The deepest borehole ever drilled, on the Kola Peninsula, reached about 12 km deep and stopped because the heat and pressure made it impossible to go on. It is about $6{,}371\ \text{km}$ to the center of the Earth, so the drill barely scratched the surface. Yet when an earthquake shakes the island, the National Seismology Center can tell what lies thousands of kilometers beneath our feet. How is that possible if nobody has ever been there?
Methods for studying Earth's interior
The deepest mines in the world reach about 4 km and boreholes about 12 km. Even so, we know that at $2{,}900\ \text{km}$ deep there is a layer of liquid metal. How do you map a planet we have only scratched?
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The deepest mine in the world reaches a depth of barely $4\ \text{km}$, while the radius of the Earth is about $6\,370\ \text{km}$. Explain why almost everything we know…
The deepest mine in the world reaches a depth of barely $4\ \text{km}$, while the radius of the Earth is about $6\,370\ \text{km}$. Explain why almost everything we know about the Earth's interior comes from indirect methods, and give one example of a direct method and one of an indirect method.
An earthquake generates waves that travel through the whole interior of the Earth. When they pass into a different material, their speed and direction change. Anyone who records them at several stations can reconstruct what they passed through.
Calculating the distance to the earthquake
P waves arrive before S waves. The farther away the earthquake is, the greater the delay between the two. This lets us calculate the distance.
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S-type seismic waves do not pass through the outer core, whereas P-waves do, although their speed is reduced. What does this fact tell us about the physical state of the…
S-type seismic waves do not pass through the outer core, whereas P-waves do, although their speed is reduced. What does this fact tell us about the physical state of the outer core? Reason your answer.
At a seismological station, the arrival of P waves is recorded at 10:14:20 and that of S waves at 10:14:49 (a delay of $\Delta t = 29\ \text{s}$). It is known that in…
At a seismological station, the arrival of P waves is recorded at 10:14:20 and that of S waves at 10:14:49 (a delay of $\Delta t = 29\ \text{s}$). It is known that in this region of the crust, P waves travel at an average velocity $v_P = 8.0\ \text{km/s}$ and S waves at $v_S = 4.5\ \text{km/s}$.
a) Deduce the algebraic expression relating the time interval $\Delta t$ to the distance to the epicentre ($d$) and calculate the exact distance from the station to the epicentre (assume the depth of the hypocentre is negligible).
b) A technician states: "Since we know the exact distance, we can now locate the epicentre on the map by drawing a straight line". Refute their statement by explaining what geometric indeterminacy exists, and design the method that seismologists must use (minimum number of stations and graphical procedure) to locate the epicentre accurately.
Order the following layers of the geosphere from the surface to the center of the planet: inner core, mantle, outer core, crust. Then state which of them occupies the…
Order the following layers of the geosphere from the surface to the center of the planet: inner core, mantle, outer core, crust. Then state which of them occupies the largest part of the Earth's volume.
Look at the diagram of the Earth's interior. Name the layers marked with numbers 1 to 4 and state one characteristic of each (physical state or composition).
Look at the diagram of the Earth's interior. Name the layers marked with numbers 1 to 4 and state one characteristic of each (physical state or composition).
During an exploration mission on a rocky planet of similar size to Earth, a seismograph records an earthquake generated at a depth of $100\ \text{km}$. It is observed…
During an exploration mission on a rocky planet of similar size to Earth, a seismograph records an earthquake generated at a depth of $100\ \text{km}$. It is observed that, starting at a depth of $2\ 900\ \text{km}$, the velocity of P waves drops abruptly from $13.7\ \text{km/s}$ to $8.1\ \text{km/s}$, and S waves cease to be detected entirely.
a) Interpret what physical changes (density and state of aggregation of matter) occur at that depth and name the analogous terrestrial seismic discontinuity, indicating which layers it separates on our planet.
b) Predict what types of waves a second seismic station located at the planet's antipodes ($180^{\circ}$ relative to the seismic focus) would record and justify why a seismic "shadow zone" exists for S waves.
In an earthquake, rocks at the surface suddenly break. In mountain ranges, on the other hand, there are strata folded in gentle curves, like modeling clay. Why does the same rock behave so differently?
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Explain in your own words what the lithosphere is, which layers of the geosphere form it, and how it differs from the crust.
Explain in your own words what the lithosphere is, which layers of the geosphere form it, and how it differs from the crust.
In the deep gold mines of South Africa, at about 4 km, the rock is above 50 °C and the air has to be cooled with ice. At what rate does the Earth heat up as you go down, and what does that heat do to the deep rocks?
Convection currents
The figure and the particle simulation show the same idea: when heated, the particles move around more, spread apart, and the matter becomes less dense; that is why it rises.
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Convection currents exist in the mantle: hot material rises and cold material sinks. Explain what causes these currents and how they relate to the movement of the…
Convection currents exist in the mantle: hot material rises and cold material sinks. Explain what causes these currents and how they relate to the movement of the lithosphere.
In a mine at a depth of $2\ \text{km}$, the temperature of the rocks is $75\ ^{\circ}\text{C}$, while at the surface it is $15\ ^{\circ}\text{C}$. Calculate the…
In a mine at a depth of $2\ \text{km}$, the temperature of the rocks is $75\ ^{\circ}\text{C}$, while at the surface it is $15\ ^{\circ}\text{C}$. Calculate the geothermal gradient (the variation in temperature per kilometer of depth) for that area and explain why the temperature increases with depth.
Back to the beginning: the Kola borehole stopped because, at 12 km, the temperature reached only about $180\ ^{\circ}\text{C}$ (a lower gradient than the typical $30\ ^{\circ}\text{C/km}$), but the hot rock, under enormous pressure, behaved plastically and tended to close the hole. But seismic waves do reach all the way to the center, and thanks to them we know the layers of the Earth.
Summary
Let's practice! 3 medium exercises in this section