The Geologic Timescale A calendar of geologic time

Subdivisions of Human Time Millennium Century Decade Year Month Day longer shorter

Subdivisions of Geologic Time Eon Era Period Epoch Stage Substage longer shorter

0 Ma Phanerozoic 540 Ma C Modern Geologic Time Scale M P Proterozoic Four

0 Ma Phanerozoic 540 Ma Proterozoic C M P Modern Geologic Time Scale “Visible

P Pleistocene Pliocene Miocene Oligocene Eocene Paleocene Cretaceous Mesozoic Proterozoic 2500 Ma Holocene Paleogene

What records the passing of geologic time? • Formation of rock layers • Sediments

Grand Canyon Rock layers are grouped into formations and given formal names by geologists.

Problem - how do you determine the order in which rock layers formed? At

circa 1790 post-Diluvial Tertiary Secondary Transitional Primary Earth History, 1700’s Flood Gravels Layers composed

Within a local region, rock layers can be correlated on the basis of their

Cretaceous System D’Omalius d’Halloy, 1822

Regional correlation of chalk deposits based on lithology.

circa 1790 circa 1820 British Isles post-Diluvial Continental Europe alluvium gravels Sicilian strata Tertiary

Correlation - the matching-up of rock layers between different places. • We can put

How can we correlate different systems if the layers cannot be correlated based on

William Smith (1769 -1839) surveyor, civil engineer

Smith made the first large scale geologic map showing the distribution and order of

In his work as a surveyor, Smith noticed that the rock layers seemed to

Goes Extinct T I M E Unique interval of time Exists Evolves

Fossils are the key to correlating regional systems Great Britain Continental Europe

Geologic Systems and Geologic Time • Once a particular regional system was formally named

Geologic Systems and Geologic Time • For example, the Jurassic System was originally named

circa 1790 circa 1870 British Isles post-Diluvial Transitional Modern Time Scale alluvium Quaternary gravels

How do we assign sedimentary layers to their correct place in time? • fossils

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The Geologic Timescale A calendar of geologic time

Subdivisions of Human Time Millennium Century Decade Year Month Day longer shorter

Subdivisions of Geologic Time Eon Era Period Epoch Stage Substage longer shorter

0 Ma Phanerozoic 540 Ma C Modern Geologic Time Scale M P Proterozoic Four Eons of Geologic Time 2500 Ma Archean 3800 Ma Hadean 4600 Ma

0 Ma Phanerozoic 540 Ma Proterozoic C M P Modern Geologic Time Scale “Visible Life” “Earlier Life” 2500 Ma Archean “Ancient Eon” 3800 Ma Hadean 4600 Ma “Hidden Eon”

P Pleistocene Pliocene Miocene Oligocene Eocene Paleocene Cretaceous Mesozoic Proterozoic 2500 Ma Holocene Paleogene Neogene Cenozoic 540 Ma M Quat. Phanerozoic Modern Geologic Time Scale C Tertiary 0 Ma Jurassic Triassic Archean 3800 Ma Carb. Paleozoic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian Hadean 4600 Ma Ma 0. 01 1. 6 5 23 35 57 65 146 208 245 290 323 360 408 439 510 540 Quat. = Quaternary Carb. = Carboniferous RIP Eons

What records the passing of geologic time? • Formation of rock layers • Sediments are deposited over time in layers. • Each layer traps and records information about the time during which it formed. • Sedimentary layers are analogous to the pages that compose the book of Earth History.

Grand Canyon

Grand Canyon Rock layers are grouped into formations and given formal names by geologists.

Problem - how do you determine the order in which rock layers formed? At a single place, layers can be ordered using the law of Superposition. youngest even less old oldest East Devonshire time younger

circa 1790 post-Diluvial Tertiary Secondary Transitional Primary Earth History, 1700’s Flood Gravels Layers composed of unconsolidated sediment Rock layers with abundant fossils Partly crystalline rock layers with sparse fossils Crystalline rock

Within a local region, rock layers can be correlated on the basis of their lithology (physical characteristics) to define a geologic system. SE Coast of England NW Coast of France

Cretaceous System D’Omalius d’Halloy, 1822

Regional correlation of chalk deposits based on lithology.

circa 1790 circa 1820 British Isles post-Diluvial Continental Europe alluvium gravels Sicilian strata Tertiary London clay Parisian gypsum beds English chalk Secondary Oolites Lias Coal Measures Parisian chalk Perm strata Muschelkalk - Trias Magnesian Limestone New Red Sandstone Transitional Old Red Sandstone Devonshire strata Welsh Greywackes Mountain Limestone Wenlock Limestone Primary Crystalline (metamorphic) strata Jura Mt. strata Geologic Systems

Correlation - the matching-up of rock layers between different places. • We can put local rock layers in the correct time order because we can see how they are stacked on each other. • We can use the physical features of rock layers to correlate them into a regional system. • The Problem: How can we correlate different regional systems so that they are in the correct time order if we can’t directly match their layers?

How can we correlate different systems if the layers cannot be correlated based on their physical features? ? ? Great Britain Continental Europe

William Smith (1769 -1839) surveyor, civil engineer

Smith made the first large scale geologic map showing the distribution and order of rock layers in Great Britain.

In his work as a surveyor, Smith noticed that the rock layers seemed to contain a unique sequence of fossil species that appear and disappear through time. Even when the rocks look different, the sequence of fossils is always the same.

Goes Extinct T I M E Unique interval of time Exists Evolves

Goes Extinct Exists T I M E Evolves

T I M E

Fossils are the key to correlating regional systems Great Britain Continental Europe

Geologic Systems and Geologic Time • Once a particular regional system was formally named and its fossils described, other regional systems with the same fossils were correlated to it and given the same name. • The original system names thus came to stand for particular intervals of geologic time.

Geologic Systems and Geologic Time • For example, the Jurassic System was originally named for the rocks and fossils of the Jura Mountains between France and Switzerland. • Now the Jurassic Period refers to the time interval during which the fossil species of the Jurassic System lived. • Any rock layers with these fossils can be identified as Jurassic in age.

circa 1790 circa 1870 British Isles post-Diluvial Transitional Modern Time Scale alluvium Quaternary gravels Sicilian strata Tertiary Secondary Continental Europe Tertiary London clay Parisian gypsum beds English chalk Parisian chalk Cretaceous Oolites Lias Jura Mt. strata Jurassic New Red Sandstone Muschelkalk - Triassic Magnesian Limestone Perm strata Permian Coal Measures Mountain Limestone Carboniferous Old Red Sandstone Devonshire strata Devonian Wenlock Limestone Silurian Welsh Greywackes Ordovician Cambrian Primary Crystalline (metamorphic) strata Precambrian

How do we assign sedimentary layers to their correct place in time? • fossils • each time interval in Earth history is defined by a unique set of species that existed at that time. • Species evolve, live for a short time, and go extinct. • The same species never evolves twice (extinction is forever). • Evolution provides a “biological calendar” that geologists use to keep track of time. • Fossils allow us to put the individual scenes from Earth history into the correct order to tell the full story.