Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Evolution of the
Earth
Seventh Edition
Prothero • Dott
Chapter 8
Cryptozoic History: Introduction to the Origin of Continental Crust
Figure 4.7
Figure 4.8
Complete Geologic Time Scale
Hadean to Recent
Phanerozoic –
“visible life”
Geologic Time Scale for 1st 3.8 Billion Years of Earth Existence
Proterozoic - “hidden life”
Archean – life first appears (?) and remains viable
Hadean – meteorite bombardment, life started and restarted?
Chap. 8 - Origin of Continental Crust
• Main Topics
– Earth cooled sufficiently to
permit formation of early
continental (granitic) material
– Isotopic age dates within
continents “cluster”
suggesting several periods of
“orogeny”
– Early continents seem to
represent “partial melts” of
andesitic volcanics or early
sediments.
– Most of the present-day
volume of continental
material had formed by ~2.5
billion yrs. ago.
Chap. 8 - Origin of Continental Crust
• Main Topics (cont.)
– Archean (3800 – 2500 Bya) rocks
characterized by “greenstone”
belts and texturally immature
sediments (graywackes), largely
form oceanic arcs. Suggesting
plate tectonics may have started?
– Proterozoic (2500 – 540 Bya)
rocks are texturally and
compositionally mature, include
chemical sediments (carbonates
and evaporites). Stromatolites are
present showing life had evolved
while evaporites suggest that sea
water had also evolved to its
present composition
Fig. 8.1
Atrists conception of what surface of
earth looked like during its first 500
million years.
Surface was largely molten, with a few of
the original microcontinents beginning to
form.
Intense meteorite bombardment heated
surface to melting.
Moon was twice as close, exerting a very
strong gravitational pull.
Early atmosphere had no O2, but probably
consisted of N2, CH4, NH3, CO2 and H2O.
Note no oceans.
Evidence of Crustal Development from Igneous
and Metamorphic Rocks
• Importance of Granite
• Rock-types surviving from early Cryptozic are
mainly granitic in composition and they are
arrangemed in highly deformed orogenic belts.
• This has led to hypothesis of continential
accretion of early granitic masses into
protocontinents and then continents.
Evidence of Crustal Development from Igneous
and Metamorphic Rocks
• However field evidence suggests that granitic
continental crust was not original and must have
increased in volume through time.
• Original crust was thin and mainly basalt.
Weathering, erosion and igneous activity converted
some of the original crust to granite to form
embryonic continents.
• Embryonic continents persisted on surface of earth
and accreted slowly to form larger continents.
Fig. 8.10
Archean granite (light) intruding metavolcanic
(metamorphosed volcanic ash, etc.) sediments. Nestor
Falls. Ontario. Granite is about 2.5 By (Algoman
orogeny).
Fig. 8.2
Cryptozoic (“hidden life”) Eon
Sondre Stromfjord,
SW Greenland.
Age of rocks in this
picture are ~3.8 By.
High-grade metamorphic rock (gneiss) typical
of ancient “shield” regions.
Fig. 8.6
Cross-section from N. Shore of L. Superior to northern Michigan.
Numbers refer to relative age (1 = oldest).
Development of a Cryptozoic Chronology
• Age dating of ancient rocks showed patterns of
old rocks bounded by younger rocks in
patterns that suggested accretion of younger
material onto a core of older, mostly granitic,
rock.
• Thus the modern continents have a history of
growth by addition of smaller granitic masses,
which persisted through time because of their
greater buoyancy.
Fig. 8.3
Map showing locations of all Cryptozoic and early Paleozoic rocks in
the world. Numbers refer to age in By.
Fig. 8.11
These geologic
provinces form the core
of the North American
craton.
The older rocks probably
accreted about 1.8 - 1.9
Bya. The Grenville
Province was sutured
about 1.0 Bya.
(craton = stable nucleus
of a continent)
Isotopic age dates show great discordance when mapped
over the entire N. American craton.
Greenstone Belts
• “Greenstone Belts” are basically metamorphosed
basalts and graywacke (discussed below) sandstones
deposited as pillow lavas and turbidity flows on the
floors of ancient seas.
• When protocontinents collided and accreted, the
ocean floors filled with these basalts and graywackes
collapsed, forming greenstone belts that also accreted
to the growing protocontinent.
• Thus some of the early seafloor survived destruction
(by subduction) and became part of the stable craton.
Fig. 8.12
Evolution of greenstone belts. A. Basins between protocontinents fill
with basalts, B. when protocontinents collide, they “collapse” the
oceans filled with basalts and graywackes, forming greenstone belts.
Fig. 8.13
Hypothetical scenario for assembly of N. American craton during
Proterozoic. Based on dates and tectonic patterns in previous
figure.
Interpretation of Crustal Development from
Sediments
• Terrigenous vs. nonterrigenous sediments
• Composition of sedimentary rock reflects
source
– Clastic sediments – primarily silicates, derived
from erosion of older rocks in land areas
– Chemical sediments – evaporites (salt – NaCl,
gypsum – CaSO4) and carbonates. Precipitates or
bio-precipitates in warm, shallow seas
Fig. 8.14
Stages in the development of textural maturity in a sand through
abrasion and sorting of grains. Size tends to decrease with time and
transport distance. Clay minerals form, from from chemically
unstable minerals such as feldspars and amphiboles and quartz is
concentrated in residue. Final stage is a pure quartz sandstone, but
often only after several tectonic (erosion, burial, uplift) cycles.
Fig. 8.15
graywacke
arkose
quartzite
Steps in the evolution of a mature sand from initial weathering of a granite.
Texturally mature sand is mono-minerallic (quartz), well-rounded and of a uniform
grain size. This indicates a long time spent in transport or washing around on a
beach. It may also be 2nd or even 3rd cycle. Graywacke suggests rapid transport
and burial (why?) while arkosic sands suggest longer transport or more intense
weathering in place, since most unstable minerals (amphiboles) are missing.
Fig. 8.16a
Photomicrograph of a
graywacke sandstone showing
lack of textural maturity
(angular grains, many
unstable minerals and poor
sorting (a wide range of grain
sizes.
This rock is 1st cycle,
deposited rapidly, perhaps as a
turbidite and spent little or no
time in a high-energy
environment such as a beach.
This type of rock would be
expected to be common on the
early (Archean) earth.
Fig. 8.8a
Graded bedding (grain size decreases upward in the gray
beds) in Archean graywacke from Ely, Mn.
Fig. 8.8b
Archean graywacke
showing multiple graded
beds and interstratified
limestones.
East of Great Slave Lake,
Northwest Territories,
Canada.
Fig. 8.20
Fig. 8.16b
Photomicrograph of a pure
quartz sandstone characterized
by good sorting (monominerallic, one dominant grain
size) well-rounded grains and
absence of clay and unstable
minerals.
This type of rock would be
expected to be found on a stable
craton where it could spend a lot
of time (millions (?) of years )
washing around as loose grains
on a beach.
This rock could be 2nd or 3rd
cycle from pre-existing
sediments as they were buried,
consolidated and then uplifted
and eroded.
• Sediment composition triangle
The diagram shows the range of
sedimentary rock types represented
as mixtures of three components:
calcium (plus magnesium)
carbonates, clay minerals
(represented by the hypothetical
hydrated aluminum and iron oxides
as the end member), and silica
(silicon dioxide). Sediments and
sedimentary rocks have the same
ranges of composition.
Iron-rich laterites and aluminum-rich
bauxites are the products of intense
weathering.
•
Sandstones are primarily composed
of indurated sandy sediments, in
many cases dominantly quartz.
Argillaceous rocks are formed by
lithification of clay-rich muds.
Sediments or sedimentary rocks
rarely, if ever, have compositions
represented by the white area of the
triangle.
One example of a classification chart for
sedimentary rocks
• Cherts are the sedimentary rock
equivalent of biologically
deposited siliceous deposits.
During the transformation into
rock, the amorphous silica,
originally deposited by diatoms
and radiolarians, is transformed
into very hard microcrystalline
quartz-rich rock.
QFL = Quartz, Feldspar, Lithic fragments
A simple model showing how different tectonic regimes lead to different
types of sandstone deposition. QFL triangular diagrams are usual method
of depicting sandstone composition and hence provenance (source) and
history.
SEDIMENTARY DEPOSITIONAL ENVIRONMENTS
“Long” vs “short” system models for sedimentary deposition
environments. Note both systems eventually result in submarine fans but
long reach has more and varied environments.
Fig. 8.9
Cross-bedded 1.75 By sandstones from the Big Bear Formation,
Coppermine River, NW Territories, Canada. Cross-beds are
produced when coarse sand is deposited by water (fluvial) or
wind (aeolian). These are probably aeolian._
Fig. 8.17
Ripple marks in early Proterozoic (Huronian) quartzite. 30 miles east
of Sault Ste. Marie, Ontario. Ripple marks contain information on
direction of sediment transport as well as being “tops” indicators.
Block diagram showing origin of cross-stratification by migration
of ripples. Cross-bedding reveals top and bottom as well as current
direction.
Fig. 8.19
Comparison of relative sorting of sand grain sizes by
different sedimentary processes. Sorting can help determine
the origin of a sandstone.
Origin of Life - Stromatolites
• A special type of rock exists throughout the geologic record,
called stromatolites, which record the very first visible
evidence of life, as early as 3.465 billion years ago.
• These rocks are actually comples colonies of different types of
bacteria, each type occuping a special niche in the colony. The
most important are the photosynthetic cyanobacteria (formerly
blue green algae) common pond scum.
• These amazing life forms are highly adaptable and form the
base of the first food chain. Oh yes, they also are responsible
for all the oxygen in the air. O2 is a waste product of their
photosynthesis.
• Plants later likely simply incorporated a version of
cyanobaterial to carry out their photosynthesis. Nature rarely
reinvents a wheel.
Fig. 8.22
Outcrop of a stromatolite “reef” from 1.6-billion year
old Proterozoic carbonate in the Wopmay orogen.
These reefs were formed by colonies of
photosynthetic “blue-green” algae, cyanobacteria and
represent some of the first life forms on earth.
Fig. 8.22
Shark Bay – A Glimpse into the Archean
Modern algae from Shark Bay
Australia. They survive in the
hypersaline lagoons because
predators cannot tolerate the high
salt content.
Fig. 8.28
Model showing schematically how cyanobacteria changed the world. Note the
iron minerals (BIFs) in A and the oxygen segregation in the oceans (B).
Fig. 8.7
Banded Iron Formation (“BIF”) near Jasper Nob,
Ishpeming MI. Chert (red) iron (gray).
Fig. 8.30
Oolites in Banded Iron Formation (BIF), N. Michigan. Oolites are now
chert (SiO2) but were most likely originally deposited as carbonate
(CaCO3). Jolter’s Key in the Bahamas may be a modern analog for the
original depositional environment.
Modern habitat of ooids
• Jolter’s Cay in
Bahamas (Island
in center of
picture). Modern
ooids form in the
warm, shallow
waters in the lee
of the island
Fig. 8.29
SEM photographic of
chert showing the
sponge spicules that
make up the bulk of the
rock. Magnification
160x.
Fig. 8.23
Fig. 8.24
Continental growth by
accretion of small
plates (“strange
terrains”). Note the
“suture” zone between
the two colliding
granitic masses.
The following slides of
E. Africa show a
modern “aulacogen” in
the process of
developing.
Fig. 8.26
Another product of a
failed rift, the midcontinent gravity high
thought to be a result of
a failed arm back in the
Keweenawan (1Bya).
The floor of the high is
largely dense basalts that
poured out of the upper
mantle before the arm
failed, again similar to
what is happening in E.
Africa today.
Fig. 8.33
Global distribution of
late Proterozoic
(Varangian) glacial
deposits (triangles)
showing their
occurrence in
equatorial regions. The
glacial deposits are
interbedded with
limestones which
further suggest a low
latitude origin. The
Earth may have
narrowly escaped
freezing over
completely in the
Varangian.
Fig. 8.31
Mud cracks in red
shales in the Chuar
Group of the Grand
Canyon. 1.8 Bya.
Rocks like these
indicate hot, dry
conditions
(mudcracks) while
the red color
indicates that there
was not enough
oxygen in the
atmosphere to turn
the rocks rusty red.
Fig. 8.32
Laminated
mudstone with
scattered pebbles
and sand grains
dropped from
above. Gowganda
Formation, Blind
River Ontario.
This textures
suggests the stones
dropped from a
drifting iceberg.
Fig. 8.34a
Fig. 8.34b
Fig. 8.5
Pillow basalts in Archean “greenstones” 15 km west of Marquette,
MI. “Protusions” on lower side of several of the pillows indicate
(point to) bottom.
Fig. 8.21
Fig. 8.25
Fig. 8.27
Fig. 8.4
Early field geologists working on Lake Mistassini, Quebec, 1885.