Even though direct temperature measurements deep within Earth remain beyond our technological reach, scientists utilize ingenious methods to understand and compare the temperatures of Earth’s distinct layers. Seismic waves, vibrations that travel through the Earth, provide crucial insights into these thermal variations. These waves, generated by earthquakes, volcanic activity, or even controlled explosions, act as messengers from the Earth’s depths.
There are two primary types of seismic waves: primary waves (P-waves) and secondary waves (S-waves). Primary waves are compressional waves, moving in the same direction as the wave travels, much like a slinky being pushed and pulled. Secondary waves, on the other hand, are shear waves, vibrating perpendicular to their direction of travel, similar to shaking a rope.
SF Fig. 7.1. (A) Primary or “P” waves show longitudinal compression similar to a slinky.
SF Fig. 7.1. (B) Secondary or “S” waves have motion perpendicular to the direction of the waive, similar to a rope.
Seismometers, sophisticated instruments, are used to detect and measure these seismic waves. The data recorded, known as a seismogram, plots wave velocity against time. An important observation from seismograms is that P-waves arrive before S-waves, indicating their faster travel speed.
SF Fig. 7.1 (C) Primary or P waves (on top) and secondary or S waves (on bottom) in motion
The velocity of seismic waves is not constant; it changes depending on the material they travel through. Denser materials generally allow seismic waves to travel faster. Furthermore, P-waves can propagate through solids, liquids, and gases, while S-waves are restricted to solids. This difference in behavior becomes critical when investigating Earth’s layers and their temperature profiles.
Temperature significantly influences the properties of Earth materials, including density and rigidity, which in turn affect seismic wave velocities. Generally, higher temperatures can lead to lower densities and increased plasticity in rocks, influencing wave speed. While not a direct temperature measurement, changes in seismic wave velocity as they pass through different layers provide indirect evidence of temperature variations.
For instance, the Earth’s crust, the outermost layer, is relatively cooler compared to the deeper layers. As seismic waves penetrate into the mantle, a significant increase in temperature is inferred due to the increasing velocity of P-waves. However, S-waves are unable to travel through the Earth’s outer core, creating an “S-wave shadow.” This observation strongly suggests that the outer core is liquid. The liquid state implies a temperature high enough to melt the materials present at those depths.
SF Fig. 7.2. Seismometers are used to measure seismic waves.
SF Fig. 7.3. A seismogram shows the data from a seismograph. Wave velocity is measured on the y axis, and time in seconds is measured on the x axis. P waves are recorded earlier than S waves, because they travel at a higher velocity.
The inner core, despite being incredibly hot, is solid due to immense pressure. P-waves passing through the inner core show a further velocity increase, hinting at compositional and possibly temperature differences compared to the outer core and mantle.
SF Fig. 7.4. This diagram shows hypothetical S and P wave propagation through the earth from an earthquake. P waves (arrows in yellow) can penetrate through the mantle and core, but S waves (arrows in red) can only travel through the mantle.
In summary, while we cannot directly measure the temperature of Earth’s layers, analyzing the behavior of seismic waves, particularly their velocities and propagation patterns, allows us to compare and contrast the thermal characteristics of the crust, mantle, and core. Variations in seismic wave behavior provide compelling evidence for temperature differences and phase changes within our planet, shaping our understanding of Earth’s internal structure and dynamics.
SF Table 7.1. Table of various minerals and their P and S wave velocities and density
| Mineral | P wave velocity (m/s) | S wave velocity (m/s) | Density (g/cm3) |
|---|---|---|---|
| Soil | 300-700 | 100-300 | 1.7-2.4 |
| Dry sand | 400-1200 | 100-500 | 1.5-1.7 |
| Limestone | 3500-6000 | 2000-3300 | 2.4-2.7 |
| Granite | 4500-6000 | 2500-3300 | 2.5-2.7 |
| Basalt | 5000-6000 | 2800-3400 | 2.7-3.1 |
Courtesy of Stanford Rock Physics Laboratory
