Dynamic Earth Flashcards
What is cosmic time and how does it start geological time?
Lecture 2: Time
Geological time starts when the first solid material condensed in our Solar Nebula.
The Solar Nebula is a floating interstellar cloud where the process of planetary formation begins, where:
1. small ice and dust particles condense in a nebula,
3. Ice: volatile material (in a gaseous state, due to pressure and temperature conditions in the protoplanetary disk)
4. Dust: refractory material ( solids containing silicate minerals which form most of the rock on Earth)
5. other atoms or molecules attach to these particles which build into masses large enough to be attracted to one another by gravity (i.e., planetesimals),
6. gravity then pulls a swirling nebula inwards and merges into a “spinning disk” (with a bulbous centre)
7. in the context of the Solar System, the Sun was the central bulb whilst the planets formed from material in the flattened outer part (i.e., protoplanetary disk) which is moving fast enough to stay in orbit.
These solids accreted over and over, forming planetesimals (asteroids) which then formed planets
- What is the principle of uniformitarianism?
Lecture 2: Time
According to the principle of uniformitarianism:
- The physical processes that we observe today also operated in the past at approximately the same rates.
- These geological processes were responsible for the formation of geologic features that we now see in outcrops.
- Due to the slow nature and rate of most geologic processes, the development of individual features take a very long time.
- The Earth had to be much older than generally thought because observed geological processes work very slowly and at different times, therefore a succession of slow events must be included in the Earth’s history.
- What is the principle of superposition?
Lecture 2: Time
According to the principle of superposition,
1. In a sequence of sedimentary rock layers, each layer must be younger than the one below.
2. A layer of sediment cannot accumulate unless there is already a substrate on which it can collect (the layer at the bottom of a sequence of strata is the oldest and the layer at the top is the youngest)
3. When beds are tilted, this must have occurred after deposition.
4. The symbol (⅄) shows the direction of ‘younging’ (the direction in which sedimentary deposits or rock layers become progressively younger)
- What is the principle of original continuity?
Lecture 2: Time
According to the principle of original continuity:
- Sediments generally accumulate in continuous sheets in a given region.
- If you see a sedimentary layer cut by a valley, one can assume that the layer once spanned the valley and has been eroded by the river that formed the valley.
- What is the principle of cross-cutting relations?
Lecture 2: Time
According to the principle of cross-cutting relations:
1. If one geologic feature cuts across another, the feature that has been cut must be older.
2. If an igneous dike cuts across a sequence of sedimentary beds, the beds existed before the dike.
3. If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers.
4. In contrast, if a layer of sediment buries a fault, the layer is younger than the fault,
- What is the geological column?
Lecture 2: Time
No one locality on the Earth provides a complete record of our planet’s history as stratigraphic successions can contain unconformities.
- By correlating strata and rocks from locality to locality around the world, geologists have pieced together a composite stratigraphic column (the geological column), representing the entirety of Earth’s visible history.
The succession of fossils in the geological column define the course of life’s evolution through Earth’s history.
- What are the differences between relative and numerical age?
Lecture 2: Time
- Relative age: the age of one feature relative to and in respect to another in a sequence.
- Numerical age: the age of a feature given in years (absolute age).
- Numerical ages in years can be abbreviated to the units:
- Ka (-kilo) for thousands of years ago,
- Ma (-mega) for millions of years ago,
- Ga (-giga) for billions of years ago.
- Ways of determining relative age were developed in the 19th century by geologists such as Charles Lyell long before ways of defining numerical age.
- What is radioactive decay?
Lecture 2: Time
- Atomic number: all atoms of a given element have the same number of protons in their nucleus
- Atomic weight: however, not all atoms have the same number of neutrons in their nucleus — thus not all will have the same atomic weight.
> Atomic weight = (number of protons + neutrons)
- Isotopes (different versions of an element) have the same atomic number but a different atomic weight.
- Stable isotopes will last forever (i.e., 3He, 12C, 28Si, 56Fe).
- Unstable isotopes will undergo radioactive decay which converts them into a different element.
- Radioactive decay rates are commonly stated in terms of an element’s half-life (the time needed for half of a group of isotopes to decay)
- Who were able to find out the age of the Earth and how?
Lecture 2: Time
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Arthur Holmes: Measured the ages of rocks from the content of radioactive Uranium (radiometric dating):
1911: 1640 Ma years
1943: 300 Ma years -
Clair Patterson: Conduted radiometric dating of an iron meteorite:
1953: 4560 Ma years
- What is the definition of geochronology?
Lecture 2: Time
Geochronology: the overall determination and interpretation of numerical ages of rocks.
- What is the definition of a mass extinction and what must occur for it to happen?
Lecture 2: Time
Mass extinction: a relatively sudden, global decrease in the diversity of life forms.
To be a mass extinction, the following must occur:
1. Extinctions occur all over the world.
2. A large number of species go extinct.
Why did this happen?
Throughout time, there are events where over 100,00 km2 of lava has erupted quickly, geologically-speaking.
Lecture 2: Time
- Describe a brief overview of the Earth’s structure.
Lecture 3: Earth’s Structure
- Mantle: Composed of silicate (silicate dominated system, also dominated by other minerals, i.e., garnet, peridot, mica, etc.) and is subdivdied into an upper and lower mantle.
- Core: Inner and outer core predominantly composed of iron and nickel as a metallic core.
- The outer core is a liquid with the convection of that liquid providing us with a magnetic field as:- heat escapes from the inner core that causes the iron and nickel to expand and become less dense,
- creating electrical currents that induce a new magnetic field.
- This magnetic field protects us from some forms of solar ionised particles
- What is a fault?
Lecture 3: Earth’s Structure
Fault: a fracture in which one body of rock slides past another which can generate earthquakes and seismic energy.
- How are earthquakes and seismic waves produced?
Lecture 3: Earth’s Structure
- Rupture of intact rock or frictional slip along a fault prduces an earthquake and seismic waves.
- These move outwards in all directions.
- Some travel:
- through the Earth (P- and S- waves)
- or along the surface (R- and L- waves)
- Some travel:
- The ability of a seismic wave to travel through a material and the velocity at which this occurs depends upon the character of that material (i.e., liquid and solid, different types of solids, etc.)
- Factors, such as:
- density (mass per unit volume)
- rigidity (stiffness of a material = resistance to shearing)
- and compressability (how easily a material’s volume changes in response to squashing)
- Factors, such as:
- The speed of waves is proportional to rock density:
- The amount of pressure released is proportional to the amount of overburden pressure.
- By using seismology, we can see inside the Earth to help us an understand the Earth’s interior, by measuring the ground and magnitude shaking.
- How do P- and S- waves function?
Lecture 3: Earth’s Structure
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P-waves: compressional body waves (i.e., pressure)
The P-wave is moving through by compressing the material, therefore the denser the quicker.
- P-waves will travel:- at 8km/sec in dense igneous rock (e.g. peroditite)
- but 3.5km/sec in sandstone (a sedimentary rock).
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S-waves: shear body waves
Particles of materials move back and forth perpendicular to the direction in which the wave itself moves.
- Seismic waves travel faster in solids than liquids (so more slowly in magma than solid rock)
