A Look at the Earth’s Interior.

Eruption! Our active planet as seen from the Aqua satellite… (Credit: NASA).

(Note: The essay that follows it part of a series of papers I wrote in my quest for my science teaching degree… I always hate the fact that school writing only makes it to two sets of eyeballs, mine and the graders, so I re-worked my writing a bit for the blog format…)

The Earth is the only terrestrial planet that we have the availability to reach out and examine up close. By use of painstaking scientific processes, we can monitor the inner workings of our world and create a model of its interior structure that presents a high degree of accuracy with what is observed. Still; while we live on its surface, we have never penetrated the shell of even its outer crust or sampled its deep interior… just how do we know what’s within?

To map the Earth’s interior perhaps no tool is more essential than the seismograph. Also sometimes referred to as the seismometer, this device is essential for recording and monitoring seismic waves in the Earth’s crust and their passage through the interior. This device usually consists of an internal inertial mass that is deflected relative to an external frame, usually anchored to the surrounding bedrock. As the device is shaken, the mass is moved as waves pass through it, deflecting a needle against a scrolling spool of paper marking the passage of time. Early detectors were constructed during the Chinese Han Dynasty. Modern detectors may be on the classic needle deflection type or digital and utilize the precise measurement of laser beams or a mass magnetically suspended generating a negative feedback loop. Key waves detected are P, or primary waves, S, or secondary waves, and surface seismic waves.  P waves are the initial “push-pull” waves of an earthquake. These are the fastest waves, and thus the first recorded during a seismic event. An elastic wave, P waves can travel through any medium, be it solid, liquid or gaseous. These waves are compressional and can also vary with the subsurface depth of the earthquake. Next waves to arrive are the S, or shear waves. Also known as transverse waves, these are slower moving and can only travel through solid material. Finally, the surface waves are the last to arrive at a given detector, as they are slower moving and generally cause the most damage. If these waves can be recorded by three separate detectors spaced out on the Earth’s surface, a precise epicenter can be pinpointed. Also, the fact that an Earthquake shadow zone is generated where only P waves are seen is prime and well documented evidence that the Earth’s outer core is not solid, but molten. Indeed, the magnetic shear or torsion generated by the interplay of Earth’s iron-nickel solid core, and liquid molten outer core, is further evidenced by our relatively strong magnetic field. In comparison to the Moon and other terrestrial or rocky planets, the interior of the Earth is a dynamic place, and seismology helps us understand this differentiated structure.

(Created by Author).

Elements of seismograph construction may include a digital strong-motion accelerograph or several inter-connected seismometers working to create one coherent output. A classic earthquake will first register on a seismogram as a series of short spikes marking the initial P-waves. Minutes later, the first S-waves will arrive spanning a slightly longer period of time. Finally, the largest and most damaging surface waves arrive. As seen on a seismograph, the timing and spacing recorded at an individual station may vary depending on the depth and distance of the earthquake epicenter.

The science of seismology is crucial to understanding the interior structure of the Earth as well as predicting where damaging earthquakes or tsunamis are likely occur. This study is vital to the whole Earth model because although we cannot directly sample interior layers of the Earth, we can model them by examining the speed and types of waves that transverse the crust, silica rich mantle, and inner and outer cores.

The outer surface of the Earth is composed of tectonic plates that either converge, or subduct one under another, diverge or separate, or strike slip or grind past one another. This type of surface material recycling drives what’s known as the rock cycle. The outer-most rigid crust is known as the lithosphere, which is comprised of the crust and a layer of brittle and solid rock about 100 km thick. This crust is thickest on the continents, and thinnest underneath the oceans. Of special interest is a separation known as the Mohorovi?i? Discontinuity, first discovered in 1909. This boundary between the crust and the Earth’s mantle was deduced by studying the refraction pattern of earthquakes by shallow p-waves.

Farther down, the lithosphere rides along top of the flexible and molten asthenosphere. This layer extends down to a depth of about 400 miles and is mechanically detached from the deep lower mantle. Again, only P-waves can travel through molten regions of the inner Earth; S-waves cannot. This key fact is prime evidence that the outer core of the Earth is fluid, or molten. Likewise, the refraction and reflection of seismic waves can also provide us with a “look” inside the Earth to probe its interior.

But beyond the probing of Earth’s interior, the study of seismology is crucial to other applications, both scientific and economic. The study and conduction of seismic waves can be applied to locating large fossil fuel deposits as well as prime aquifers or areas of potential sink hole activity. Again, this utilizes our understanding of the transmission, reflection, and refraction of P- and S- waves through solid versus liquid and gaseous material. Seismology is also used to detect nuclear weapons testing, and to assure compliance with test ban treaties. The liquid outer core is further evidenced by the creation of Earth’s magnetic field. When we look at smaller, cooler bodies such as the Moon and Mars, little evidence for a magnetic field is seen; in fact, the low density of the Moon versus its size is prime evidence that it was once part of the Earth’s crust and mantle ejected by a massive impact. Igneous basaltic rocks brought back from the Moon by Apollo astronauts support this theory. Finally, seismology demonstrates evidence for plate tectonics by showing observational proof that the plates of the lithosphere are active and in motion. Plates snapping back into place or grinding past each other all generate massive amounts of seismic waves in what we know as earthquakes. Over time, these cause the raising of great mountain ranges such as the Himalayas or massive earthquakes such as were recently witnessed in Haiti, Chile and Japan.

In conclusion, seismology and the study of seismic waves are key examples of how we can study something in science without directly examining it. Beyond just scientific interest, this has given us such benefits as the Pacific Tsunami Warning Center that has saved countless lives. As we move out and study other planets in our solar system, knowledge of the interior structure of the Earth will give us some insight into comparative planetary science and just how common or rare a dynamic place like Earth truly is.