Exploring Earth’s Past

3 Basic Types of Rocks• Igneous

– Crystalline– Cooling and hardening of magma– Make up the majority of crust– E.g., pumice

• Sedimentary– result of the accumulation of small pieces broken off of pre-existing

rocks• Clastic: little pieces of broken up rock which have piled up and

compacted• Chemical: minerals left behind from water evaporation• Organic: debris caused by organic processes

– E.g., sandstone• Metamorphic

– Rock that has undergone chemical or structural changes due to heat and pressure

– E.g., marble

Some Vocab• Subsidence - downward shift of a surface

• Uplift - upward shift of a surface

• Plate tectonics - theory which describes the large scale motions of Earth's lithosphere

Uniformitarianism versus Catastrophism

• Catastrophism– James Ussher (1581–1656)– Earth's history has been dominated by cataclysmic events

rather than gradual processes acting over long periods of time.

– E.g., formation of Rocky Mountains due to a single rapid event such as a great earthquake

• Uniformitarianism ("The present is the key to the past.”)– James Hutton (1726–1797)– Geologic processes operate at the same rates and with the

same intensity now as they did in the past.• Weathering of rocks and erosion of sediment

– Problem: Impact craters and fossil record

The Theory of Actualism

• Laws of nature do not change with time • Earth's history can be explained in

terms of currently observable processes.

• But the rates of geologic change are not constant over long periods of time

• There have been some catastrophic geologic events

Finding Clues about Earth’s Past

• Fossils

• Relative Dating

• Unconformities

• Rock Layer Correlation

• Radiometric Dating

Fossils

Fossils

Formation of Fossils– Original remains

• Entire organism is preserved in its entirely• E.g., frozen organisms, insects trapped in sap

– Replaced remains• Soft part of organism is replaced with minerals• E.g., Fossil bones, teeth and shells

– Molds and casts• Hollow depression is formed in the rock - Mold• Minerals seep into the mold - Cast• E.g., shell fish

– Trace Fossils• Impressions• E.g., footprints, burrows

– Carbonaceous film• Silhouette composed of a carbon film• High T and P cause organic compounds to undergo chemical change

• Organism must die in or near water– Remains are insulated from elements that contribute to decomposition– Only soft part decomposes

• Sedimentation– Remains are buried (soil, mud and land slides)– Rapid sedimentation --> great for fossilization– Clay versus sand

• Permineralization– Lower sediment layers become compacted– Pressure on fossil increases, turning sediments to rock– Minerals glue particles of sediment together – Minerals may replace hard remains

• Uplift – Movement of plates

• Formation of mountains• Sea floors become dry land

• Erosion– Reveals fossils

Estimating Time Periods Using Relative Dating

• Relative Dating - placement of events in sequence (not actual dates)– Stratigraphic Laws

• Embedded Fragments

Stratigraphic Laws

• Law of Superposition– Oldest layer is at the bottom

of the sequence

• Law of Original Horizontality– Sediments and rock layers

were deposited horizontally– Topography controls the

angle at which sediments are deposited locally.

• Law of Lateral Continuity– Deposits originally

extended in all directions

• Cross-Cutting Relationship– Rock which intrudes by

magma flow into existing rock is always younger than the rock it invades.

Stratigraphic Laws

Igneous intrusion

Embedded Fragments

• Rocks embedded in another rock are older than the rock in which they are found– Pebbles in a conglomerate must have

existed before the conglomerate formed.

Unconformities

• Gaps in the geologic record that may indicate episodes of deformation, erosion, and sea level variations. – times when deposition stopped, an interval

of erosion removed some of the previously deposited rock, and finally deposition was resumed.

Angular Unconformity• Older package of

sediments has been tilted, eroded, and then erosion, younger package of sediments was deposited on this erosion surface. – Sediment deposition– rocks are uplifted and tilted

(deformation)– erosion removes the

uplifted mountain range; – sea covers the land surface– new sediments are

deposited

Disconformities• Erosion surface between

two packages of sediment• lower package of

sediments was not tilted prior to deposition of the upper sediment package.

• Subsidence and sediment deposition;

• uplift and erosion; • Third: renewed

subsidence and deposition.

Correlating Rock Layers• Matching layers from two locations

– Walking the Outcrop• Outcrop - part of a layer that can be seen at the surface• Easy way fo find if two layers are the same• Difficulty if there is vegitation, erosion or thick soil

– Matching Rock Characteristics– Matching Key Beds

• Single rock layer that is unique, easily recognizable and widespread

– Bentonite (clay material formed from volcanic eruptions)– Dust and debris from impact craters

– Index Fossils• Unique• Abundant• Found over a broad geographical area• Occur only in a few rock layers (same time period formation)

Nonconformity

• Sedimentary layers are deposited on igneous or metamorphic rock– Indication of long periods

of erosion prior to deposition

– Record of major episodes of uplift, erosion and subsidence during growth of the continents

– Evidence for mobility of the crust

Measuring Absolute Time• Rates of Erosion and Sedimentation

– Not constant– Only the ages of young geologic features

• Counting Tree rings– 1 ring = 1 year– Width = rainfall and temperature

• Varve– Sediment that is deposited on a yearly cycle– Clearest in glacial lakes formed during an ice age– Thick, light colored sandy layer in summer– Thin, dark-colored clay layer in the winter

• Radiometric Dating

Atomic StructureSubatomic

ParticleLocation Charge Mass (g) Mass (u)

Proton Nucleus Positive 1.67 x 10-24 1

Neutron Nucleus Neutral 1.67 x 10-24 1

Electron Orbiting the nucleus

Negative 9.28 x 10-28 0.00055 = 0

u = atomic mass unit (1 u = 1.67 x 10-24 g)

The mass number of an element is represented as relative mass numberssince the atoms are so small. The scale used is that of C-12 (C-12 = 12u).

Standard Atomic Notation: AX Z

So, mass number (A) = #p+ + # n0

Isotopes• Notice the masses in the periodic table!• They are not integers!• The masses found in the periodic table are

actually called atomic masses. They are weighted average of the mass numbers of the atoms of an element.

• Atoms of an element have different number of neutrons. They have the same number of protons, because Z is unique to the element

• Isotopes are forms of an element in which the atoms have the same number of protons but different number of neutrons.

Isotopes

Symbol Isotope Protons Neutrons Electrons Abundance

35Cl17

Chlorine-35

17 18 17 75.77%

37Cl17

Chlorine-37

17 20 17 24.23%

Atomic mass = 75.77% x 35 + 24.23% x 37 = 35.45

Protons and electrons are largely responsible for the chemical behaviourof an element, therefore isotopes may have the same chemical properties.

But physical properties may vary. E.g., heavy water (H-2 aka deuterium) Is used in nuclear reactors.

Radioisotopes

• The isotopes of some elements are very unstable; they emit radiation when they decay, changing the composition of its nucleus

• These isotopes are called radioisotopes

• The radiation emitted can be either harmless or very dangerous

Types of Radiation

Radiation Approximate Speed

Penetration in air

Effective Barrier

Alpha (, 4He2+) 2

Variable, but relatively slow

A few cm A sheet of paper

Beta (, e-) Variable, but relatively fast

A few m 1-2 mm of metal

Gamma () Very fast (speed of light)

Unlimited 1 m of lead or concrete

Radioisotopes and Half-Life

• Radioisotopes have characteristic half-lives.• A half-life (T1/2) is the time it takes half of the nuclei in a radioactive sample

to decay.• Radiocarbon Dating (C-14 --> C-12)

– Great for dating fossils (C-14… T1/2 = 5730 years!). Ratio of C-14 to C-12: time elapsed since the death of the organism

• Uranium-Lead Dating (U-238 --> Pb-206)– T1/2 for uranium is 4.5 billion years– Dating of oldest rocks

• Rubidium-Strontium Dating (Rb-87 -->Sr-87)– T1/2 is 47 billion years– Dating of extremely old rocks

• Potassium-Argon Dating (K-40 -->Ar-40)– T1/2 is 1.3 billion years– K is very common in certain mineral rocks