LEC.171 Earth's interior Flashcards
What is the age of Earth in relation to the universe?
Earth is thought to be roughly a third of the age of the universe.
Why do geoscientists think that there was no oxygen in the atmosphere of early Earth?
No sediments yet formed on land displayed the telltale signs of an oxygen-rich atmosphere.
What happens when an S-wave travelling into the Earth’s interior reaches the core-mantle boundary?
It reflects back up as an S-wave and it refracts into the lower layer as a P-wave.
What is the order of the eons from youngest to oldest?
Hadean, Archean, Proterozoic, Phanerozoic.
What triggered the dramatic increase in oxygen levels about 580 Ma ago?
Continental collisions which lead to increased rates of carbon burial by seafloor sediments.
What is the Moho the boundary between and which is above/below?
The crust (above) and the upper mantle (below).
What is the relationship between S-wave velocity and density?
S-wave velocity is inversely proportional to the square root of density.
Where does water on Earth mainly originate from?
Volcanic outgassing and comets that bombarded the early Earth.
What is the Shadow Zone?
The area on Earth’s surface between ~105° and 142° away from an earthquake where no direct P-waves are recorded.
What seismic wave would be labelled as PKJKP?
One which travels as a P-wave through the mantle and outer core, but is transformed to an S-wave in the inner core (shear wave).
What is the difference between linear and angular speed of a plate?
The linear speed of a plate varies with distance from its Euler pole, but the angular speed is constant for the entire plate.
What does oceanic lithosphere converging with continental lithosphere create?
An active continental margin.
What does continental lithosphere diverging from continental lithosphere create?
Continental rift zone.
What is the chemical composition of magnetite?
Fe3O4.
What is a typical depth of the ocean near the crest of a mid ocean ridge?
2km.
What does GPS use to function?
GPS uses radio waves emitted by a network of satellites to a ground-based receiver.
What is the temperature below which magnetism gets locked into basalt?
The temperature below which magnetism gets locked into basalt is 500°c.
Does an aseismic ridge have active volcanism?
Yes but only at one end of the ridge (present day hotspot).
What are the layers of ocean crust composed of?
Layer 1 of ocean crust is formed of sediments, layer 2 is formed of basalt, layer 3 is formed of gabbro.
In the present day, where is the greatest volume of magma produced?
Mid-ocean ridges.
What does heat flow by conduction involve?
Transfer of vibrational energy at an atomic level.
Define solidus.
The temperature at which the mantle begins to partially melt, at a given pressure.
What is the order of igneous rocks in increasing silica content?
Peridotite, basalt, andesite, rhyolite.
What produces magma at subduction zones?
Magma is produced at subduction zones due to the dehydration of the subducted plate.
What is one of the first minerals to melt when the mantle is partially melted?
Clinopyroxene.
What is the main heat flow mechanism for young oceanic crusts?
Hydrothermal circulation (fluid advection).
What is the main heat flow mechanism for the lower mantle?
Convection by solid-state creep.
What is the main heat flow mechanism for the outer core?
Convection of molten material.
True or false, the mantle is made of molten peridotite?
False, peridotite is the correct rock type, but it is not molten. The fact that the mantle transmits S waves shows that it is solid, because S waves cannot travel through liquids or gases.
Is there evidence for hotspot type volcanism on Mars and Venus?
Yes
The climate impact of an explosive volcanic eruption depends on the latitude because?
The troposphere is 2x thicker at the equator than at the poles, this means that a much bigger eruption is needed to inject aerosols into the stratosphere if the volcano is close to the equator, compared to if it is close to one of the poles.
Britain’s geological history (500Ma, 320Ma, 250Ma, 100Ma)
500Ma: North and South separated by the Iapetus Ocean
320Ma: Britain was at the equator
250Ma: Pangea
100Ma: Mostly submerged underwater
At ~670km deep what happens inside earth?
The Benioff zone ends
How long ago did the most recent super plume (flood basalt) erupt?
16Ma, Columbia River Basalt
Island arc magmas (subduction related, not hotspot islands) are produced by what type of partial melting?
Fluid-induced
When did the lithosphere develop? When was the differentiation complete?
During the Hadean. By 4.4Ga
What did the lithosphere look like in the Hadean eon?
Solidified rocky outer shell, completely igneous with no continents.
When did the hydrosphere develop, and when were oceans formed?
Archean eon, ~4.5-3.8Ga
Where did the water for the hydrosphere come from?
Volcanic outgassing, comets and asteroids.
What had to happen for the water to stop evaporating and be able to form oceans?
Volcanic eruption frequency/density as they used to boil off the water, when it cooled down the water was able to form oceans.
What is the origin of the biosphere?
Hot springs and deep hydrothermal vents are believed to be the environments were life first originated.
What type of organisms are probably the earliest forms of life?
Hyperthermophilic chemoautotrophs. These organisms can tolerate extreme heat and obtain energy through oxidation.
What type of metabolism do early life forms likely utilized?
Anaerobic metabolism. This is due to the absence of O2 in their environments.
What role did photosynthetic cyanobacteria play in Earth’s history?
They may have produced O2 in the oceans since 2.7 billion years ago.
What significant geological formations were formed due to increasing O2 levels?
Banded iron formations (BIFs) formed from iron oxide minerals precipitating from seawater between 2.7 and 2.4 billion years ago. This is because iron is soluble in water when the O2 concentration is low
How did the conditions of the oceans change over time?
The oceans alternated between oxic and anoxic conditions while the atmosphere remained anoxic (Archean).
When did O2 begin to accumulate in the atmosphere?
Around 2.4Ga (proterozoic)
What potentially triggered the Snowball Earth glaciation in the Proterozoic?
O2 rxn with CH4, removing CH4 cooling the Earth, ~2.43Ga
When was O2 stabilised at 1% of the atmosphere, evidence?
2Ga during the proterozoic, iron oxide rich redbeds deposits on land
Life in the Proterozoic?
Eukaryotes had evolved, single cell life for the next Ga, photosynthesis-respiration was balanced.
O2 in the Phanerozoic?
Rises to 21% (modern levels) ~580Ga
What caused the build up of atmospheric CO2 in the Phanerozoic?
Mountain formation. Sediment production and burial of organic C removed C from the surface environment, hich allowed for more O2 in the atmopshere.
What happened during the Cambrian explosion?
O2 increase coincided with evolutionary radiation, first multicellular animals, rapid evolution of shells and skeletons, first ‘proper’ fossils.
What does 542Ma mark?
Beginning of the Cambrian period within the Phanerozoic eon - all major animal groups appeared within 10Ma
When were the earliest plants on land?
850Ma
How many mass extinctions in the Phanerozoic?
5
When were the earliest hominids?
5Ma
When were Homo sapiens first on Earth?
0.2 Ma
Where is it thought that Earth’s magnetic field derives from?
Core convection currents
What do seismic waves reveal about the structure of Earth?
Layering in Earth, composition and structures
Roughly what depth is the core-mantle boundary (CMB)?
2900km
How long do P and S waves take to make it from origin to the other side of the world?
P - 20mins approx. S waves don’t because they don’t travel through liquid.
Wave classification: what do the following refer to : K, I, J, c, i
P wave in outer core, P wave in inner core, S wave in inner core (only in the case of PKJKP as an S wave wouldn’t get through the outer core), reflection off outer core, reflection off inner core
Where are PKIKP waves found, and what characteristics of this suggest the presence of a discontinuity?
P wave shadow szone, weak arrivals and long travel times are indicative of a discontinuity
What element is the majority of the core’s composition?
Fe
Inner core growing as Earth cools??
Molten iron in the outer core freezes and ‘snows’ onto the inner core surface, theory that in the future the whole core could be solid?
How far is the North magnetic pole from the North geographic pole?
11.5°, moves slightly through time
Magnetic field strength varies with latitude, where is it strongest and weakest?
Strongest - magnetic poles. Weakest - equator
What is a dipmeter?
compass whose needle can move in the vertical plane , angle of dip (magnetic inclination) is measured relative to the horizontal.
Why must magnetism be generated by convection currents than iron in the core?
Curie temp 770°c for iron takes away its magnetic property. The geomagnetic field must be moving - connection currents - in an electrically conducting (metallic) liquid in the outer core, the fluid movement creates electric current that then generates the magnetic field.
What evidence can be used to determine past magnetic fields of Earth?
Rocks canf reeze, particles containing Fe will generally line up with the magnetic field, looking at the orientation in frozen rocks can indicate the magnetic field direction.
Describe P waves
Pressure and volume change. Rocks being squashed and then recovering from compression, zone of compression that propagates through the rock
Describe S waves
Shaking motion, changing shape of the rock, a ‘shear’ motion
What types of rocks will have higher wave velocities?
Rocks with interlocking particles because there isn’t the energy dissipation and friction that loose particle rocks have.
What is the bulk modulus (K)
How much pressure you have to impose and how much the volume changes as a result.
What is the shear modulus (u)
A measure of a material’s resistance to shear deformation, or the ratio of shear stress to shear strain.
Snells’s law? What does it mean
Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the wave velocities in the two materials. This means that seismic waves bend toward vertical when traveling from a fast material to a slow material, and toward horizontal when traveling from a slow material to a fast material.
What is the Moho?
A sharp boundary (sudden change in composition) between the crust and upper mantle.
Composition of the crust?
Gabbro - olivine, pyroxene, feldspar
Composition of the mantle?
Peridotite - olivine, pyroxene
What is the lithosphere-asthenosphere boundary?
Thermal boundary layer, dependent on the local temperature gradient rather than the chemical composition.
Plate tectonics involve moving the lithosphere - what does the asthenosphere do for this?
It is a weak layer with low elasticity so it flows to accommodate the movement of the lithosphere.
What are some oceanographic research methods that have helped discover the Earth’s bathymetry?
Depth sounding, echo sounding, seismic reflection profiling, dredging and dating of samples, drilling
Ocean floor layer 1?
~0.4km ocean sediments. terrigenous sediment, pelagic clays, pelagic oozes
Ocean floor layer 2?
basalt - low silica mama generated at ocean ridges. ~1.0-2.5km thick. Pillow lavas - blobs of lava that cooled rapidly on contact with seawater. Sheeted dykes - vertical blades of cooled magma.
Ocean floor layer 3?
Gabbro ~5km thicl. Slowly cooled, coarse grained igneous rock, same chemical comp as basalt. Layered at the bottom due to crystal setting.
Evidence for continental drift?
- Jigsaw fit of contintenal coastlines (or continental rises)
- Similar geological histories 350-150Ma (Carboniferous to Jurassic)
- Southern hemisphere glacial deposits 200Ma, South America, Antarctica, South Africa, India and, Australia. Glacial striations in the direction of ice flow, kind of similar, if continents back together it’d be quite a consistent flow of the ice. Gondwanaland (super continent)
- Plant and animal fossil distribution, including species that could not have crossed the present day oceans
- Paleomagnetism, in ancient lavas magnetic poles appear to have wandered over time. Different continents have different apparent polar wander paths.
On the map symbol for a convergent (destructive) margin, which plate are the triangles pointed towards?
The plate that is overriding the other.
What is a Euler pole?
A point on Earth’s surface that represents the axis of rotation for a tectonic plate that moves on Earth’s surface
How can we measure absolute plate velocities?
Hotspots. Plate motions carry extinct volcanoes away from hotspots. Isotopic dating of lavas from different islands (e.g. Hawaii), plot against distance. S=D/T
How are present day global plate motions monitored?
GPS
What is the Earth’s main method of cooling?
Mantle convection
Why are there variable geothermal gradients?
Heat flow is spatially variable.
Do plates have an active or passive role in Earth’s mantle convection?
Active
What is the main force driving plates?
Subduction
Sources of heat for Earth?
Sun, radioactive decay
Describe conduction
neighbouring particles share heat energy, inefficient because atoms are static
Describe convection
material must flow, more efficient than conduction because atoms can move, hot material rising buoyantly, relies on density difference between hotter and cooler mantle. mantle convection main heat flow mechanism in most of the Earth.
What is heat flow in ocean basins correlated with?
The age of oceanic crust, highest at newly created MORS, lowest at trenches
Wilson cycles are the suggested reason for?
Assembly and self-destruction of super continents
Origins of continental crust?
~3.1Ga, silica-rich magma formed and became buoyant
8 stages of the Wilson cycle?
- uplift, extension and thinning of continental crust, formation if time, of the young spreading ridge
- young passive margin
- mature passive margin, well established MOR (current day North Atlantic stage)
- ocean closing phase, oceanic subduction, currently in Atlantic in Caribbean
- continents on either side of ocean basin getting destroyed, collision of continental plates
- uplift and thickening of the crust, mountains, horizontal shortening
- stable continent
- cratonic sag basin, subsidence
