novel idea, is that #1 

232
00:14:06,600 --> 00:14:10,080
continental crust subducts. 
OK, we didn't think it did, but 

233
00:14:10,080 --> 00:14:13,760
yes, it does. 
And #2 if you subduct 

234
00:14:13,760 --> 00:14:17,520
continental crust, some of it 
comes back to the surface. 

235
00:14:17,520 --> 00:14:19,640
And that's how we know it's 
subducts because we find 

236
00:14:19,640 --> 00:14:22,800
diamonds, for example, in some 
ultra high pressure metamorphic 

237
00:14:22,800 --> 00:14:25,720
rocks, we find high pressure 
polymorphs of silica like 

238
00:14:25,720 --> 00:14:28,640
coacite. 
So it's subducts. 

239
00:14:29,600 --> 00:14:32,280
But what if not all of it comes 
back to the surface? 

240
00:14:32,680 --> 00:14:34,920
And the stuff that's not going 
to come back to the surface is 

241
00:14:34,920 --> 00:14:38,240
the dense stuff. 
And so you take a differentiated

242
00:14:38,240 --> 00:14:42,560
cross with a more felsic buoyant
upper portion and a more mafic 

243
00:14:42,840 --> 00:14:46,000
dense lower portion. 
You subduct it, you bring the 

244
00:14:46,000 --> 00:14:48,880
buoyant stuff back up, and you 
get rid of the dense stuff. 

245
00:14:48,920 --> 00:14:51,040
I see. 
I think of it as a version of 

246
00:14:51,040 --> 00:14:54,160
recycling the mafic recycling. 
It's just a different way of 

247
00:14:54,160 --> 00:14:57,680
doing it than dropping things 
off the bottom of the crust. 

248
00:14:57,680 --> 00:15:00,920
So the buoyant stuff that comes 
up gets mixed back in or re 

249
00:15:00,920 --> 00:15:03,280
laminated. 
Yeah, re laminated. 

250
00:15:04,840 --> 00:15:07,680
It just conjures up a kind of 
plainer image of a slab being 

251
00:15:07,680 --> 00:15:10,000
stuck to the bottom, but that's 
not really what's meant. 

252
00:15:10,080 --> 00:15:13,680
Well, I think some version of 
that is one version of re 

253
00:15:13,680 --> 00:15:15,440
lamination. 
There's many versions of re 

254
00:15:15,440 --> 00:15:18,720
lamination, but the one that 
most people are talking about in

255
00:15:18,720 --> 00:15:21,400
the new interesting idea, I 
think, was this idea of a 

256
00:15:21,400 --> 00:15:25,600
diapiric rise of more buoyant 
material and the sinking of the 

257
00:15:25,880 --> 00:15:29,200
less buoyant material. 
I'm reminded of the podcast 

258
00:15:29,200 --> 00:15:32,920
episode we did with Peter 
Molnar, which he talked about 

259
00:15:33,320 --> 00:15:37,480
the thickness of the Tibetan 
Plateau and why it's so high. 

260
00:15:37,480 --> 00:15:41,480
And one of the ideas he 
mentioned was just exactly what 

261
00:15:41,480 --> 00:15:44,640
you alluded to earlier, the 
foundering of the bottom of the 

262
00:15:44,640 --> 00:15:48,240
crust that maybe gets converted 
into eclogite or some denser 

263
00:15:48,880 --> 00:15:52,560
material that then breaks off 
and falls like a BLOB into the 

264
00:15:52,560 --> 00:15:55,120
mantle. 
Is that what is still being 

265
00:15:55,120 --> 00:15:57,600
visualized as one of the ideas? 
Exactly. 

266
00:15:58,000 --> 00:16:03,200
And this idea came around in the
early 1980s, and I have to say, 

267
00:16:03,200 --> 00:16:07,440
I was extremely skeptical that 
this could be an important way 

268
00:16:07,440 --> 00:16:09,080
in which the crust 
differentiates. 

269
00:16:09,080 --> 00:16:11,760
And I've really come around to 
thinking that yes, for sure it's

270
00:16:11,760 --> 00:16:14,520
happening in some places. 
And is that because you've 

271
00:16:14,520 --> 00:16:16,840
looked at things like seismic 
tomography? 

272
00:16:17,240 --> 00:16:20,520
You know, it's really hard to 
catch these blobs on seismic 

273
00:16:20,640 --> 00:16:23,360
images because they sink 
relatively rapidly. 

274
00:16:23,760 --> 00:16:27,600
So it's more places like the 
North China Kraton where we see 

275
00:16:27,600 --> 00:16:31,440
evidence of fundamental change 
in the lithosphere between a 

276
00:16:31,440 --> 00:16:34,880
Paleozoic when we had an 
Archaean, typical Archaean 

277
00:16:34,880 --> 00:16:38,440
lithosphere to present day. 
It's completely different. 

278
00:16:38,680 --> 00:16:42,160
And I think that's one of the 
areas where for sure there has 

279
00:16:42,160 --> 00:16:44,000
been removal of the deep 
lithosphere. 

280
00:16:44,080 --> 00:16:46,960
But how do we know there's been 
a fundamental change in the 

281
00:16:46,960 --> 00:16:49,040
lithos sphere, the North China 
Craton? 

282
00:16:49,320 --> 00:16:52,640
So in eastern China, there's an 
Archaean craton, and they have a

283
00:16:52,640 --> 00:16:56,440
particular type of lithospheric 
route that's very distinctive. 

284
00:16:56,520 --> 00:16:59,640
And in the Art of vision in 
eastern China, kimberlites came 

285
00:16:59,640 --> 00:17:01,680
up and they brought diamonds and
they brought pieces of the 

286
00:17:01,680 --> 00:17:04,480
lithospheric group. 
And we said, all right, we could

287
00:17:04,480 --> 00:17:08,119
see that at that time it was a 
typical Archaean craton. 

288
00:17:08,480 --> 00:17:12,280
But then in the Cenozoic, the 
salts come up and bring up a 

289
00:17:12,280 --> 00:17:15,480
completely different type of 
mantle lithosphere, completely 

290
00:17:15,480 --> 00:17:19,560
changed composition. 
And so the idea there is that, 

291
00:17:20,000 --> 00:17:24,319
well, the North China Kraton got
hit on three sides by subduction

292
00:17:24,319 --> 00:17:27,119
and collision. 
And so that happened during the 

293
00:17:27,119 --> 00:17:29,960
Mesozoic. 
And probably it was during the 

294
00:17:29,960 --> 00:17:32,560
Mesozoic that this 
transformation occurred. 

295
00:17:32,720 --> 00:17:35,960
And subduction was probably at 
least part of the reason why it 

296
00:17:35,960 --> 00:17:38,600
happened, although there's a lot
of debate about exactly how it 

297
00:17:38,600 --> 00:17:41,240
happened. 
So for sure the mantolithosphere

298
00:17:41,240 --> 00:17:44,120
was removed because the alkali, 
the salts are sampling in the 

299
00:17:44,120 --> 00:17:46,400
exact same place that the 
kimberlite sampled in the ore 

300
00:17:46,400 --> 00:17:49,920
division. 
And so we see, oh, back in the 

301
00:17:49,920 --> 00:17:53,760
ore division, it's Archaean. 
And then in the Tertiary, oh, 

302
00:17:53,800 --> 00:17:56,480
it's not Archaean. 
So something happened, something

303
00:17:56,480 --> 00:17:59,800
very fundamental happened. 
And with my colleague GAO Shan, 

304
00:17:59,800 --> 00:18:04,400
we also thought we were seeing 
some magnetism in the Mesozoic 

305
00:18:04,680 --> 00:18:08,480
that was produced by melting of 
these foundering blobs, which 

306
00:18:08,480 --> 00:18:10,920
have a very distinctive chemical
composition. 

307
00:18:11,200 --> 00:18:14,360
So that would imply that not 
only the mantle lithosphere, but

308
00:18:14,360 --> 00:18:17,600
also the lower cross might have 
been removed during this event. 

309
00:18:18,200 --> 00:18:20,840
But there are other places in 
the world where we see similar 

310
00:18:20,880 --> 00:18:23,240
types of evidence for this 
happening. 

311
00:18:23,600 --> 00:18:27,760
So just in terms of the tectonic
context where these ideas about 

312
00:18:27,760 --> 00:18:31,280
foundering a thought to take 
place, is it then that you could

313
00:18:31,280 --> 00:18:34,400
get it on the thickened 
continental crust where the base

314
00:18:34,400 --> 00:18:37,800
is heated and compressed As for 
example we see in the present 

315
00:18:37,800 --> 00:18:41,080
day in the Tibetan situation? 
Or is that also something that 

316
00:18:41,080 --> 00:18:44,720
we think is associated with 
continental subduction like you 

317
00:18:44,720 --> 00:18:47,360
mentioned earlier? 
When there is subduction 

318
00:18:47,360 --> 00:18:50,960
underneath continents, we also 
tend to see a lot of crustal 

319
00:18:50,960 --> 00:18:52,480
thickening and lithospheric 
thickening. 

320
00:18:52,760 --> 00:18:56,160
For the example, the Andes have 
some of the thickest crust on 

321
00:18:56,160 --> 00:18:59,400
Earth, and that's because 
there's compression going along 

322
00:18:59,400 --> 00:19:01,080
with the subduction. 
I see. 

323
00:19:01,080 --> 00:19:03,960
So the two tectonic contexts are
actually related. 

324
00:19:04,520 --> 00:19:07,160
The subduction creates a thick 
crust and then the thick crust 

325
00:19:07,200 --> 00:19:08,840
is where this foundering can 
happen. 

326
00:19:09,920 --> 00:19:13,040
So let's turn to the second 
category that you talked about, 

327
00:19:13,120 --> 00:19:17,120
the idea that slab melting 
during the hot Archaean can 

328
00:19:17,160 --> 00:19:21,560
produce anti cytic crust. 
So that's motivated by the fact 

329
00:19:21,560 --> 00:19:24,920
that when you look at Archaean 
cratons, you see a type of 

330
00:19:24,920 --> 00:19:28,800
granite that is sodium rich. 
It's called a tonolite and 

331
00:19:28,800 --> 00:19:32,480
there's this suite called 
tonolites, trunge mites and 

332
00:19:32,480 --> 00:19:37,160
granodiarites, otherwise known 
as TTGS, that are very typical 

333
00:19:37,360 --> 00:19:40,520
of arcane kratons. 
So the difference between that 

334
00:19:40,520 --> 00:19:43,920
and what we see later is that 
later we see more potassium rich

335
00:19:43,920 --> 00:19:46,680
granites. 
And so these tonalites, there 

336
00:19:46,680 --> 00:19:49,520
have been a lot of people 
exploring this experimentally. 

337
00:19:49,520 --> 00:19:52,600
How do you make a tonalite? 
Well, you melt a basalt to 

338
00:19:52,600 --> 00:19:56,720
create a tonalite. 
OK, so where was that basalt? 

339
00:19:56,880 --> 00:20:00,760
Well, if it was in a subducting 
slab and the slab was actually 

340
00:20:00,760 --> 00:20:04,160
melting because it heated up to 
the point where it could melt 

341
00:20:04,160 --> 00:20:08,040
before it was fully dehydrated 
when it was subducted, then you 

342
00:20:08,040 --> 00:20:11,240
could generate a tonalite and 
move that tonalite. 

343
00:20:11,680 --> 00:20:15,520
Through the mantle above it to 
create continental crust, and 

344
00:20:15,520 --> 00:20:19,280
then the residue is removed once
the slab continues to subduct. 

345
00:20:19,520 --> 00:20:24,680
So the very presence of these 
TGS is viewed as evidence that 

346
00:20:24,760 --> 00:20:27,760
they were formed out of basaltic
crust and that the more mafic 

347
00:20:27,760 --> 00:20:31,280
part is no longer there. 
And what we're seeing in the 

348
00:20:31,280 --> 00:20:35,320
crust is this more solistic, 
more evolved TTG. 

349
00:20:35,400 --> 00:20:38,160
That's correct. 
And there's still debate though 

350
00:20:38,200 --> 00:20:41,720
about TTGS because you could do 
it by slab melting or you could 

351
00:20:41,720 --> 00:20:44,920
do it by differentiation with 
the crust and then removal of 

352
00:20:44,920 --> 00:20:48,440
the dense complement through 
delamination or density 

353
00:20:48,440 --> 00:20:51,120
foundering and. 
If it's the latter, why don't we

354
00:20:51,120 --> 00:20:53,720
see them being formed today? 
Or do we? 

355
00:20:53,760 --> 00:20:58,880
Well, we do see some tonalites, 
so Sierra Nevada for example has

356
00:20:58,880 --> 00:21:03,000
some sodium rich granites. 
But probably that means that the

357
00:21:03,520 --> 00:21:06,120
source rock for the granite has 
changed over time. 

358
00:21:06,280 --> 00:21:10,600
So going from strictly mafic in 
the Archaean to something a 

359
00:21:10,600 --> 00:21:12,840
little more evolved after the 
Archaean. 

360
00:21:13,640 --> 00:21:17,400
OK, let's talk about the third 
category you mentioned, which is

361
00:21:17,400 --> 00:21:19,880
the weathering, chemical 
weathering. 

362
00:21:19,880 --> 00:21:21,960
How does that help resolve the 
paradox? 

363
00:21:22,440 --> 00:21:26,720
Well, as I said earlier, some 
elements are very soluble during

364
00:21:26,720 --> 00:21:30,520
chemical weathering and 
magnesium is one of those and 

365
00:21:30,520 --> 00:21:33,680
what we really need to do is 
remove magnesium from the salt 

366
00:21:33,840 --> 00:21:36,040
in order to create a non 
basaltic crust. 

367
00:21:36,040 --> 00:21:38,800
Also calcium and calcium's also 
soluble. 

368
00:21:39,200 --> 00:21:42,960
So the weathering hypothesis is 
that you remove some of these 

369
00:21:42,960 --> 00:21:46,400
soluble elements and leave 
behind things like silica and 

370
00:21:46,400 --> 00:21:50,640
aluminum and that could drive 
your bulk composition across 

371
00:21:50,640 --> 00:21:55,320
towards a non mafic composition.
But the problem is, well, 

372
00:21:55,320 --> 00:21:58,240
they're removed, they go into 
the oceans and then what happens

373
00:21:58,240 --> 00:22:00,760
to them? 
Well, maybe they get taken up 

374
00:22:00,760 --> 00:22:02,720
through alteration of oceanic 
crust. 

375
00:22:02,840 --> 00:22:06,360
Magnesium, for example, may be 
enriched in altered oceanic 

376
00:22:06,360 --> 00:22:09,720
crust, also calcium. 
So then you subduct it. 

377
00:22:10,000 --> 00:22:13,000
But then sodium, we got plenty 
of sodium, we haven't really 

378
00:22:13,000 --> 00:22:15,000
lost sodium. 
So you have to have the 

379
00:22:15,000 --> 00:22:18,760
mechanism get sodium back into 
the continents if the crust has 

380
00:22:18,760 --> 00:22:23,200
been influenced by weathering. 
So maybe sodium as that ultra 

381
00:22:23,200 --> 00:22:26,080
oceanic crust subducts. 
Maybe sodium comes back up 

382
00:22:26,120 --> 00:22:29,520
through arc magnetism 
preferentially to magnesium and 

383
00:22:29,520 --> 00:22:31,600
calcium. 
That's more or less the 

384
00:22:31,600 --> 00:22:34,680
weathering hypothesis. 
It's not particularly a well 

385
00:22:34,680 --> 00:22:39,800
developed hypothesis, but 
there's one observational fact 

386
00:22:40,080 --> 00:22:44,880
that really supports it and this
is that if you look at stable 

387
00:22:44,880 --> 00:22:49,800
isotope composition, things like
oxygen isotopes of any crustal 

388
00:22:49,800 --> 00:22:53,960
rock, we know that rocks out of 
the mantle should have a 

389
00:22:53,960 --> 00:22:57,840
particular composition around 
six per mil relative to the 

390
00:22:57,840 --> 00:23:01,400
standard. 
And yet almost all of the 

391
00:23:01,400 --> 00:23:04,240
continental crust, any Rock You 
look at is heavier than that, 

392
00:23:04,360 --> 00:23:06,960
has a higher, what's called 
delta OH 18. 

393
00:23:07,840 --> 00:23:10,800
And that's produced by chemical 
weathering. 

394
00:23:11,400 --> 00:23:13,800
And you see that right through 
the crust, even into the base of

395
00:23:13,800 --> 00:23:17,320
the crust. 
And so it really is compelling 

396
00:23:17,360 --> 00:23:21,280
that weathering has had an 
imprint on the crust, throughout

397
00:23:21,280 --> 00:23:24,160
the crust. 
So we're pretty sure that the 

398
00:23:24,160 --> 00:23:26,680
weathering has had an impact, 
but you say there are still some

399
00:23:26,680 --> 00:23:29,440
residual problems about what 
happens to the stuff that's 

400
00:23:29,440 --> 00:23:32,000
weathered and how we wind up 
with the present day composition

401
00:23:32,000 --> 00:23:33,440
in that model? 
That's right. 

402
00:23:33,960 --> 00:23:37,760
So I said earlier that there's 
no consensus, but do you have a 

403
00:23:37,800 --> 00:23:41,080
favorite theory or favorite 
approach to this problem? 

404
00:23:41,600 --> 00:23:44,360
I think chemical weathering has 
had an influence, but I'm not 

405
00:23:44,360 --> 00:23:46,880
sure how strong of that 
influence is. 

406
00:23:47,120 --> 00:23:49,360
I don't rule any of them out. 
I think all of them have 

407
00:23:49,360 --> 00:23:53,360
happened, but I think this 
return of mafic to ultra mafic 

408
00:23:53,360 --> 00:23:57,680
cumulus is probably the key 
process by which the continental

409
00:23:57,680 --> 00:24:02,680
cross has gone from up assault 
to and quote and aside full 

410
00:24:02,680 --> 00:24:05,440
composition. 
And any particular mechanism of 

411
00:24:05,440 --> 00:24:07,400
the return of the cumulus? 
Then you're thinking more of the

412
00:24:07,400 --> 00:24:09,400
thick, thick and continental 
crust context. 

413
00:24:09,400 --> 00:24:15,000
I am agnostic on that. 
Delamination, relamination, all 

414
00:24:15,000 --> 00:24:19,040
of those actually, even the 
subduction and subduction zone 

415
00:24:19,040 --> 00:24:21,600
melts, they're all basically are
doing the same thing. 

416
00:24:21,600 --> 00:24:24,520
They're taking mphic, ultra 
mphic materials and returning it

417
00:24:24,520 --> 00:24:28,400
to the mantle, leaving behind 
the more evolved compositions. 

418
00:24:29,400 --> 00:24:34,320
What observations or theoretical
work do you think would help us 

419
00:24:34,320 --> 00:24:38,280
distinguish the various theories
and reach a consensus on why 

420
00:24:38,280 --> 00:24:40,920
this paradox exists? 
I don't think there's ever going

421
00:24:40,920 --> 00:24:46,160
to be a consensus in my 
lifetime, which is fine because 

422
00:24:46,200 --> 00:24:50,040
debate is what invigorates 
science and also spurs people to

423
00:24:50,040 --> 00:24:53,000
new discoveries. 
I think case studies are 

424
00:24:53,000 --> 00:24:56,440
critically important. 
And as I said, I started out 

425
00:24:56,440 --> 00:25:01,400
being very skeptical of the idea
of the delamination or return of

426
00:25:01,400 --> 00:25:04,280
mafic, ultra mafic materials, 
but now I'm convinced that it 

427
00:25:04,280 --> 00:25:06,760
has to happen. 
There have been good case 

428
00:25:06,760 --> 00:25:10,320
studies demonstrating that 
probably happens in some areas. 

429
00:25:10,320 --> 00:25:13,520
So that's one approach. 
There will be continued efforts 

430
00:25:13,520 --> 00:25:16,840
to refine crust composition, but
I can tell you that that's not 

431
00:25:16,840 --> 00:25:18,960
going to change. 
We've known it's Andasite bulk 

432
00:25:18,960 --> 00:25:22,760
composition since the 1880s and 
it hasn't changed since then. 

433
00:25:23,120 --> 00:25:25,680
Details may change, but that's a
fundamental. 

434
00:25:26,240 --> 00:25:29,800
I think just continued case 
studies is probably how we're 

435
00:25:29,800 --> 00:25:32,160
going to make headway and new 
hypotheses. 

436
00:25:32,160 --> 00:25:34,600
When Brad Hacker and his 
colleagues came up with a re 

437
00:25:34,600 --> 00:25:38,360
lamination idea, it was a new 
idea and it was great and it's 

438
00:25:38,360 --> 00:25:40,640
probably happening in some 
places. 

439
00:25:40,960 --> 00:25:45,000
People have been working hard to
identify areas where it's 

440
00:25:45,000 --> 00:25:48,840
happened. 
Do we know enough about how the 

441
00:25:48,840 --> 00:25:52,920
surface of other rocky planets 
in the solar system formed to 

442
00:25:52,920 --> 00:25:56,760
know if it's consistent with 
their parent Magma's or not? 

443
00:25:56,880 --> 00:26:00,040
In other words, is there the 
potential for a similar crustal 

444
00:26:00,040 --> 00:26:03,040
composition paradox elsewhere in
the solar system? 

445
00:26:04,840 --> 00:26:08,640
I mean, yes, we know the 
composition, a pretty good idea,

446
00:26:08,640 --> 00:26:11,360
the compositions of the crusts 
of terrestrial planets in our 

447
00:26:11,360 --> 00:26:13,840
solar system. 
And there is no similar 

448
00:26:13,840 --> 00:26:17,320
dichotomy anywhere else, which 
is very interesting. 

449
00:26:17,560 --> 00:26:20,640
Venus is considered a sister 
planet in many ways in terms of 

450
00:26:20,640 --> 00:26:23,960
mass density, that's a little 
closer to the sun, but it 

451
00:26:23,960 --> 00:26:26,040
evolved very differently from 
Earth. 

452
00:26:26,120 --> 00:26:30,320
And so a lot of this you could 
take one step back and say, OK, 

453
00:26:30,320 --> 00:26:32,560
why do we have continents and 
why do we have continental 

454
00:26:32,560 --> 00:26:35,240
crust? 
And I think the answer is 

455
00:26:35,240 --> 00:26:38,520
because we have plate tectonics 
and we have subduction, and 

456
00:26:38,520 --> 00:26:41,080
that's critically important for 
creating continents. 

457
00:26:41,440 --> 00:26:44,120
But then why do we have plate 
tectonics? 

458
00:26:45,080 --> 00:26:48,240
We don't see evidence for plate 
tectonics on any of the other 

459
00:26:48,240 --> 00:26:51,520
terrestrial planets. 
Is it because we have liquid 

460
00:26:51,520 --> 00:26:54,520
water? 
So Venus probably never had 

461
00:26:54,520 --> 00:26:57,080
liquid water. 
Mars definitely did have liquid 

462
00:26:57,080 --> 00:26:59,560
water in the past. 
We can see the evidence for that

463
00:26:59,560 --> 00:27:02,520
in the sedimentary deposits. 
Some people think they see 

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00:27:02,560 --> 00:27:05,120
former lakes from the surface of
Mars. 

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00:27:05,120 --> 00:27:09,040
And yet Mars also did not seem 
to develop plate tectonics. 

466
00:27:09,520 --> 00:27:13,200
So these are big questions and 
really interesting questions, 

467
00:27:13,200 --> 00:27:15,880
and I hope it continues to 
motivate scientists well into 

468
00:27:15,880 --> 00:27:18,120
the future to explore these 
ideas. 

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00:27:19,560 --> 00:27:21,520
Roberto Rudnick, thank you very 
much. 

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00:27:21,800 --> 00:27:24,240
It's been a pleasure. 
Thank you for the invitation to 

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00:27:24,240 --> 00:27:27,200
talk to you today. 
To see pictures and 

472
00:27:27,200 --> 00:27:32,600
illustrations that support this 
podcast, go to geologybytes.com,

473
00:27:33,160 --> 00:27:36,440
where you'll also find 
transcripts and a subject matter

474
00:27:36,440 --> 00:27:39,800
index of all the episodes. 
There you can also give me 

475
00:27:39,800 --> 00:27:43,720
feedback which I welcome, as 
well as sign up to get my emails

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00:27:43,720 --> 00:27:44,920
about new episodes.