The Geologic Time Scale

The Geologic Time ScaleGeologic time is divided into Eons.

1. Precambrian Eon

a) Archean Eon

b) Proterozoic Eon

2. Phanerozoic Eon

The Geologic Time ScaleEons are further divided into Eras.

1. Archean Eon• Eoarchean Era• Paleoarchean Era• Mesoarchean Era• Neoarchean Era

2. Proterozoic Eon• Paleoproterozoic Era• Mesoproterozoic Era• Neoproterozoic Era

3. Phanerozoic Eon• Paleozoic Era• Mesozoic Era• Cenozoic Era

The Geologic Time ScaleThe Phanerozoic Eon is further divided into Periods.

1. Phanerozoic Eon• Paleozoic Era

• Cambrian Period• Ordovician Period• Silurian Period• Devonian Period• Carboniferous Mississippian Period• Carboniferous Pennsylvanian Period• Permian Period

• Mesozoic Era• Triassic Period• Jurassic Period• Cretaceous Period

• Cenozoic Era• Tertiary Period• Quaternary Period

The Geologic Time ScaleThe Cenozoic Era is further divided into Epochs.

• Cenozoic Era• Tertiary Period

• Paleocene Epoch• Eocene Epoch• Oligocene Epoch• Miocene Epoch• Pliocene Epoch

• Quaternary Period• Pleistocene Epoch• Holocene Epoch

Early Evolution of EarthFormation of Earth’s Layered

Structure o As Earth formed, the decay of

radioactive elements and heat from high-velocity impacts caused the temperature to increase.

o Iron and nickel began to melt and sink toward the center

o Lighter rocky components floated outward, toward the surface

o Gaseous material escaped from Earth’s interior to produce the primitive atmosphere

The Precambrian Eon The Precambrian

Eon, which is divided into the Archean and the Proterozoic Eons, spans about 88% of

Earth’s history. Much of Earth’s stable continental crust was created during this time.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 Ba 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 .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.