Geologic time ( الزمن الجيولوجي )

Determining geological ages Determining geological ages Relative age dates (التأريخ النسبي) – placing rocks and geologic events in their proper sequence Relative age dates (التأريخ النسبي) – placing rocks and geologic events in their proper sequence of formation of formation Numerical dates (التأريخ المطلق) – define the actual age of a particular geologic event (termed absolute age dating) Numerical dates (التأريخ المطلق) – define the actual age of a particular geologic event (termed absolute age dating)

Steno’s Laws Nicolaus Steno, 1669: Nicolaus Steno, 1669: Principle of original horizontality Principle of original horizontality Principle of superposition Principle of superposition Principle of original continuity Principle of original continuity Powerful tools for: Powerful tools for: Analysis of outcrops. Analysis of outcrops.

Principle of Original Horizontality ( قانون التأفق المبدئي ) Sedimentary rocks are formed in layers (strata) which were originally horizontal Sedimentary rocks are formed in layers (strata) which were originally horizontal Flat strata are probably undisturbed by folding and faulting Flat strata are probably undisturbed by folding and faulting Tilted strata have been affected by tectonics. Tilted strata have been affected by tectonics.

Principle of Original Horizontality

Principle of Superposition ( قانون تعاقب الطبقات ) Principle of Superposition ( قانون تعاقب الطبقات ) Developed by Nicolaus Steno in 1669 Developed by Nicolaus Steno in 1669 In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top

Principle of Superposition Oldest sedimentary rocks at the bottom: C deposited first Oldest sedimentary rocks at the bottom: C deposited first Younger sedimentary rocks on top: B then A. Younger sedimentary rocks on top: B then A.

Superposition illustrated by strata in the Grand Canyon

Principle of cross-cutting relationships ( قانون التقاطع ) Magma under pressure squeezes into nearby rock whenever fractures form in the rock Magma under pressure squeezes into nearby rock whenever fractures form in the rock Then the magma slowly cools and becomes rock itself. Then the magma slowly cools and becomes rock itself. This newly formed igneous rock is, of course, younger than the rock it intruded This newly formed igneous rock is, of course, younger than the rock it intruded Younger features cut across older ones. Younger features cut across older ones. When a fault cuts through other rocks, or when magma intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected. When a fault cuts through other rocks, or when magma intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.

Principle of cross-cutting Cross cutting igneous rocks are younger than what they intrude. Cross cutting igneous rocks are younger than what they intrude. Faults are younger than what they cut. Faults are younger than what they cut.

Principle of Inclusion ( قانون الجذة القاطعة ) Inclusions are pieces of one rock unit that are contained within another. Inclusions are pieces of one rock unit that are contained within another. Erosion surfaces exist in the rock record... Erosion surfaces exist in the rock record... Fragments within strata above are derived from the older strata below  they are older than the strata containing them. Fragments within strata above are derived from the older strata below  they are older than the strata containing them. The strata containing the fragments are younger than the strata the fragments came from The strata containing the fragments are younger than the strata the fragments came from

Included Fragments If a rock contains a fragment or particle of another rock, then the fragment is older than the rock. If a rock contains a fragment or particle of another rock, then the fragment is older than the rock. Example Example A pebble that is part of a conglomerate rock is older than the rock. The pebble had to exist first in order to become part of the conglomerate when it formed. A pebble that is part of a conglomerate rock is older than the rock. The pebble had to exist first in order to become part of the conglomerate when it formed.

Conglomerate (sedimentary)

Breaks in stratigraphic record Sometimes the sedimentation stops and the strata become exposed at the surface Sometimes the sedimentation stops and the strata become exposed at the surface The beak in deposition may result from a regional uplift or sea level falling The beak in deposition may result from a regional uplift or sea level falling The strata exposed at the surface are subjected to weathering and erosion The strata exposed at the surface are subjected to weathering and erosion Sedimentation restarts when the area becomes submerged in water again Sedimentation restarts when the area becomes submerged in water again The breaks (gaps) are marked by a rough and eroded surface called unconformity The breaks (gaps) are marked by a rough and eroded surface called unconformity Unconformity is an indication of environmental conditions which cause the sedimentation to stop. Unconformity is an indication of environmental conditions which cause the sedimentation to stop.

Unconformities (loss of rock record) ( عدم التوافق ) An unconformity is a break in the rock record produced by erosion and/or nondeposition An unconformity is a break in the rock record produced by erosion and/or nondeposition Types of unconformities Types of unconformities Angular unconformity – tilted rocks overlain by flat- lying rocks (عدم التوافق الزاوي) Angular unconformity – tilted rocks overlain by flat- lying rocks (عدم التوافق الزاوي) Disconformity – strata on either side of the unconformity are parallel Disconformity – strata on either side of the unconformity are parallel Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement)

Conformable Contact ( متوافقة ) Layers of rock that have been deposited without any interruption. Layers of rock that have been deposited without any interruption. No gaps in time. No gaps in time. No missing record due to erosion, non- deposition, etc. No missing record due to erosion, non- deposition, etc.

Formation of an angular unconformity

An angular unconformity at Siccar Point, England Unconformity

Cross Cutting Relationships in strata

Development of a Nonconformity

Nonconformity in the Grand Canyon - Strata deposited over Schist

Disconformity A break in the rock record across which there is little change in stratal orientation. A break in the rock record across which there is little change in stratal orientation. Often just a pause in deposition (subtle). Often just a pause in deposition (subtle). May also be obvious erosion surface. May also be obvious erosion surface.

Correlation of rock layers Matching strata of similar ages in different regions is called correlation Matching strata of similar ages in different regions is called correlation

Correlation of strata in southwestern United States Sections are incomplete

Correlation of rock layers with fossils Correlation relies upon fossils Correlation relies upon fossils Principle of fossil succession – fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it Principle of fossil succession – fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it

Determining the ages of rocks using fossils

Geologic time scale The geologic time scale – a “calendar” of Earth history The geologic time scale – a “calendar” of Earth history Subdivides geologic history into units Subdivides geologic history into units Originally created using relative dates Originally created using relative dates Structure of the geologic time scale Structure of the geologic time scale Eon – the greatest expanse of time Eon – the greatest expanse of time

Geologic Timescale Divisions based on fossils

Geologic time scale Structure of the geologic time scale Structure of the geologic time scale Names of the eons Names of the eons Phanerozoic (“visible life”) – the most recent eon, began about 540 million years ago Phanerozoic (“visible life”) – the most recent eon, began about 540 million years ago Proterozoic Proterozoic Archean Archean Hadean – the oldest eon Hadean – the oldest eon

Geologic time scale Structure of the geologic time scale Structure of the geologic time scale Era – subdivision of an eon Era – subdivision of an eon Eras of the Phanerozoic eon Eras of the Phanerozoic eon Cenozoic (“recent life”) Cenozoic (“recent life”) Mesozoic (“middle life”) Mesozoic (“middle life”) Paleozoic (“ancient life”) Paleozoic (“ancient life”) Eras are subdivided into periods Eras are subdivided into periods Periods are subdivided into epochs Periods are subdivided into epochs

Geologic time scale Precambrian time Precambrian time Nearly 4 billion years prior to the Cambrian period Nearly 4 billion years prior to the Cambrian period Not divided into small time units because the events of Precambrian history are not know in detail Not divided into small time units because the events of Precambrian history are not know in detail Immense space of time (Earth is ~ 4.5 Ga) Immense space of time (Earth is ~ 4.5 Ga)

Radioactivity (Used to age date rocks) Radioactivity (Used to age date rocks) Spontaneous changes (decay) in structure of atomic nuclei Spontaneous changes (decay) in structure of atomic nuclei Types of radioactive decay Types of radioactive decay Alpha emission Alpha emission Emission of 2 protons and 2 neutrons (an alpha particle) Emission of 2 protons and 2 neutrons (an alpha particle) Mass no.= +4 Mass no.= +4 Atomic no.=-2 Atomic no.=-2 Beta emission Beta emission An electron (beta particle) is ejected from the nucleus An electron (beta particle) is ejected from the nucleus Mass no.= constant Mass no.= constant Atomic no.=+1 Atomic no.=+1 Electron capture Electron capture An electron is captured by the nucleus An electron is captured by the nucleus The electron combines with a proton to form a neutron The electron combines with a proton to form a neutron Mass no.= constant Mass no.= constant Atomic no.=-1 Atomic no.=-1

Neutron capture (A) and Beta emission (B) Neutron capture (A) and Beta emission (B)

Using radioactivity in dating Using radioactivity in dating Parent – an unstable radioactive isotope Parent – an unstable radioactive isotope Daughter product – isotopes resulting from decay of parent Daughter product – isotopes resulting from decay of parent Half-life – time required for one-half of the parent isotope in a sample to decay Half-life – time required for one-half of the parent isotope in a sample to decay

A radioactive decay curve A radioactive decay curve

Dating with carbon-14 (radiocarbon dating) Dating with carbon-14 (radiocarbon dating) Half-life only 5730 years Half-life only 5730 years Used to date very young rocks Used to date very young rocks Carbon-14 is produced in the upper atmosphere Carbon-14 is produced in the upper atmosphere Useful tool for geologists who study very recent Earth history Useful tool for geologists who study very recent Earth history

Using radioactivity in dating Importance of radiometric dating Importance of radiometric dating Allows us to calibrate geologic timescale Allows us to calibrate geologic timescale Determines geologic history Determines geologic history Confirms idea that geologic time is immense Confirms idea that geologic time is immense