Planet Earth/6a. Journey to the Center of the Earth: Earth’s Interior and Core

The Interior of the Earth
It is likely that you do not often think about the 6,371 kilometers below you, the distance to the center of the Earth. And you likely take for granted the Earth is solid all the way down to the core of its center. The solid interior of the Earth is nearly impossible to observe, and so it is no surprise that science fiction writers such as Jules Verne, who wrote the classic book Journey to the Center of the Earth in 1864, have dreamed of the mystery beneath our feet. Henry Cavendish’s measurement of big G in 1798 suggested that Earth was not hollow, but dense and solid. Measurements by Lord Kelvin showed that the Earth becomes hotter the deeper you travel down along the geothermal gradient. The observation of molten magma and lava that bubbled up through volcanoes attested to an interior that was extremely hot and containing molten rock, that was driven by convection. But the structure of the interior of the Earth appeared to be something that could never be discerned. The deepest well ever drilled into the Earth was the Kola Superdeep Borehole, which goes down just over 12 kilometers beneath the Earth’s surface, only 0.2% of the distance to the Earth’s central core.

The woman who found another planet inside Earth
Inge Lehmann was an extremely shy student with a fondness for science. In 1893, a school was opened in Denmark near where Lehmann lived. The school was created by an ambitious woman named Hanna Adler, who had just returned from a trip to America with a new idea. Her school would be unique in the that “boys and girls will always be taught and brought up together.” Despite being a girl, Lehmann enjoyed an excellent education, and was able to attend college at the age of 18 at the University of Copenhagen, then transferred to the University of Cambridge in England to study physics and mathematics. It was an extremely intense study, and four years into her studies at the university, she was exhausted, burnt out and in poor health. She returned to Denmark and took a job to make ends meet. At thirty years of age, she decided to go back to school at the University of Copenhagen, with the dream of becoming a true scientist and teaching at a university. Her life-long dream was achieved in 1923, and she went on in 1928 to begin working with a team of scientists at the Geodetical Institute of Denmark. Lehmann was tasked with setting up seismological observatories across Denmark and Greenland. Seismometers were a new scientific instrument that measure the motion of the Earth. A seismometer is simply a heavy weight dangling above a fixed base which is firmly attached to the ground. During an earthquake, the fixed base moves with the shaking of the Earth, while the dangling heavy weight, due to inertia, does not. The motion of the base with respect to the dangling weight is transformed into an electrical voltage, which is recorded on paper with an inked needle. This squiggly line is proportional to the motion of the earth in respect to the dangling weight above it. Mathematically, the squiggly line is converted to a record of the absolute motion of the ground, and is used to record the intensity of Earthquakes, which produce seismic waves.

Seismic waves
Seismic waves are waves of energy that passes through solid rock resulting from earthquakes, explosions or other loud noises. Similar to sound waves, seismic waves are mostly compressional waves, where the wave is compressing the volume of the rock by alternating the compression and expansion of the rock within discrete frequencies. The same occurs with sound waves, which compress and expand the particles in the air (a gas) that allow you to hear sounds. Since these waves can pass through material blocked by light waves, they can image the inside of materials that are opaque to light waves. An example of this you might be more familiar with is in ultrasound, which uses a high frequency sound wave to send waves into the uterus of a pregnant mother, this allows the imaging of the growing fetus before birth inside a mother before the baby is born. It is also used in many other medical applications and does not require exposing the patient to radiation. The same tools can be used on a more global scale by geophysicists to image the interior of the Earth. This requires large events, such as massive Earthquakes or nuclear detonations to produce these seismic waves of energy, and seismographs are able to pick up these waves of energy scattered around the planet from monitoring stations that listen to the interior of the Earth. There are two types of compressional seismic body waves, the first one distorts the volume of the material, simply called primary waves (P-Waves), and the other that distort the shape of the material, called shear waves (S-Waves). P-Waves compress material parallel to the direction of travel, while S-Waves compress material perpendicular to the direction of travel. An analogy to think of is a row of rowdy boys in a tight line, when the boy at the end of the line pushes the boy in front of him, who then pushes the person in front of him, causing a wave to travel down the line, while the push also results in the boys to fall, or move in response. The energy of the initial push will travel the fastest, while the effect of this push, of the boys falling or stagger, will be slower. Thus P-Waves always travel faster than S-Waves, especially since P-Waves travel parallel to the direction of travel.

There is a third type of seismic wave associated with Earthquakes, called Surface Waves, that do not penetrate the Earth’s interior, and remain on the surface. They are the slowest type of seismic waves, but cause the most amount of damage to buildings and other structures during earthquakes. P-Waves, S-Waves and Surface Waves can be detected using a seismograph which measures the motion of the solid Earth below the device, which is fixed to the solid ground. P-Waves are the first waves to arrive during any event, and are quickly followed by S-Waves and then much later Surface Waves. Both P-Waves and S-Waves are called Body Waves, since they can move through the body or interior of the Earth. While surface waves only remain on the Surface of the Earth. The difference in time that it takes the P-Waves and S-Waves to arrive at a seismogram is known as the S-P interval; the S-P interval is important in detecting the location of Earthquakes. One critical aspect separates S-Waves from P-Waves: S-Waves cannot pass through liquids, they are absorbed by any liquid material that they encounter.

The Moho Discontinuity
One of the greatest insights about the nature of the interior of the Earth, came about with a massive earthquake that occurred on October 8th 1909 centered over the Kupa Valley in Croatia. A year before, at the Zagreb observatory in Croatia installed a new seismograph. With a massive Earthquake only 30 kilometers away, the readings on the seismograph revealed a clear signal to Andrija Mohorovičić, who maintained the scientific instrument at the observatory. He published a series of papers in 1910 noticing the difference in the arrival times of the P-Waves, which were faster than he had predicted based on mathematics. The short arrival time of the P-Waves appeared to be caused by the compressional seismic waves bouncing down into a material that facilitated a quicker travel time than expected if they travel through solid rocks. This lower material in the Earth appeared to have different physical properties that allowed seismic P-Waves to travel faster. Seismic waves travel at different speeds depending on the acoustic impedance of the material. Some materials facilitate a faster travel time of seismic waves than others. For example, a gelatinous substance of Jell-O, has a reduced acoustic impedance. If you place a loud speaker next to a bowl of Jell-O and blast music at it, the Jell-O will wiggle and jiggle as the soundwaves are converted to compressional P-Waves and S-Waves. While if you place a loud speaker next to a bowl of salt and blast music at it, the salt would barely budge. Salt has a higher acoustic impedance than Jell-O. Mohorovičić postulated that there was a Jell-O like layer beneath the Earth that allowed the P-Waves to travel faster at deeper depths in the Earth. This idea was formed with the recent realization of a surface layer called the crust and a deeper mantle layer from the German word for coat. Mohorovičić envisioned a double layer, something like peanut brittle (the Crust of the Earth) on top of Jell-O (the Mantle of the Earth). Mohorovičić advocated for a boundary layer known today as the Moho Discontinuity, which separates the upper crust from the lower mantle, between depths of 5 to 90 kilometers, which actually varies considerably over the sub-surface of the Earth. The mantle can be viewed as a material of dough-like ductile solids that facilitates easy deformation, when compared to the solid brittle crust of the Earth’s near surface, where rocks break into jagged pieces. Earthquakes are limited to depths above the Moho discontinuity, since the ductile mantle below does not break under stress, but plastically deforms. The Moho discontinuity is closely associated with the Brittle‐Ductile Transition Depth, the depth at which the material below is ductile, like bread-dough.

A Liquid Core and the S-Wave Shadow
In 1908, Karl Zoeppritz, a brilliant early seismologist at the University of Göttingen in Germany died, leaving behind his unpublished notes. These notes were passed by his supervisor at the University, Emil Wiechert, then to Wiechert’s student Beno Gutenberg. Gutenberg was interested in using the new seismographs to measure tiny motions within the Earth, such as those caused by the motion of crashing waves at the beach, but the notes left behind by Zoeppritz contained the motions caused by far away earthquakes, such as the 1906 Earthquake in San Francisco. For his thesis research Gutenberg compiled large sets of data from an ever-expanding number of seismographs, observing discontinuities in the subsurface. His advisor argued that the core of the Earth must be made of iron, although the nature of the core was a mystery. Gutenberg figured that he could solve its mystery with his large sets of data. In 1913, Gutenberg reported on the absence of the S-Waves from earthquakes that originated on the other side of the earth from the seismograph. This S-Wave shadow appeared to be caused by the absorption of S-Waves in the core of the Earth because it was liquid. Gutenberg set about determining the depth of this lower mantle/core boundary, which today is placed at a depth of 2,891 km. The molten iron core, was a liquid, while the ductile mantle above was a solid. Gutenberg was just getting started with his research on the interior structure of the Earth, when he was drafted into the German army at the outbreak of World War I. Less than a year into his research, in August of 1914 he was an infantryman in the trenches on the western front, his experience was put to action by the army by measuring the travel times of enemy cannons to determine their position. He nearly dogged death when a grenade nearly killed him with shrapnel. After the war, although celebrated for his discovery of a liquid core, there was no work for him as a scientist. He returned home, and took over the soap factory that his father ran. His research was reduced to evenings and weekends, as the soap factory provided an income to his family in post-war Germany in the 1920s. It was during this time that Lehmann visited him, to learn what she could from the war-veteran about how to read seismographs. It must have been exciting to have a colleague interested in his research and to value his contributions toward the study of the interior of the Earth. Lehmann learned much from him about the liquid molten iron core that he had proposed in 1913. During the 1920s and into the 1930s, Lehmann studied the signals transmitted through the Earth’s core. She copied the recorded arrival times of P Waves from Earthquakes from all around the world onto index cards which she filed in oatmeal boxes. Using these records, she postulated that some of the P-Waves passed through the core of the Earth and bent, forming what she called P′ waves, as she summarized in her paper: Seismic waves that travel through the Earth’s mantle and crust only are represented by P, while P′ represents P-waves that pass through the mantle into the core, and then pass through the mantle again. Both P′-Waves and P-Waves exhibit a shadow ringed zone between 112.5 degrees and 153.9 degrees where no evidence of arrival times was detected in this area of the Earth. Inside this ringed zone, there was only P′-Waves, waves that traveled through the core of the Earth, these would slow as they passed through the liquid core from 10 km/sec to 8 km/sec. In the subset of data in this ringed zone, she noticed that arrival times suggested some P′-Waves were slightly faster than expected, like they traveled through a solid, while others were slower. She also noticed that some of these P′-Waves could be observed faintly in the shadow zone, as if they bounced off some solid inner core. These waves she called P′2-Waves (today they are called PKiKP-Waves) and they both reveal a solid inner core inside the Earth. Her research was published in 1936, with the shortest title of any scientific paper, P'. She had discovered a planet inside the molten iron liquid core of the Earth, what is known as the solid inner core. Her paper did not need to be verbose. This inner core has a radius of about 1,220 kilometers, about the size of Pluto. It rotates in an orbit within a liquid outer core of molten sulfur, iron and nickel at the center of the Earth. Much of the variability of Earth’s magnetic field is a result of this motion between the inner and outer core of the Earth, which is known as the Earth’s dynamo. Inge Lehmann had found a planet deep inside the Earth.

The complexity of Earth’s Interior
Since the pioneering work of Lehmann, Gutenberg and Mohorovičić, other discontinuities have been found using seismic waves produced by earthquakes to image the inside of the Earth. The Moho (short for Mohorovičić) Seismic Discontinuity separates the brittle Crust from the ductile Mantle, the Gutenberg Seismic Discontinuity separates the ductile Mantle from the liquid molten Outer Core, while the Lehmann Seismic Discontinuity separates the liquid molten Outer Core from the solid Inner Core. However, there are several other seismic discontinuities that have been observed in the Mantle. In the upper mantle (100 to 250 kilometers depth) seismic waves are slower than the lower mantle and the S-Waves appear weakened when traveling through this low velocity zone. While the faster seismic waves in the lower mantle are likely a result of increasing pressure and density of rocks below, the weakened S-Waves indicate that this zone of the upper mantle is partially composed of molten liquid rocks; a weak mushy soft layer compared to the harder more rigid lower mantle below. This layer in the upper mantle is known as the Asthenosphere, “the weak sphere,” which is capped by the Lithosphere, “the rock sphere,” which includes the crust, and some portions of the uppermost mantle. The tectonic plates that move continents are lithospheric plates, which ride over the asthenosphere, driven by convection of liquid magma and other molten rocks in the upper mantle. The asthenosphere, is not a liquid, but likely a ductile solid with many chambers of large regions of liquid magma deep under the Earth’s surface. The asthenosphere plays an important role in mountain building, the distribution of volcanoes on Earth, and the break-up and movement of tectonic plates. Deeper in the mantle are two other discontinuities that are described by their depth. The 410-kilometer seismic discontinuity appears to be caused by a phase transition in the crystalline structure of the rock to become more densely packed at these high pressures, while the 680 seismic discontinuity (sometimes referenced to depths of 660 or 670 km) is believed to be another phase transition of the minerals in the rock. The 680 seismic discontinuity is often used to distinguish the upper mantle from the lower mantle. The velocity of seismic waves increases across these boundary zones, when they increase their speed through the more densely pack crystalline rocks at these lower depths.

Shallow Seismic Waves
In 1976, Andy Hildebrand was hired by Exxon in Houston, Texas, to use seismic waves to image the subsurface along the north shore of Alaska in the pursuit of oil. Exxon would drive large thumper trucks that would send seismic waves through the ground, which were recorded by geophones (a type of seismography), hoping to image the underground region in the search for oil. Hildebrand, as an electrical engineer spent his time developing a new tool that tuned those seismic waves using different frequencies. This tuning resulted in better resolution to the images of the shallow subsurface of the Earth. These were shallow images allowing geologists to virtually see underneath the Earth about 5 kilometers down into the crust, and made the company an enormous fortune, when they found oil using them. With his retirement in 1989, Hildebrand set about to pursue his true passion; playing the flute. He took classes and majored in music as he pursued a second career in the music industry, but he was a still a scientist at heart. The same technology that could image the inside of the Earth, could be used to auto-tune musical notes. In 1997 he introduced software designed to tune the pitch of musical notes and people’s voices. This became known as Auto-Tune, which was first used on Cher’s Believe album in 1998. Today, Auto-Tune can turn any speaking voice into a song, used by the Rapper T-Pain, to the Gregory Brothers on their YouTube channel Schmoyoho to John D. Boswell on his YouTube channel Symphony of Science, it illustrates the strange way science in one field can lead to innovations in another.

Gravity Anomalies
In 1671, the French astronomer named Jean Richer observed that a pendulum swung in French Guyana near the equator recorded less swings in a day than the same pendulum swung back home in France. This suggested that the gravity of the Earth (g) was slightly greater near the equator in French Guyana due to a greater distance from the center of the Earth, than back in Paris, France. There is more mass beneath French Guyana, then under Paris; in other words, there is a greater distance between French Guyana and Earth’s center than between Paris and Earth’s center. This was evidence that the Earth was not a perfect sphere in shape, but was wider and budged around the equator, in a spheroid shape. The experiment sparks much interest in the study of the geodetic shape of Earth, because this budge at the equator would have great effect on navigation and map making.

In 1735, the French Academy of Sciences embarked on an expedition to Ecuador to carry out experiments to verify this experiment and carry out other experiments near the equator. Pierre Bouguer, was selected for the trip, as well as other French and Spanish scientists of the day. Bouguer was already a celebrated genius, who began teaching naval navigation and ship building at the age of only 16. The team sailed across the Atlantic Ocean arriving at Panama, and crossed the Isthmus of Panama by land, and then ferried down the coast of South America to Ecuador which was under Spanish control as the Real Audiencia of Quito. It was an exotic place to carry out experiments, but the team set to work, much of their knowledge left a lasting expression on the town of Quito, which today maintains the oldest South American Astronomical Observatory. For Bouguer, the expedition would last ten years before he returned to France, and wrote of his experience in a book entitled La figure de la terre (The Shape of the Earth) published in 1746. While in Quito, Bouguer measured the number of pendulum swings during the sidereal day, which as predicted swung slower, proving that the Earth did in fact bulge at the equator slightly. To verify these results, Bouguer decided to repeat the experiment by climbing to higher altitudes on the peaks of the nearby mountains of Pichincha and Chimborazo. Bouguer reasoned that these higher mountains would slow the pendulum down even more, as they added additional mass between him and the center of the Earth. He predicted that the pendulum on top of the highest peak, Pichincha, would slow down by 0.15%. In August of 1737 the team climbed to the top of Pichincha and attempted to measure a swinging pendulum. The wind howled and it rained upon the rocky peak, as the group did their best to count the swings of the pendulum. They observed that the pendulum slowed down by a factor of 0.12%, close to the predicted value. Bouguer demonstrated that the higher your altitude or elevation on land the greater the force of gravity will be due to the increase in mass because of the higher topography below you, something known as the Bouguer Gravity Anomaly. However, there was something odd, in that the pendulum slowed down less than predicted. Bouguer surmised that the interior of the Earth was not uniformly dense, and that the least dense rocks were the ones directly below the mountain. Rocks become denser as they get closer to the center of the Earth and were subjected to more and more pressure, so the rocks near the surface and directly below the mountain were less dense. This led to the discrepancy of 0.03%. Bouguer had discovered a way to measure the Earth’s interior density using gravity anomalies.

Gravimeters
Gravity anomalies allow scientists to see into the interior of the Earth by measuring the density of the solid Earth below each point on the surface. Early gravimeters were developed to measure the gravity over large areas using the length of displacement of a weight held freely by a spring. If gravity was stronger, the weight was pulled lower and closer to the Earth. Often the gravimeter was corrected by measurements from barometers to remove the effect of topography (the Bouguer Gravity Anomaly), as gravity increases with elevation.

Imagine that you are measuring gravity using a simple gravimeter on a flat surface of ground. If you pass over underground features with lower density (say a cave), the gravitational field will be slightly less, while if you pass over an underground feature with higher density (say a thick vein of gold), the gravitational field will be slightly more. Superconducting gravimeters have recently been developed that can detect minute changes in the surface gravity, which due to their size are fixed. They record the daily Earth tides caused by the pull of the moon, which stretches the interior of the Earth, making it less dense when the moon is directly overhead or directly opposite the station, and denser when the moon is at the perpendicular axis. These Earth tides also have to be corrected for in the data to be able to record gravitational changes at each station due to subsurface features. However, less sensitive spring-based gravimeters can measure large regions, sometimes even by flying over an area in an airplane, and recording the signal from the air. New technologies have allowed gravity anomalies to be measured from outer space on board orbiting satellites above Earth, such as the twinned satellites of the United States and Germany; GRACE (Gravity Recovery and Climate Experiment), which measured Earth’s global gravity anomalies between 2002 to 2017, including recording the dramatic melt of Greenland’s ice sheets during that span of time. Gravity anomalies (corrected from the Bouguer anomaly due to topography) are mapped using different colors to highlight the density of the Earth below each location, revealing pockets of Earth’s surface which are above denser rocks. With seismic measured depths to the Moho discontinuity (ranging from 10 to 90 kilometers), gravity anomalies are found to be high over regions with shallow depths to the Moho (10 to 20 kilometers), and low gravity anomalies over regions that exhibit deeper depths to the Moho (greater than 20 kilometers). Rock layers below the Moho Discontinuity are denser. If the crust is thin, there will be more of these dense layers below the region, while a thicker layer of crust will result in less dense layers below the surface. If the topography is the same, a thinner crust will result in greater gravity. Mapped gravity anomalies across the state of Utah reveals a pattern, in which Western Utah and the Great Basin overlays a thin crust (with higher gravity), while Eastern Utah and the Colorado Plateau and Uinta Mountains overlays thicker crust (with lower gravity). Thinner crusts are more susceptible to earthquakes and volcanoes, but are also found beneath the ocean basin, which exhibit the thinnest crust and shallowest Moho discontinuities on Earth’s surface; they are also some of the densest rocks. Thus, the Earth’s crust can be divided into the thin crust under the ocean basins, and the thicker crust under the continents. Ocean crust and continental crust are physically and chemically very different.

Gravity anomalies are frequently utilized by precious metal prospectors, since they can reveal regions where the denser rocks of the mantle have reached the Earth’s surface, resulting in siderophile elements like gold, iron, and nickel being more prevalent in these areas.

Isostasy and Isostatic Rebound
Seismic and gravity anomalies reveal a variably thick crust which extends over the entire surface of Earth. In some places, like ocean basins, the crust is thin, with a shallow Moho discontinuity, while over continents the crust is thicker, with a deeper Moho discontinuity. The thickest crusts are often found below high mountain ranges, while the thinnest are found in mid-ocean ridges. Mid-Ocean Ridges are extremely long ridges that form deep under the ocean and rise above the sea floor; here the Moho discontinuity is almost at the surface. The crust and uppermost layers of the mantle form the brittle zone called the lithosphere, beneath the lithosphere is the mushy doughy soft layer called the asthenosphere, which is much denser than the lithosphere. In a sense, the lithosphere floats upon of this weaker and softer asthenosphere layer. The asthenosphere is soft enough it can convect heat from the Earth’s interior by the very slow motion of this material. One simple analogy, is to imagine that the lithosphere is wood that floats on denser water, with water representing the asthenosphere. If the wood is thick, it will be more buoyant and will float higher above the water, while if the wood is thin, it will be less buoyant and will float lower in the water. This buoyancy of the lithosphere on top of the asthenosphere is called Isostasy. This explains why the crust (and hence the lithosphere) is thicker in regions with mountains, because the mountains are supported by a deeper root of lighter crust, and are held up by this thicker layer of crust. Now imagine that the wood is slowly whittled away by the process of erosion over millions of years from the top surface. The weight of the wood will decrease, but since it has a deeper root, its buoyancy will cause it to rise, this is called Isostatic Rebound. Even as the mountain is eroding, it is continually rising because of this isostatic rebound. Another example of isostatic rebound is when large ice sheets melt, taking weight off the lithosphere, which responds by rising the topography upward. This uplift can significantly change the landscape, although the process is slow. Often near hogbacks, piedmonts and valleys on the edge of mountain ranges, rivers will incise deep canyons, as the mountain range rise while the rivers and erosion work to cut down into the margins of these mountain ranges. One of the best examples of this is the Royal Gorge in Canon City, Colorado, but many other examples exist where steep canyons and gorges have formed on the margins of mountain ranges, due to the continued uplift of the mountains because of isostatic rebound. If the mountain range continues to rise, and continues to be eroded, the thicker crust below the region will not be enough to support the region’s higher topography, and will result in the mountain range to begin to sink and collapse.

One of the more dramatic examples of this can be found in Utah in Brown’s Park, where the eastern range of the Uinta Mountains sank, forming a wide valley, in which the Green River passes through today, but in the past this region of Utah was topographically very high in elevation. Seismic and gravity surveys reveal that the crust is particularly thin here, and was unable to support a tall mountain range in this area, and hence resulted in a sinking broad valley, once the mountain was fully eroded down. Isostasy became a powerful way to explain the rise and fall of mountains. For decades geologist believed that mountains were the product of thickening and thinning crust, and the buoyancy of the lithosphere above a weak soft asthenosphere. Continents did not move. One scientist argued that they did move, his name was Alfred Wegener, and although he was not a geologist, his crazy idea would lead to a revolutionary idea that would transform the field of Earth Science, and led to the theory of plate tectonics.