Oline Quiz parts 1 & 2

Online Quiz part 3

Part.1: Mountains Building

The origin of continents Theories

The birth of the moon: The Pacific ocean is a scar from which the moon was torn

Catastrophic events: continental crust broken & redistributed by Catastrophic events

The expanding theory: the earth was expanded to double its original size, & the ocean basins formed as cracks between the ruptured continents

Continental drift: Parallelism of outlines of the opposing coasts of the S-Atlantic was observed & In 1859, the first map fitting S-America against Africa was published → the concept of large-scale rearrangement of continents, or Continental drift

A geosyncline

elongate belt of thick strata that linked closely with the formation of mountains

It shows tightly compressed folds

low angle overthrust faults

Most contain granitic batholiths & regionally metamorphosed rocks

Developed within structurally unstable orogenic belts located between stable regions, such as:
  – Modern oceanic island arc-trenches
  – Volcanic arcs, especially in midoceans

The Geosynclinal Concept

Hall’s Theory
1. Thick sedimentation along the edge of the continent depressed the crust as deposition occurred
2. Down bending crust stretched & faulted
3. Overlying sediments compressed & folded
4. Last result that a topographic mountains were raised at the surface

Dana’s theory
1. Bending of crust as the interior cooled & shrinkage
2. Bending was concentrated at continental margins where erosion of upraised genaticline provided sediment to an adjacent geosyncline
3. Finally, crust failed, mountains raised, & the geosyncline was welded to the continent
Repetition of this process caused growth of the continent through mountain building

Until now, mountain belts are primarily the results of profound (deep) lateral compression across relatively narrow zones

type of forces required to Mountain Building

1. A cooling & Shrinkage of Earth
2. Continental Drift
3. Thermal Convection

A cooling & Shrinkage Earth

proposed by Dana

The earth cooling from a molten origin, so
1. The earth must be contracting
2. cold, & rigid crust must be bend or ruptur as interior shrinks

The most of the compressive adjustment of the crust would occur at the boundaries between continents & ocean basins, exactly where N-American mountains occur now

Thermal Convection

Proposed by A. Holmes

Mechanism: If more heat generated in one portion of deep mantel due to irregular distributions of isotopes

1. This material plastic flow upward, & cool
– ruptured the crust

2. then flow laterally beneath the lithosphere
– drag the crust along conveyor belt fashion

3. as it cool the mantle below, it sink (move downward)
– buckle up to form mountains

Continental Drift

Taylor suggested that Drifting continents had caused wrinkling of the crust to produce Cenozoic mountain systems

mechanism:
1. The leading edges of the moving continents depressed the oceanic crust a head to form troughs in which sediments could accumulate
2. Further movement then compressed & up-heaved those strata to produce the present mountains rimming the Pacific

The catastrophic tidal action as a force source was rejected

Up-heaval of the Alpine-Himalayan resulted  by the direct collision of continents

Part.2 : Paleomagnetism

Paleomagnetism-Drift’s Renaissance

Young lavas should show magnetization parallel to that of the earth’s present field

The magnetic minerals retain their fossil, or magnetism unless heated above the demagnetization temperature (500ºC)

many rocks, containe these minerals today retain evidence of orientation of magnetic field when & where they were formed

Part.3 : Sea Floor

General Nature of the Sea Floor

Before 1960s, geologists assumed that the crust beneath ocean basins was:
1. very old
2. topographically feature less
3. structurally tranquil
4. essentially fixed in place
– All assumptions appear to be incorrect

Profiles established by reflection of low-frequency waves from shown that:
1. Ocean floor not smooth & characterized by ridges, deep trenches, & sea mountains
2. rugged & younger than continental crust

Oceanic Ridges

Oceanic ridges: zones of extension along which the crust is being to torn open
– scars of a pre-drift configuration of crust
– made mainly of basaltic lavas
– Show a high volcanic activity
– display shallow seismicity beneath axes
– characterized by greater than average heat flow through the crust along their axes
– had a narrow depressions that extends along their axis for thousands of kilometers

Submarine ridges: The most striking features of ocean floor (Atlantic & Indian)

Rift: any branch in ridge (Ridge patterns may be complicated by branching)

Triple Junction: juncture of 3 separate rifts

sea-floor spreading hypothesis

Proposed by Hess

2 opposing convection cells rising beneath ocean ridges cause the abnormal heat flow & produce tension in crust

convective flow laterally away from ridge axes causing the spreading of sea floors

Examples/application for Hess hypothesis:
1. Atlantic & Indian Oceans
2. E-Africa & Arabia experiencing the beginning of new phase of disruption in response to the shift of mantle convection
3. Spreading along the extension of W-Indian Ocean tore open the Aden rift in crust

Mechanism by which rising hot mantle plumes drive rifting & continental drifting

1. Continents situated over mantle (hot spot)
2. Uplift or flat continental plate form triple junction with 3 cracks, & igneous dikes
3. As 3 cracks get wider, 2 arms open up to form ocean basin & third (failed) arm becomes sediment-filled trough (Aulacogen)

Aulacogen is a trough formed by a rift that has failed to develop
– Ex. southward-trending East African rift

E-African rifts are tectonically active, they have not opened up as has Red Sea-Aden system, because They have continental crust beneath rather than oceanic

The Aulacogens have a special important
1. Many great rivers flow down it
2. Important petroleum resources
3. Ore deposits formed where hot solutions rise along the faults bordering the troughs

Confirmation of Sea-Floor spreading

Wilson proposed that oceanic islands symmetrically distributed as to relative age outward from ridges: youngest islands near the axes of ridges, & oldest away

Sea-floor Spreading was confirmed by magnetic Anomalies

anomalies strips show 2 striking features
1. Parallel closely the ridge axis
2. Have bilateral symmetry (mirror plane)

anomalies reflect successive reversal of polarity of the magnetic field while the sea floor was spreading away from the ridges

Plate Collisions

Erosion able to reduce continents to sea level in a mere 10Ma, the persistence of continents requires some rejuvenation mechanism

Isostasy equilibrium among the larger units of the crust according to their total mass
1. thinner, denser units (ocean) stand lower
2. thicker, less dense (continents, mountains) stand higher

Crust sink if load of sediment or ice added
Crust rise if rock were removed by erosion

Sedimentary basins

Areas of thick sediments vary in:
1. size
2. shape
3. tectonic setting

Sedimentary basins include
1. trench, forearc, & foreland
3. intracratonic, & passive margin
4. rift or aulacogen basins

Accumulation of thick sediments require one or both of the following
1. sea level rose
2. underlying lithosphere subsided during deposition

The sea level variation can not explain the presence of thick sediments, so they are indirectly related to plate tectonics

Causes of Subsidence

1. Subduction subsidance
2. Thermal or cooling subsidence
3. Crustal-thinning subsidence
4. Sediment-loading subsidence
5. Thrust loading

Subduction subsidance: caused by the profound forcible depression of one lithosphere plate as it is subducted beneath another

Thermal or cooling subsidence: caused by the cooling of a lithosphere plate as it moves away from a hot, spreading ridge

Crustal-thinning subsidence: can provide space for thick sediment accumulation as a result of isostatic subsidence of any crust that has been thinned

Sediment-loading subsidence: This type of loading is limited by the initial depth of water & ratio of densities of the loading material & of the crust & mantle beneath

Thrust loading: occurs when a collision of 2 lithosphere plates happened, so over- thrusting of thick slabs of rock onto the edge of a craton formed.

Thick sediment can accumulate in the foreland sedimentary basin formed by the resulting isostatic subsidence

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Distribution of the elements

Only ¼ of 100 known elements are common

> 95% of the universe being composed of only the 2 lightest elements (H & He)

15-20 elements can be called constituents of the earth, & some aren’t very conspicuous

major elements in solid earth & meteorites: H, C, N, neon Ne, & argon Ar

Helium (He) is almost totally missing from the earth because it has been lost to space

why is there any H (lighter than He!)? Because much of the earth‘s original H combining with O to make relatively heavy water molecules

The abundance of water is the most unique chemical characteristic of the earth

Probable Origin of the Earth

1. Buffon hypothesis gravitational pull of passing comets had torn away hot masses from sun, then cooled to form planets
– rejected because:
   So many near collisions are improbable
   Comets have very week gravity fields

2. Solar nebula condensation of the planets from a hot, gaseous cloud (surrounding the sun) rather than their being derived from the sun itself

3. aggregation of cold clouds of dust & gases after 1900, the internal heating of the earth occurred after its aggregation, The energy produced by such heating continues to disturb the earth, as evidenced by earthquakes, volcanoes, & mountains

4. Planetesimal hypothesis: first important suggestion of a cold origin of the planets, close passage of another star past our sun, The pull of intruder‘s gravity field extracted solar gaseous material, which first condensed in space to form small, & solid bodies (planetesimals) & Then number of these cold asteroid-like aggregated to form planets

In the mid-20th century, von Weizacker & Kuiper modified the planetesimal concept they tried to explain the simultaneous origin of the entire solar system in a unified nebular hypothesis

nebular hypothesis

Heat & Composition distribution

Atmosphere

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Dating of the Earth

Early Attempts to Date the earth

Scriptures: the earth was about 6000yr old

Buffon in 1760: the earth was 75,000yr old

based on the uniformity doctrine, Hutton, Lyell thought that the earth must be far older than the above dates

C. Darwin in 1859 needs millions of years before the appearance of human beings based in his evolution theory

In 19th century, a British geologist estimated that all the time since the beginning of the Cambrian ≈ 75 MY, He based his conclusion upon the max. known thickness of strata of that age span multiplied by an assumed average rate of sedimentation (derived from study of modern depositional rates)

In 1899 Joly used the accumulation rate of salt in the sea, he found that it would taken 100MY to develop the present salinity

All the above attempts at assessing the age of the earth in terms of numerical time were based upon shaky assumptions, just all of them showed an evolution of thought in one direction, toward older & older estimates

In middle of 19th century, most geologists thought that the earth date is several hundred MY but they had no numerical method to prove that

Kelvin’s Dating

Deep mines showed that Temperature increase with depth, this leads to one fact that the earth losing heat from its interior

The heat lose could be a result of:
1. It cooling after an initial, very hot stage
2. It contains internal source of heat energy

Lyell appealed to chemical reactions in the interior to produce heat in an endless cycle, allowing for a steady-state earth

In 1846 Kelvin refused Lyell assumption because he reasoned the heat to another assumption that earth originally molten

By using the rate of cooling of the earth backward to a time when the earth was molten, Kelvin thought that the total age of the earth is between 20-30MY

Chamberlin Dating

In 1899 Chamberlin challenged Kelvin assumption that the earth had begun as a molten body

Chamberlin introduced new hypothesis that planets formed by the accretion, or collecting together, of cold, solid chunks mater

So earth must have heated up sometime after its initial formation via some internal process quite independent of solar heat

Summery

Scriptures → 6000yr old

Buffon → 75,000yr old

Hutton, Lyell →>> 75,000yr

C. Darwin → millions of years “evolution”

British geologist →preambrian 75 MY, based on average rate of sedimentation

Joly → 100MY “accumulation rate of salt”

19th, Most geologists → several hundred MY

Kelvin → 20-30MY “cooling rate of the earth”

Radioactivity

Important Terms

Radioactivity: elements changing into other by emitting charged particles (Spontaneous changes in structure of atomic nuclei)

Isotopes: elements have the same atomic number & almost same chemical properties, but atomic weight are different (designated by Mass Number)

Atomic Number: number of proton

Mass Number: number of the total sum of neutrons & protons, written at the top left of the chemical symbol

Parent isotopes: isotopes with unstable nuclei, & undergo spontaneous changes until a permanent, stable configuration is reached

Daughter isotopes: isotopes with stable configuration nuclei

Radioactive decay: process of change Parent to Daughter isotope

Dating: determining the ratio of parent to daughter atoms & calculating a mineral’s age by multiplying by decay rate of the parent & This leads to half-time units

half-time time required for one-half of the parent isotope in a sample to decay

Radioactivity

discovered by Henri Becquerel & Curies in 1896, by the films story

2 unknown elements were soon discovered (Radium & Polonium) which form from U by changes in the atomic nucleus that involve unknown types of radiation & This lead to the Isotope definition

Radioactive decay

results in one or more of 3 types of Emissions from the nucleus at a fixed average rate characteristic of any isotope

The Emission types
1. Alpha rays: helium nucleus (He+2e-)
2. Beta rays: free electrons (e-)
3. Gamma rays: similar to X rays

The radioactive decay types
1. α-emission: emission α-particle, 2p⁺+2n±
2. β-emission: e- ejected from the nucleus
3. Electron capture: e- captured by nucleus (combines with p⁺ to form n±)

RA-decay can be written like reaction
Unstability Isotope→ Stability Isotopes
Parent → Daughter + (α, β, γ, e) + heat

Emissions result in increasing or decreasing of the number of the neutrons
Ex. U238 decay α particles He, this reduce the neutrons & protons 2 of each, the resulting isotope has amass number 234 (Thorium)

First Dating of Minerals

In 1902, Rutherford & Soddy stated: total emission activity was proportional to number of unstable parent atoms

Based on this statement:
1. the emission decrease regularly which can be expressed mathematically
2. the progressive accumulation of daughter isotopes might provide basis for the numerical measurement of geologic time

Dating of Minerals

time of the crystallization of a mineral dated by isotopes (not the time of origin of isotope in the universe, which was earlier than origin of the earth), such as the time of incorporation of unstable isotope into that mineral

When mineral crystallized, a closed chemical system was formed such that any daughter product then present in the mineral was formed only from the decay of the original parent isotope therein

Accuracy of Isotopic Dates

There is several factors limit the accuracy of isotopic dates:
1. The statistical nature of decay even under the best conditions, only average decay rates are determinable (introduces uncertainty)
2. uncertainty originates in the laboratory analyses of the isotope ratios: mass spectrometer provides the most sensitive analyses of isotope abundance, with from ±0.2 to ±2.0% accuracy possible

Isotopic Time Scale

the bulk of isotopic dating has been confined to igneous or metamorphic rocks, Because most datable minerals containing radioactive elements originate in such rocks

Dating sedimentary rocks by isotopes: date of crystallization of the minerals in parent rocks prior to erosion, transportation & deposition in a sediment

Direct isotopic dates of sedimentary rocks are possible for only a few minerals that:
1. contain unstable species
2. crystallize in depositional environment
– Ex. Gluconite contain K40, so the Mineral can be dated by the K-Ar method

Because most dating must be done on igneous or metamorphic rocks, it necessary to relate such ages indirectly to the relative time scale in order to establish a numerical scale

the basic principles of stratigraphy come into play, especially when:
1. A volcanic rock interstratified within fossiliferous sediments
2. An intrusive igneous body cut or lay the sedimentary strata

Using radioactivity in dating importance of radiometric dating
– Allows us to calibrate geologic timescale
– Determines geologic history
– Confirms idea: geologic time is immense

radiocarbon dating

Is the Dating with carbon-14 (¹⁴C)
Half-life of ¹⁴C = 5730yr
Used to date very young rocks
¹⁴C is produced in upper atmosphere
Useful tool for geologists who study very recent Earth history

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Important concepts

Stratigraphy: the study of stratified rocks, their nature, occurrence, classification, & relationships to each other

Historical Geology: the study of the history of the earth

Historical Geology includes stratigraphy

History of The Relative Geologic Time scale

How geologic mapping in Europe led to the relative time scale

1. by the principle of superposition
2. by the principle of fossil correlation

After publication of Smith’s & Cuvier’s geological maps in the 1800s, the case was:

Wernerian Chronology continued to used
استمر استخدام التسلسل الزمني
Local names of strata became more numerous
اصبحت الاسماء المحلية للطبقات اكثر عددا
Naming of distinctive rock bodies was a natural by product of mining & mapping
كانت تسمية الاجسام الصخرية المميزة امرا طبيعيا بالتعدين والخرائط
Names developed as shorthand & reflected geographic localities or rock types
تم تطوير الاسماء بطريقة مختصرة وتعكس هذه الاسماء المواقع الجغرافية وانواع الصخور
these names was extended more widely as a result of the lateral continuity
تم تعميم هذه الاسماء على نطاق اوسع
At the same time fossils were being collected & widely separated strata were correlated
بنفس الوقت جُمعت الاحافير، وربُط طبقات منفصلة بالمقارنة
The result Geological map of all western Europe gradually developed
النتيجة: تطورت الخريطة الجيولوجية لاوروبا الغربية تدريجيا

Jura

A group of secondary strata (named for the Jura mountains of France & Switzerland)

When superposition principle applied, the Jura were found to overlie another group:
1. Trias in Germany
2. Cretaceous in France 
3. Carboniferous coal-bearing strata, Britian

in 1829 the name Quaternary was proposed to include the young deposits

This scheme displaced the older Wernerian scheme

Modern Relative Time Scale

In 1835 Sedgwick & Murchison named
1. Cambrian (roman name of Wales)
2. Silurian (for an ancient welsh tribe)
3. Devonian: Based on the correlations & fossils content they defined the relative age of Devonshire strata (between Silurian & Carboniferous)

Sedgwick proposed a large division which include smaller subdivisions (such as Paleozoic Era)
Mesozoic & Cenozoic Eras named Based on relative percentages of living species
The Permian Period was named next

abstract geological time & rock record

There is a clear distinctions between the abstract geological time & rock record by: time is continuous, but rock record is riddled with unconformities of varying magnitude

So what we known of history must be gleaned from the imperfect rock record

If elsewhere a more complete sequence of Cambrian rocks were found with fewer & smaller unconformities & more fossils, it might provide a better world standard of reference than does that of Wales, Thus, what we have are time & rock divisions

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The Formation

Important stratigraphical terms

Arranged from the largest to smallest
Group: rock unit consisting of more than one formations that are next to each other in a succession & related to each other
Formation: distinctive series of strata that originated through same formative processes, is the most basic local units of stratigrphy
Member: a part of a formation
Bed or strata: sedimentary rocks layer that is marked off above & bellow by surfaces that can be seen & made up of material that is the same in all parts
Lamina: thin layer (<10mm) in sedimentary, separated from material above & bellow it

Formation must be

distinctive in appearance in order to be easily recognizable

named for type locality where they normally displayed in well-exposed type section

Characteristics chosen to define a formation include one or more of the following (Lithology)

1. Composition of mineral grains
2. Color
3. Textural properties (Ex. size of grain)
4. Thickness & geometry of stratification
5. Character of any organic remains
6. Outcrop character

– Formation include > one stratum
– Group contains > one Formations

Problems arise when a new formation defined

1. The name
2. The type section
3. Designation of lower & upper limits
4. formations might vary in sediment type, or lithology, laterally or vertically, depending on deposit environments

• 

formations might vary

The stratigraphic divisions being named in Europe were universally present & of the same age worldwide

In another word as long the geologist found the same fossils in different & far stratified rocks they assigned the same age for both of them (Ex. the Cambrian strata in Wales)

This mean that Geologist thought that any named formation should laterally extend without change

This principle was the cornerstone of Werner’s time scale

In 1789 French writings show that: similarity of fossils in similar sedimentary rocks might reflect environmental factors rather than strict age equivalence

the sedimentary products of each environment have aunique characteristics, even though they accumulate at the same time & grade gradually into one another

The result in other words is: formations might vary in sediment type, or lithology, laterally as well as vertically

• 

The last result was supported by sedgwick & Murchison in the 1830s

sedgwick & Murchison determined that the dominantly non- marine deposits of the Old Red sandstone of Wales are Synchronous with marine Devonian deposits farther south based on the intertonguing interpretation

inter-tonguing: 2 different lithologies can grade laterally into one another in complex manner

• 

Depositional Environment & Sedimentary Facies The lateral changes of lithology & fossils contents brings the sedimentary facies concept after a well-exposed example of lateral changes of both lithology & fossils was described in 1883 in Switzerland 

Sedimentary Facies

characters of a sedimentary rock, especially those that indicate the deposition environment
– Facies identified by their dominant lithology
– the study of sediments & organisms in modern environments provides important clues for understanding ancient facies, Because the facies concept relates sediments to their depositional environments

The sedimentary environment

The conditions under sediment deposited
– affect texture, composition, & structure of sediments
– for example
1. The depth & temperature of the water
2. The strength & direction of the currents

Preservation of large volumes of most strata requires one or both of following conditions

Subsidence of earth crust during deposition

a rise of sea level

2 types of facies patterns

1. Transgressive facies pattern
2. Regressive facies pattern

Transgression

the advance of the sea over the land
– reflect shrinkage of land area
– preceded by an erosional unconformity
– show landward shift of facies via time
– tend to become finer upward at any one geographic locality
caused a continuous shift of environments & their sedimentary & biological products landward

Regression

retreat of the sea from land area, it has the opposite effects than of Transgression
– reflect enlargement of land area
– followed by an erosional unconformity
– show seaward shift of facies through time
– tend to become coarser upward at any locality
results from
1. relative fall of sea level
2. rise of land level

Walther’s law

Vertical progression of facies will be the same as corresponding lateral facies changes

result of this fact Environments shifts position through geologic time & the respective sedimentary facies of adjacent environments succeed one another in vertical sequences

So.
1. in TFP, just as the sequence becomes finer upward, the finer facies also spreads laterally in the direction of transgression
2. just as a regressive sequence becomes coarser upward, coarser facies also spreads laterally in the direction of regression

Local Versus Worldwide

Sea level changes may result from:
1. fluctuating continental glaciation
2. large-scale warping of deep ocean basins

– such as Netherlands & California

Rapid Sedimentation

Very rapid sedimentation result from:
1. uplift of a distant inland area
2. climatic change that accelerates erosion

The above factors cause
1. local seaward retreat of shoreline independent of structural warping of crust
2. worldwide changes of sea level

Biostratigraphic Concepts

Fossil zone

An interval of strata characterized by a distinctive index fossil

Close attention must be paid to overlapping stratigraphic ranges of index fossils

Both range & abundance of a single species vary from place to place

Environmental changes may have been more rapid than evolutionary ones

Lithologic formation boundaries need not have any relationship to the biostratigraphic boundaries of fossil zones

Facies fossils

fossil type which restricted to lithology

Bottom dwelling organisms tend to be restricted to one facies, so they aren’t recommended as index fossils for facies correlation

Un conformities

Un conformities

Traced & mapped to established their physical continuity

Studied from the standpoint of age such as unconformity surface varies in age from place to place

The total time interval represented by the discontinuity may vary greatly

An unconformity may even disappear laterally into a continuous, unbroken, or conformable sequence of strata, this means unconformities show lateral & vertical differences as important as of rock units

Types of Unconformities

Angular unconformity: there is angle of discordance between older& younger strata
– This type shows that severe deformation occurred before unconformity was buried

Disconformity: no discordance between strata below & above discontinuity surface

Nonconformity: when the underlying rocks are igneous or metamorphic

Unconformity-Bounded Sequences

new way of studying stratigraphic record & formed additional type of stratigraphic division
– suggested by Sloss In the 1950

Additional Relative Time Scales

Index fossils aren’t only things that provide worldwide, synchronous punctuations of the rock record with potential for correlating & subdividing that record

There are some alternatives that provides relative-age scales with special applications:
1.The sequence unconformities which reflect the global sea- level changes
2. Worldwide magnetic polarity reversals
3. Chemical analyses for rare or trace elements & isotopes in sedimentary rocks also provide potential for relative geochemical time scales

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Online Quiz

Quizlet

Important Terms

Relative: Know Order of Events But Not Dates, Such as Bedrock Formed Before The Glaciers Came

Relative age dates: placing rocks & geologic events in their proper sequence

Absolute or Numerical: Know Order & Dates of events

Absolute or Numerical dates: termed absolute age dating, actual age of geologic events

2 Conceptions of History

Catastrophism
– earth history dominated by violent events
– As.: great effects require great causes
– Catastrophes do happen but uncommon

Uniformitarianism earth history dominated by small-scale events typical of the present 
– Assumption: we can use cause & effect to determine causes of the past events

principles of relative dating

– Developed by Nicolaus Steno in 1669
1. Superposition Law: In undeformed sequence of sedimentary or volcanic rocks, the oldest rocks at base; & youngest at top

2. Principle of original horizontality: Layers of sediment deposited horizontally
– flat strata not disturbed by folding or faulting

3. Cross-Cutting Relationship Principle Younger features cut older once

correlation

Matching strata of similar ages in different regions

Principle of fossil (or faunal): Correlation relies upon fossils
succession fossil organisms succeed one another in a recognizable order, thus any time period is defined by the type of fossils in it

Unconformities

loss (or break) of the rock record

produced by erosion & nondeposition

Types of unconformities

Angular unconformity
tilted rocks overlain by flat-lying rocks
– between 2 sequences of sedimentary rocks

Disconformity
strata on either side of the unconformity are parallel (but time is lost)
– between 2 sequences of sedimentary rocks

Nonconformity (basement)
sedimentary rocks deposited above metamorphic or igneous rocks
– between sedimentary rocks (above) & non-sedimentary rocks (below)

The geologic time scale

Is the calendar of Earth history, Subdivides geologic history into units, Originally created using relative dates

Structure of the geologic time scale:
Eon→Era→Period→Epoch→Age→Chron
– Era subdivision of an eon & subdivided into periods, Periods subdivided into epochs…atc

Eras of Phanerozoic eon:
1. Cenozoic (recent life)
2. Mesozoic (middle life)
3. Paleozoic (ancient life)

Precambrian time
– 4BY prior to Cambrian period
– Not divided into small time units because the events aren’t known in detail
– Immense space of time (Earth is ≈ 4.5BY)

Introduction to Radioactive Decay

Work Sheet Click on page 2