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Earth Interior and Plate Tectonics
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Earth Interior and Plate Tectonics
Earth Interior and Plate Tectonics
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1
Question
What is the primary method scientists use to study Earth's interior structure?
Answer
Seismic waves from earthquakes, nuclear explosions, and explosives.
2
Question
How hot was Earth initially compared to today during its formation 4.5 billion years ago?
Answer
About six to seven times hotter than today.
3
Question
What mainly supplies Earth's internal heat today?
Answer
Radioactive decay of long-lived isotopes like uranium-238, thorium-232, uranium-235, and potassium-40.
4
Question
What were the major early sources of Earth's internal heat during formation?
Answer
Heat from colliding particles during accretion, gravitational contraction, decay of short-lived isotopes like aluminum-26 and iron-60, Mars-sized impact forming the Moon, and iron crystallization in the inner core.
5
Question
Why do P-waves cause particles to move parallel to the wave direction?
Answer
They are compressional waves that expand and contract material in the direction of propagation.
6
Question
How do S-waves differ from P-waves in particle motion?
Answer
S-waves are shear waves where particles move perpendicular or up and down relative to the wave path.
7
Question
What are body waves and which types travel through Earth's deep interior?
Answer
P-waves and S-waves, also called body waves, that penetrate Earth's layers unlike surface waves.
8
Question
What is the focus or hypocenter of an earthquake?
Answer
The site of initial slippage along the fault where the earthquake originates.
9
Question
How is the epicenter of an earthquake defined relative to the focus?
Answer
The point on Earth's surface directly above the hypocenter or focus.
10
Question
In what order do seismic waves arrive at a distant seismograph station?
Answer
P-waves first (fastest), then S-waves at about half the velocity, followed last by slowest surface waves.
11
Question
What determines the arrival times of P, S, and surface waves at a seismograph?
Answer
The distance of the seismograph station from the earthquake focus.
12
Question
What is the typical thickness range of oceanic crust?
Answer
3 to 15 kilometers thick.
13
Question
What composition characterizes oceanic crust?
Answer
Mafic rocks like basalt and gabbro, rich in iron and magnesium.
14
Question
What is the thickness range and composition of continental crust?
Answer
40 to 65 kilometers thick, primarily granitic composition that is less dense and more buoyant.
15
Question
Why is continental crust more buoyant than oceanic crust?
Answer
Granite in continental crust is less dense than the iron-magnesium rich mafic rocks of oceanic crust.
16
Question
What layers compose the lithosphere?
Answer
Crust plus the solid uppermost mantle, forming a rigid, strong layer about 100 km thick.
17
Question
What role does the lithosphere play in plate tectonics?
Answer
Broken into rigid plates that move and interact, driving plate tectonics.
18
Question
What defines the asthenosphere?
Answer
Weak sphere in the upper mantle beneath the lithosphere, known as the low-velocity zone due to partial melting.
19
Question
Why does the asthenosphere slow down P and S waves?
Answer
Small amount of partial melting softens the peridotite, reducing rigidity.
20
Question
What rock type comprises the asthenosphere?
Answer
Peridotite, an ultramafic rock rich in iron and magnesium minerals.
21
Question
What evidence indicates the outer core is liquid iron?
Answer
P-waves slow dramatically and S-waves stop entirely, as S-waves cannot propagate through liquids.
22
Question
What happens to seismic wave velocities in the inner core?
Answer
P-waves speed up again and S-waves reappear, confirming solid iron composition.
23
Question
What is the P-wave shadow zone on Earth's surface?
Answer
Area between 100° and 140° angular distance from the epicenter where no P-waves arrive due to refraction.
24
Question
Why is the S-wave shadow zone larger than the P-wave shadow zone?
Answer
S-waves cannot travel through the liquid outer core at all, creating a broader absent zone.
25
Question
How does Earth's magnetic field originate?
Answer
Convection of liquid iron in the outer core, driven by Earth's rotation and internal heat, generates electric currents.
26
Question
What is the average geothermal gradient in Earth's crust?
Answer
20 to 30 degrees Celsius per kilometer depth.
27
Question
How is heat primarily transferred in the inner core?
Answer
By conduction through the solid iron.
28
Question
What mechanism dominates heat transfer through Earth's mantle?
Answer
Convection of mantle material.
29
Question
What is seismic tomography?
Answer
A three-dimensional imaging technique like a CAT scan, using thousands of earthquake records to model wave velocities.
30
Question
What evidence supports continental drift?
Answer
Jigsaw fit of continents like South America and Africa, similar fossils 200-300 million years old, matching rocks and structures, paleoclimates, and paleomagnetism.