MondayNovember 26,

2012(Discussion and WS - The

Precambrian Eon:Formation of Continents

and Steps Leading to Higher Life Forms)

The Launch PadMonday, 11/26/12

Which of these organisms lived during the Precambrian Eon?

Announcements

Assignment Currently Open

Summative or

Formative?Date Issued Date Due Date Into

GradeSpeed Final Day

Quiz 11 S1 11/9 11/9 11/30Lab – The

Geologic Time Scale

F3 11/14 11/16 11/30

Quiz 12 S2 11/16 11/16 12/7

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The Precambrian Eon is the large span of time in Earth's history

before the current Phanerozoic Eon, and is further divided into the

Archaen and the Proterozoic Eons.

The Precambrian begins with the formation of Earth about

4570 Ma (million years) ago, and ends with the beginning of

the Cambrian Period, about 541.0 ± 1.0 Ma, when macroscopic hard-shelled animals first appeared in

abundance.

The Precambrian accounts for 88% of geologic time.

The Precambrian Eon

Not much is known about the Precambrian, despite its making up

roughly seven-eighths of the Earth's history, and what little is known has largely been discovered

in the past 50 years.

The Precambrian fossil record is poor, and those fossils present

(e.g. stromatolites ) are of limited biostratigraphic use.

This is because many Precambrian rocks are heavily metamorphosed,

obscuring their origins, while others have either been destroyed

by erosion, or remain deeply buried beneath Phanerozoic strata.

The Precambrian Eon

Figure 11.19

The Precambrian Eon

The Precambrian Eon Much of Earth’s

stable continental crust was created

during the Precambrian.

Partial melting of the mantle formed

volcanic island arcs and ocean

plateaus.

The Precambrian Eon These crustal

fragments collided and accreted to

form larger crustal provinces.

Larger crustal areas were

assembled into larger blocks

called cratons.Cratons form the core of modern

continents.

Formation of Continental Crust

According to one model, the growth of large continental masses were accomplished through the collision and

accretion of various types of terrains, including volcanic arcs and oceanic plateaus.

The Extent of Crustal Materials Remaining From the Archean and Proterozoic Eons

Map showing the major geological provinces of North America and their ages in billions of years

(Ga). It appears that North America was

assembled from crustal blocks that were joined

by processes very similar to modern plate

tectonics. These ancient collisions produced

mountainous belts that include remnant island

arcs trapped by colliding continental fragments.

The Precambrian Eon The

Supercontinents The supercontinents

were large landmasses that consisted of all, or

nearly all, of the existing continents.

Pangaea was the most recent supercontinent, but perhaps an even larger one, Rodinia,

preceded it.Splitting and

reassembling of supercontinents have

generated most of Earth’s major mountain

belts.Supercontinents have

also profoundly affected Earth’s climate over

time.

Possible Configuration

of the Supercontinent

Rodinia

Figure 12.15

The Precambrian Eon It was during the Precambrian Eon

that life first began on the

Earth.Although the

Precambrian Eon contains some 88%

of Earth's history, its fossil record is poor because organism were soft-bodied during this time, resulting in little

remaining evidence.

The Precambrian Eon The majority of Precambrian fossils are stromatolites that

are often heavily metamorphosed or deeply

buried.However, preserved cells have been discovered at selective sites, such as the 2.0 Bya Gunflint Formation Jasper stromatolites

from Gunflint Formation near Mackies, northern Ontario.

The Precambrian Eon

The first primitive prokaryotic single-celled organisms appeared in the

oceans in the form of bacteria

(eubacteria or Achaea).

The earliest life forms were prokaryotes

that evolved in the seas, possibly as early as 3.8 Ga.

The Precambrian Eon Earth’s first living organisms were probably

chemotrophs existing in an anoxic world and producing H2S or CO2.

The Precambrian Eon Nearly 3.5 Ga, photosynthezing

cyanobacteria began releasing oxygen into the atmosphere as a by-product of the

process of photosynthesis.

The Precambrian Eon The first multi-cellular organisms appeared

toward the end of the Precambrian Eon, sometime prior to 542 Ma.

The Precambrian Eon When the eukaryotes (single-celled

organisms with a nucleus) evolved through endosymbiosis is disputed, with claims as

early as 3.4 Ga, but with less equivocal fossils dating from 1.8 to 0.8 Ga.

The Precambrian Eon With the eukaryotes comes sexual

reproduction, enabling genetic diversity and the concomitant ability to adapt to and

survive environmental changes.

WorksheetThe Precambrian Eon:

Formation of Continents and Steps Leading to

Higher Life Forms