WEEK 5 - Our perception of Time and the Antiquity of Earth Flashcards
Human Perception of Time
- Seconds, minutes, hours
- Days, weeks, months, years
- Decades
- Maybe a century (for fortunate individuals)
🚫 Humans struggle to grasp time spans beyond a single century!
🦣 How Do We Study Life From the Past?
By examining the dirt and rocks
Example:
- Southern Ontario, ~13,000 years ago (Ice Age, Mammoths)
🌍 How Do We MEASURE Earth’s History?
Millions of years ago → Early humans in East Africa (~5 million years ago)
Tens of millions of years ago → Dinosaurs in Southern Alberta (~70 million years ago) 🦖
Hundreds of millions of years ago → Ancient marine life in Southern Ontario (~450 million years ago) 🌊
Billions of years ago → Earliest life forms on Earth (~3.5 billion years ago) 🦠
📌 Geological time spans are far beyond human perception!
How Do We STUDY Earth’s History?
Most of Earth’s history is recorded in rocks
Written human history: only 5,000 years
Geologic record: 4.6 billion years
⏳ What are The Two Types of Time Used in Geology?
- Relative time – Determines the order of events
- Absolute time – Determines the exact age using radioactive decay
How Do Events Occur in Relative Time?
Events follow a sequence (earliest at bottom, latest at top)
Example:
- Wake up in the morning
- Eat breakfast
- Eat lunch
- Eat supper
- Go to sleep at night
Law of Original Horizontality
Sedimentary & lava layers are deposited horizontally
If strata are not horizontal, they were later disturbed by tectonic forces
Law of Superposition
In undisturbed layers:
Oldest rock is at the bottom
Youngest rock is at the top
Similar to stacking term papers (oldest at bottom, latest on top)
Law of Fossil Succession
Fossils occur in a consistent vertical order in sedimentary rocks globally
Evolution is irreversible, so species appear in a specific sequence
Fossils help determine relative ages of rock layers
Index Fossils
Fossils that are useful for dating rock layers
Represent specific time periods
Found worldwide in a specific sequence
Law of Cross-Cutting Relationships
Faults & igneous intrusions cut into older rocks
The fault or intrusion must be younger than the rock it cuts
Inclusions in Geology
Magma can break off older rock fragments and incorporate them into new rock
Eroded rock fragments can be embedded into younger sedimentary layers
These fragments are called inclusions
Law of Inclusions
If Rock B contains fragments of Rock A, Rock B must be younger
Inclusions in igneous rocks → dislodged from magma chamber walls
Inclusions in sedimentary rocks → fragments eroded from underlying layers
⏳ Why is There “Missing Time” in Rock Layers?
Deposition is not continuous—pauses or erosion can remove rock
Time gaps in rock layers can range from years to billions of years
Example:
- 375 million years missing in London’s sedimentary record
Unconformities in Geology
A gap in the geological record due to erosion or non-deposition
Marks a significant time gap between rock layers
Three Types of Unconformities
Nonconformity → Metamorphic/igneous rocks below, sedimentary rocks above
Angular Unconformity → Tilted/folded rocks below, horizontal rocks above
Disconformity → Parallel layers with an erosional gap between them
Nonconformity in Geology
Sedimentary rocks overlying metamorphic or igneous rocks
Formed when underlying rocks uplift, erode, and are later buried by new sediment
Example:
- Schist (metamorphic) overlain by horizontal sedimentary layers
Angular Unconformity
Separates tilted/folded rocks from overlying horizontal layers
Formed by: Deposition → Uplift → Erosion → New deposition
Indicates significant tectonic activity before the unconformity formed
Disconformity
Parallel sedimentary layers with an erosional surface in between
Formed by uplift, erosion, and later sediment deposition
Local depressions in the unconformity may be filled by later sediments
🏜 Where Can All Three Types of Unconformities Be Observed?
THE GRAND CANYON
- Nonconformities
- Angular unconformities
- Disconformities
What Does Relative Time Tell Us About Geological Events?
Does not provide exact dates but shows the order of events
Based on principles like:
1. Superposition (younger rocks on top of older ones)
2. Cross-cutting relationships (faults and intrusions are younger than the rocks they cut)
3. Inclusions (fragments in a rock are older than the rock itself)
How Do Geologists Determine The Order of Geological Events?
By analyzing rock layers, intrusions, erosion surfaces, and unconformities
Events are ordered youngest to oldest using principles like:
- Superposition (youngest layers on top)
- Cross-cutting relationships (intrusions and faults are younger than the rocks they cut)
- Inclusions (rock fragments inside another rock are older)
Youngest vs. Oldest Events
📌 What is the youngest event in the geologic sequence?
- Present-day erosion (shaping today’s landscape)
📌 What is the oldest event in the geologic sequence?
- Deposition of Unit A (oldest sedimentary layer)
Major Geologic Events in the Sequence
📊 What are some key geologic events recorded in this sequence?
1️⃣ Deposition of sedimentary layers (Units A-K)
2️⃣ Intrusion of igneous rocks:
Unit D (gabbro) → Sill
Unit F (granite) → Dyke
3️⃣ Tilting of rock layers
4️⃣ Erosion of Units C-F → Angular unconformity
5️⃣ Extrusion of Unit J (rhyolite) → Lava flow
6️⃣ Present-day erosion
How Does Angular Unconformity FORM?
Older layers are deposited → tilted → eroded → buried by younger layers
Represents a gap in the geologic record due to missing time (erosion)
How Do We Determine The Absolute Age of a Rock?
By using atomic clocks in minerals
Each element has a fixed number of protons, but neutron numbers can vary
Atoms of the same element with different atomic weights are called isotopes
What is Radioactive Decay?
Some isotopes (parent isotopes) lose particles from their nucleus
This forms a new element (daughter isotope)
Energy is released in the process
🔄 This process is called radioactive decay
Protons, Neutrons, Electrons and Nucleus
PROTONS:
Positively charged (+) particles
Found in the nucleus of an atom
NEUTRONS:
Neutral (no charge) particles
Found in the nucleus of an atom
ELECTRONS:
Negatively charged (-) particles
Move around the nucleus in energy levels (shells)
NUCLEUS:
The dense central core of an atom
Contains protons (+) and neutrons (0)
Holds almost all the atom’s mass
How Does Radiometric Dating Work?
As a parent isotope decays, daughter isotopes accumulate
The ratio of daughter to parent isotopes increases over time
More daughter isotopes = Older rock sample
What is Half-Life in Radioactive Decay?
The time it takes for half of the parent atoms to decay into daughter atoms
Each half-life represents the halving of the previous amount of parent isotopes
Understanding Parent/Daughter Ratios
📊 What do different ratios tell us?
1:1 (½ parent left) = One half-life passed
1:3 (¼ parent left) = Two half-lives passed
1:7 (⅛ parent left) = Three half-lives passed
1:15 (1/16 parent left) = Four half-lives passed
Potassium-Argon Dating
Used to date igneous rocks
Measures the ratio of Potassium-40 (parent isotope) to Argon-40 (daughter isotope)
Decay starts when minerals crystallize from magma
How Potassium-Argon Dating Works
⚛ What does the parent-daughter ratio tell us?
More Argon-40 → Older sample
Half-life of Potassium-40 = 1.25 billion years
If 1/4 parent isotope remains (1:3 ratio) → 2 half-lives passed → Sample = 2.5 billion years old
What Materials Can Be Dated for Potassium-Argon Dating?
✅ Igneous rocks (e.g., feldspar, mica)
✅ Intrusive bodies (e.g., dykes, sills)
❌ Sedimentary rocks CANNOT be directly dated
Dating Sedimentary Rocks Indirectly
📏 How can igneous rocks help date sedimentary layers?
Igneous intrusions or ash layers in sedimentary strata provide age brackets
Example:
- Dakota Sandstone → Estimated between 160 & 66 million years old
🕰 The Geologic Time Scale
A “master timetable” for Earth’s history
Based on relative dating (fossils) & absolute dating (radiometric techniques)
Divides Earth’s history into eons, eras, periods, and epochs
Why is the Geologic Time Scale Important?
1️⃣ Provides a framework for studying Earth’s history
2️⃣ Makes scientific communication easier
3️⃣ Helps define large & small time units
How is Geologic Time Divided?
1️⃣ Eon = Largest unit
2️⃣ Era = Subdivision of eon
3️⃣ Period = Subdivision of era
4️⃣ Epoch = Subdivision of period
The Four Eons of Earth
1️⃣ Hadean (oldest) 🌋 ~4.6 billion years ago
2️⃣ Archean 🦠 ~4.0 billion years ago
3️⃣ Proterozoic 🧬 ~2.5 billion years ago
4️⃣ Phanerozoic (“visible life”) 🦖 ~541 million years ago (most recent)
Three Eras of the Phanerozoic Eon
1️⃣ Paleozoic = “Ancient Life” 🐠
2️⃣ Mesozoic = “Middle Life” 🦖
3️⃣ Cenozoic = “Recent Life” 🐘
How Are Geologic Time Periods Named?
1️⃣ Type localities – Named after locations (e.g., Jurassic from Jura Mountains)
2️⃣ Rock characteristics – Based on features like coal deposits (e.g., Carboniferous)
3️⃣ Whims & history – Inspired by cultural references (e.g., Silurian from a Celtic tribe)
What Does the Geologic Time Scale Represent?
A “master timetable” of Earth’s history
Based on relative time (sequence of events) & absolute time (radiometric dating)
Organized hierarchically into large & small time units
Six Important Geologic Periods 📅
Cambrian – Explosion of life, first hard-shelled organisms
Ordovician & Silurian – First fish, early land plants
Carboniferous – Coal-forming forests, giant insects 🦗
Jurassic – Dinosaurs dominate 🦖
Cretaceous – Flowering plants appear 🌸, dinosaurs go extinct
Quaternary – Humans appear 👣
Boundary Dates of Eons
📏 What are the major time boundaries in millions of years?
Hadean → Archean: ~4.0 billion years ago
Archean → Proterozoic: ~2.5 billion years ago
Proterozoic → Phanerozoic: 541 million years ago
