Evolution of the earth

8 - Origin of Continental Crust• Main Topics – Earth cooled sufficiently to permit formation of early continental granitic material – Isotopic age dates within continents “cluster” sugge

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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

High-grade metamorphic rock (gneiss) typical

of ancient “shield” regions

Sondre Stromfjord,

SW Greenland

Age of rocks in this

picture are ~3.8 By

Cryptozoic (“hidden life”) Eon

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

Fig 8.13

Hypothetical scenario for assembly of N American craton during Proterozoic Based on dates and tectonic patterns in previous

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

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

graywacke arkose quartzite

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

(mono-minerallic, 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

One example of a classification chart for sedimentary rocks

• 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

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 QFL = Quartz, Feldspar, Lithic fragments

SEDIMENTARY DEPOSITIONAL ENVIRONMENTS

“Long” vs “short” system models for sedimentary deposition

environments Note both systems eventually result in submarine fans but

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

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

Modern algae from Shark Bay

Australia They survive in the

hypersaline lagoons because

predators cannot tolerate the high

Shark Bay – A Glimpse into the Archean

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).

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

Fig 8.26

Another product of a

failed rift, the

mid-continent 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.

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

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.