3. Earth Flashcards
What are the different layers of the Earth and their main
properties?
- Earth’s crust (continental and ocean) is composed of low-density silicate rocks, which are rich in aluminum and potassium. The crust essentially floats on the mantle, like an iceberg floats in water.
- The mantle is composed of silicate rocks of higher density than the crust and contains more magnesium and iron. The layers of the mantle are lithosphere, asthenosphere and lower mantle. The Moho is the boundary between the crust and mantle.
- The core is thought to be metallic, composed mostly of iron and nickel. It has has an inner core (solid) and a molten outer core.
Give one characteristic to each layer of Earth:
Inner core: solid
Outer core: liquid
Lower mantle: rigid
Upper mantle: weak/plastic (asthenosphere), strong (lithosphere)
Crust: hard
What is the approximate thickness of the different layers?
Inner core = radius 1220 km
Outer core = radius 2260 km
Lower mantle = radius 2240 km
Upper mantle = 660 km
Lithosphere = 5 to 250 km
Explain why each layer have its consistence:
Inner core - High pressure from the other layers at the Earth center makes the inner core solid.
Outer core - It is liquid because the temperature is high and there is not enough pressure to make it solid.
Mantle - Since the pressure is too high for true liquids to form and the temperature is also high, the mantle has a plastic and solid texture. Plastic in the asthenosphere and solid in the lithosphere.
Crust - it is hard since pressure and temperature are low.
If the inner core is hotter, how could it be solid while the outer core is molten?
The inner core is solid due to high pressures at Earth’s center.
What is the difference between oceanic and continental
crust?
- Oceanic crust
Mainly basalt - homogeneous composition
Young: ca. 7 km thick
Density ca. 3.0 g/cm3 - Continental crust
Many rock types; heterogeneous
Old to young
Upper crust average composition: granitic
Lower crust average composition: more basaltic
Average 35-40 km thick, > 70 km in mountain areas
Average density ca. 2.8 g/cm3
Compare oceanic crust with continental crust accordingly with the elements:
thickness, density, age, composition, mixture of rocks
Oceanic crust: Thin, Dense, Young, Basaltic composition, Homogeneous
Continental crust: Thicker (than the oceanic crust), Less dense, Old to young, Granite + many other rock types, Variable
Why is Earth not cold?
Heating of early earth due to: Bombardment (friction), radioactive decay , contraction (gravity friction)
Nowadays mantle convection contributes to keep the Earth warm. Mantle convection is the slow creeping motion of Earth’s solid silicate mantle caused by convection currents carrying heat from the interior of the Earth to the surface
Why is the magnetic field important in Geology?
- The magnetic field shields Earth from dangerously high-energy solar particles - no life without the magnetic field
- The magnetic field holds the atmosphere “in place”
- no water in the atmosphere & hydrosphere
- no life without the magnetic field
- no water in the lithosphere
(crust & uppermost mantle)
- no mantle convection
- no plate tectonics
How to use the magnetic field for dating purposes?
We can measure the age of the ocean’s floor by comparing magnetic anomaly bands mapped on the seafloor with the sequence of magnetic reversals worked out on land. The procedure has been verified and extended by deep-sea drilling. Geologists can now draw isochrons for most of the world’s oceans, enabling them to reconstruct the history of seafloor spreading over the past 200 million years. Using this method and other geologic data, geologists have developed a detailed model of how Pangaea broke apart and the continents drifted into their present configuration.
Temperatures, geothermal gradients
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth’s interior. Away from tectonic plate boundaries, it is about 25 °C per km of depth (1 °F per 70 feet of depth) in most of the world.
How do we know about the interior of the Earth?
A century ago geologists began to look downward into Earth’s interior with waves produced by earthquakes. The different Earth layers have characteristic physical properties causing difference in speed of seismic waves.
- primary waves: If the Earth was homogenous, the seismic waves would travel along straight lines from the focus of an earthquake. But the Earth is not homogeneous. Pressure and temperature increase towards the center. There is increase in seismic velocities: the seismic waves are refracted.
- Secondary waves: No S waves reach the area on the opposite side of the Earth from the focus. It proves that the Earth has a molten core.
Evidence on inner core - - Refraction of P waves at mantle / outer core boundary and outer core / inner core boundary. No P waves registered in P-wave shadow zone
seismicity & xenoliths
How do different seismic waves travel?
Two types of waves travel through the Earth’s interior: P (primary/compressional) waves, which
move through all forms of matter and move the fastest, and S (secondary/shear waves) waves, which move through solids only and at about half the speed of P
waves.
The third type, surface waves, need a free surface like the Earth’s surface to ripple across, like waves on the ocean. They move more slowly than the interior waves but cause most of the destruction associated with earthquakes.
How and where earthquakes can be recorded?
By a seismograph, an instrument that makes a record of seismic waves caused by an earthquake, explosion, or other Earth-shaking phenomenon. Seismographs are equipped with electromagnetic sensors that translate ground motions into electrical changes, which are processed and recorded by the instruments’ analog or digital circuits. A record produced by a seismograph on a display screen or paper printout is called a seismogram.
