esas0104__LookBnthLandscapes-stu

esas0104__LookBnthLandscapes-stu - Looking beneath the...

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Unformatted text preview: Looking beneath the landscapes Bandelier
National
Monument, Río
Grande
Rift,
New
Mexico EARTH
SCIENCE
in ARIZONA
and
the SOUTHWEST For
millennia,
people
of
the
Southwest
have
been
interested
in what
lies,
or
what
was
thought
to
lie,
below
the
surface. The
origin
stories
of
many
Native peoples
of
the
Southwest
involve ascent
through
different subterranean
worlds
and emergence
on
the
surface
of
this world. A
reconstructed
kiva
(Ancestral
Puebloan ceremonial
chamber)
at
Alcove
House, Bandelier
National
Monument,
Río
Grande Rift,
New
Mexico. Within
the
kiva
is
a
sipapu:
a
small
pit symbolizing
the
original
place
of
emergence. (Wikimedia
Commons,
2005) Looking
beneath the
landscapes 2 For
millennia,
people
of
the
Southwest
have
been
interested
in what
lies,
or
what
was
thought
to
lie,
below
the
surface. Early
Southwestern
miners
sought
to understand
the
subsurface
geometry
of valuable
ore
deposits
as
they
dug
down to
extract
them. They
garnered
much
empirical knowledge
of
the
shallow
depths
of
the Earth
where
they
worked,
but
had
little knowledge
of
what
lay
deeper. Mexican
barreteros (miners
who
used
only
hand
tools) depicted
at
work
in
1867. (Young,
1970) Looking
beneath the
landscapes 3 Is
the
complex
topography
of
the
Southwest
reflected in
similarly
complex
structures
and
materials
beneath
the
surface? How
can
we
explore
the
geosphere
beneath
the
Southwest? (Bowen,
2005) ? Looking
beneath the
landscapes How
deep
down
can
we
look? Mean
radius
of
Earth
=
6371
km 4 How
can
we
explore
the
structure
of
the
Earth
beneath
the
Southwest? How
deep
can
we
dig? How
deep can
we
drill? (S.
Semken) (EarthScope) Proposed
Resolution
Copper
Mine, Superior,
AZ:

7000
ft
(2.1
km)
deep 2.1
km
/
6371
km
=
______% 0.033 of
Earth’s
radius. Deepest
mine
(South
Africa): 0.079 5
km
=
______%
of
Earth’s
radius. Looking
beneath the
landscapes San
Andreas
Fault
Observatory
at
Depth, Parkfield,
CA:

10500
ft
(3.2
km)
deep 3.2
km
/
6371
km
=
______%

of
Earth’s
radius 0.050 Deepest
well
(Russia): 12.2
km
=
______%
of
Earth’s
radius. 0.191 5 L
 T How
can
we
explore
the
structure
of
the
Earth
beneath
the
Southwest? Geological
processes
of
uplift
(including
mountain‐building), followed
by
erosion,
can
expose
rocks
that
were
formerly
deeply
buried. 
1.8
km
deep 6371
km 1.8 km (1.1 mi) (Beus
&
Morales,
2003) =










 

%
of 0.028 Earth’s
radius. Grand
Canyon,
Arizona (S.
Semken) Looking
beneath the
landscapes 6 L
 T How
can
we
explore
the
structure
of
the
Earth
beneath
the
Southwest? Xenoliths
are erupted
in
lava Some
volcanoes
bring
up
rock
samples (xenoliths,
“foreign
rocks”)
from
Earth’s
interior. For
example,
peridotite
xenoliths
in
lava
erupted
at Burned
Mountain
(Peridot
Mesa),
Transition
Zone on
the
San
Carlos
Apache
Nation,
Arizona. Xenoliths
break
off Molten
rock (magma)
rising These
are
thought
to
come
from
about
65
km
depth: 65
km
/
6371
km
=














%
of
Earth’s
radius. 1.02 Some
xenoliths
are
thought
to
come
from
as
deep
as 400
km:

still
only














%
of
Earth’s
radius. 6.28 (image
from
Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) Looking
beneath the
landscapes (S.
Semken) 7 L
 T How
can
we
explore
the
structure
of
the
Earth
beneath
the
Southwest …if
direct
access
to
Earth’s
interior
is
not
possible? (Bowen,
2005) ? Looking
beneath the
landscapes 8 L
 T Earth
scientists
must
use
indirect
methods
to
examine
and
interpret
the structure,
composition,
and
dynamics
of
Earth
beneath
the
Southwest. Geophysical
methods
variously
use
seismic
waves,
magnetism,
gravity,
and
heat
to image
the
interior
of
Earth. Magnetics Gravity Seismology How
can
we
explore
the
structure
of
the
Earth
beneath
the
Southwest? L im it e d 
d ir e c t 
a c ce ss ,
M ,
& orin 
Ca r ,
2 ter 010 ) (im ag o e
fr m
R ey ds nol ,
Jo hn ,
K son elly Laboratory
experiments
on
Earth
materials,
under
conditions
that
simulate
those
of the
deep
solid
Earth,
also
provide
important
clues. Looking
beneath the
landscapes 9 










A
current
effort:

The
EarthScope
Project is
 an
 intensive
 deployment
 of
 advanced
 geophysical instruments
 across
 the
 USA,
 to
 better
 investigate structure
and
dynamics
beneath
the
continent. Installing
seismograph, Mexican
Hat,
UT (EarthScope,
2007) Seismic
station
T17A,
San
Carlos,
AZ (S.
Semken) GPS
station,
Canyon
de
Chelly,
AZ (EarthScope.org) ASU
SESE
is
a
major
collaborator
in
the
EarthScope
Project. Looking
beneath the
landscapes 10 Earthquakes
release
energy
as
seismic
waves
that
travel
through
Earth. Seismologists
use
these
waves
to
interpret
subsurface
structure. EarthScope
Project
(US
Array) seismographs
across
the
West
and Southwest
receive
seismic
waves from
a
2006
quake
in
Nevada. (IRIS,
2010) Seismograph (Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) Looking
beneath the
landscapes 11 Seismic
waves
radiated
from
an
earthquake
do
not
travel
far
in
straight
paths; they
are
deflected
by
layers
they
encounter
beneath
the
surface. Earthquake Seismograph less
dense more
dense (Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) The
 refraction
 of
 seismic waves
into
curved
paths
is evidence
 of
 the
 non‐ uniform
internal
structure of
Earth. (IRIS) Looking
beneath the
landscapes Analyses
of
seismic
waves on
 a
 global
 scale
 have shown
 that
 Earth
 is concentrically
 layered
and that
 these
 layers
 differ
 by density. 12 Global
seismology
reveals
that
Earth
is
concentrically
layered,
with
a
structure caused
by
variation
in
material
density
(implying
variation
in
composition): A
 thin
 crust
 composed
 of
 low‐density rock;
 continental
 crust
 is
 thicker
 and less
dense
than
oceanic
crust. Earth
surf ace Continental
crust Oceanic
crust Mantle Crust A
 thick
 mantle
 composed
 of
 higher‐ density
 rock;
 rock
 types
 vary
 with pressure
 (depth).
 
 The
 mantle
 is ductile,
but
essentially
all
of
it
is
solid. An
 outer
 core
 of
 molten
 iron‐nickel metal
alloy
(composition
similar
to
that of
iron
meteorites). An
inner
core
of
solid
iron‐nickel
alloy. Looking
beneath the
landscapes 7 - 40
km Up m a per nt le Lo m a wer nt le The
crust‐mantle
boundary is
called
the Mohorovicic
discontinuity
 (Moho
for
short). 2900
km Ou t co e r re 5150
km (Images
from
Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) Inner
core 13 6371
km The
average
compositions
of
Earth’s
internal
layers: Continental
crust: Granite Oceanic
crust: Basalt (R.
Weller) (M.
Doiron) Earth
surf ace Continental
crust Oceanic
crust Crust Mantle Mantle:
Peridotite (Olivine‐rich
rock) (Glendale
CC) 7 - 40
km Up m a per nt le Lo m a wer nt le Ou t co e r re 5150
km Core:
Metallic
iron‐nickel
alloy Looking
beneath the
landscapes 2900
km Inner
core (Other
Images:
Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) 14 6371
km Minute,
local
variations
in
the
downward
pull
of
Earth’s
gravity
can
be
measured and
studied
to
interpret
subsurface
structure
and
composition. Newton’s
Law
of
Gravitation Force
of
attraction
at
Earth
(E)
surface:

F
=
G
m
mE
/
rE2 …depends
on
mass
m
in
device. Gravitational
acceleration
g
=
G
mE
/
rE2 …depends
on
mE
only…
g
~
9.8
m/s2
at
surface. Gravimeter
(Mussett
&
Khan,
2000) Minute
variations
in
g
reveal
local
 differences
in
crustal
mass
or
density beneath
the
surface. Looking
beneath the
landscapes Geoscience
students
conducting
a
gravity measurement
in
the
Four
Corners
area on
the
Colorado
Plateau











(S.
Semken) 15 Gravity
measurements
in
the
Southwest
reflect
topography,
but: Higher gravity Lower gravity Higher gravity Bouguer gravity
map (Sweeney
&
Hill,
2001) Lower gravity Aren’t
these
results
counter‐intuitive
? g

α

ME …Higher
mountains
=
more
mass
underneath
?

why
not
higher
gravity? Basin
and
Range
 Looking
beneath the
landscapes Transition
Zone

 Colorado
Plateau 16 (Topographic
profile
from
Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) The
principle
of
isostasy
provides
an
explanation. The
weight
of
a
floating
solid
body
equals
the
weight
of
the
“fluid”
it
displaces. Larger
bodies
must
displace
greater
volumes,
so
they
float
both
higher
and
deeper. This
is
a
reasonable
analogy,
but
remember:
the
mantle
is
not
liquid! This
implies
that
mountainous
regions
are
buoyed
up
by
thick
crustal
roots. Basin
and
Range Transition
Zone Thinner low‐density crust =
Higher
gravity Thinner crustal
root Thicker crustal
root Thicker low‐density
 crust =
Lower
gravity (Images
from
Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) Looking
beneath the
landscapes 17 Isostasy
enables
us
to
calculate
and
compare
crustal
thicknesses beneath
regions
with
different
elevations. The
weight
of
column
M
must
equal the
weight
of
column
B. Mountains Basin (Weight
=
mass
times
gravity;
Mass
=
thickness
times
density) WM
=
(H
ρc
+
T
ρc
+
R
ρc
+
D
ρm
)
g M H B WB
=
(T
ρc
+
R
ρm
+
D
ρm
)
g 
If
WM
=
WB
then (H
ρc
+
T
ρc
+
R
ρc
+
D
ρm
)
g

=

(T
ρc
+
R
ρm
+
D
ρm
)
g H
ρc
+
T
ρc
+
R
ρc
+
D
ρm


=

T
ρc
+
R
ρm
+
D
ρm H
ρc
+
R
ρc




=

R
ρm R
ρm
‐
R
ρc




=

H
ρc R
=
H
ρc
/
(
ρm
‐
ρc
) Crust density
=
ρc Crustal
root T R Mantle density
=
ρm D Thickness
of
crustal
root
R
=
H
ρc
/
∆
ρ where
 (After
an
exercise
by
Pinter,
n.d.) H
=
elevation,

ρc
=
density
of
crust,
and and
∆
ρ
=
density
difference
between
mantle
and
crust. 18 Looking
beneath the
landscapes L
 T Use
the
principle
of
isostasy
to
calculate
variation
of
crustal
thickness
across
Arizona. Equation
for
thickness
of
a
crustal
root
(read
Pinter
exercise
for
derivation): R
=
h
ρc
/
∆
ρ where
 h
=
elevation,

ρc
=
crustal
density
(assume
2800kg/m3
though
actual
density
varies) and
∆
ρ
=
density
difference
between
mantle
and
crust
(assume
500
kg/m3). Elevation
of
Phoenix
=
300
m Basin
and
Range Elevation
of
Flagstaff
=
2100
m Transition
Zone (Reynolds,
Johnson,
Kelly,
Morin,
&
Carter,
2010) How
much
thicker
is
the
crust
under
Flagstaff
than
the
crust
under
Phoenix? Looking
beneath the
landscapes 19 L
 T Now
compare
the
latest
results
from
seismic
data
collected
in
the
EarthScope
project. Moho
? (EarthScope.org) Courtesy
of
H.
Gilbert
(Paper
in
review,
2010) General
relationship
between
elevation
and
crustal
thickness
holds,
but
not
everywhere
…. Isostasy
may
not
be
sufficient
to
account
for
crustal
thickness
in
some
regions
of
the
Southwest. Looking
beneath the
landscapes 20 The
complex
topography
of
the
Southwest is
reflected
by
variations
in
crustal
thickness: Mountainous
areas
have
deep
crustal
roots, and
the
crust
beneath
areas
of
lower
elevation
is
thinner
(for
the
most
part
!). Too
shallow? Too
deep? Data
from
H.
Gilbert
(Paper
in
review,
2010) Up here on Turtle Island, we're riding along on the Earth's crust, that rides on the Earth's mantle, that rides on the sea of molten iron. Turtle on the Sea is an apt metaphor for the planet's morphology. Martha E. Ture, Native American journalist and writer Looking
beneath the
landscapes 21 Complex
topography
and
crustal
structure
together
suggest
an eventful
geologic
history
for
the
Southwest
region. Looking
beneath the
landscapes Stay
tuned
! 22 Looking
beneath
the
landscapes What
you
should
now
know •Earth
 scientists
 must
 use
 indirect methods
 (such
 as
 seismology
 and gravity
 measurements)
 to
 examine and
 interpret
 the
 structure, composition,
 and
 dynamics
 of
 Earth beneath
the
Southwest. •The
topography
of
the
Southwest
is reflected
 by
 variations
 in
 crustal thickness:
 Mountainous
 areas
 have deep
 crustal
 roots,
 and
 the
 crust beneath
 areas
 of
 lower
 elevation
 is thinner. •Complex
 topography
 and
 crustal structure
 suggest
 an
 eventful geologic
 history
 for
 the
 Southwest region. What
you
now
should
be
able
to
do •Summarize
the
limited
direct
means
that are
 available
 to
 observe
 the
 interior
 of Earth. •Summarize
 (or
 depict
 in
 a
 sketch)
 the ways
 that
 seismic
 and
 gravity
 measure‐ ments
 are
 used
 to
 explore
 crust
 and mantle
structure
and
composition. •Sketch
 a
 cross‐section
 of
 the
 crust
 and uppermost
 mantle
 of
 the
 Southwest, illustrating
 the
 differences
 in
 surface elevations
 and
 thicknesses
 among
 the topographic
provinces
of
the
region. Something from there, from down in there is talking to you. You could hear it. You listen. Listen. You can hear it. The stones in the earth rattling together. Simon
J.
Ortiz,
Poet
and
ASU
Professor 23 Looking
beneath the
landscapes 1.4



Looking
beneath
the
landscapes Oct‐8‐10 REFERENCES Beus,
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E.
(2003).
Turtles
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the
way
down.

IMDiversity.com.

Retrieved
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from http://www.imdiversity.com/villages/native/arts_culture_media/ture_turtles.asp. U.S.
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(n.d.)

Digital‐elevation,
shaded‐relief
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This note was uploaded on 03/01/2011 for the course GLG 394 taught by Professor Semken during the Fall '10 term at ASU.

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