4.22: Evidence of Earth’s magnetic field from Jack Hills Zircon crystals - confirming an early differentiation of the core and mantle

• 4.24: Zircons from Kendujhar district of Odisha in India

  • Nuvvuagittuq greenstone belt

4.2 EARTH’S CORE AND MAGNETIC FIELD

• Continual reworking of the young, hot earth material driven by the internal heat of the earth and the external heat from the sun and continued collisions of planetesimals

  • The presence of aluminum26, a radioactive element, in meteorites formed during the early solar system, provides a source of the massive amount of heat that contributed to the separation of the core from the mantle deep within the earth. Abundant aluminum26 would give off enough heat to melt the early earth many times over and is likely to still contribute heat to the system. Aluminum26 decays to magnesium26

  • PROPERTIES OF EARTHS MAGNETIC FIELD: the vast majority originates in the earths interior. It has been present for the majority of earths lifetime. The field is constantly changing direction and intensity and occasionally reverses polarity.

MODERN CORE

• By 1914 scientist had observed that something deep within the earth influenced seismic waves. 2900km below the surface there was a distinct change in velocity of P waves and a lack of S waves, but it wasn’t until 1926 that scientist understood the lack of s-waves indicated a liquid like material that could not support shear deformation. From 2890-5140km the outer core is believed to be liquid (molten) because shear seismic waves do not pass through that zone the same way shear ways to not move through liquids. By 10 years later in 1936 scientist understood earth had a solid core a little smaller than the moon with a radius of 1217 km surrounded by a liquid outer core.

• Both the liquid outer core and solid inner core are very hot, the temperature is between 6470-10,470 deg F, about as hot as the surface of the sun, and under immense pressure. Even though the core is very hot, it is the very high pressure that causes the inner core to be solid.

• The exact composition of the core material can’t be sampled because it is too hot and the pressure is much to high. The presence of Iron-Nickel meteorites in the solar system during planet formation makes it reasonable that our core is made up of those same materials. Separation of the material that makes up the core (iron and nickel) and the mantle (aluminum, potassium, sodium, calcium and magnesium) happened by 4.2 Billion years ago.

• The interaction between the liquid outer core and solid inner core is what gives earth, and some of the other planets. Mariner measured a weak magnetic field around Mercury. Probes found that Mars and Venus do not have a significant magnetic field, but likely had some magnetic field in their pasts. Jupiter, Saturn, Uranus, and Neptune all have magnetic fields much stronger than that of the Earth. Jupiter is the champion- having the largest magnetic field. The mechanism that causes their magnetic fields is not fully understood but could be due to the interaction of metallic hydrogen within the planet.

• HOW DO WE KNOW? SEISMOLOGY

• Seismic waves generated from earthquakes travel through the earth and stretch and squish the layers of rock, this process is called elastic deformation. The intensity of the waves are defined by their frequency and wavelength.

• The motion caused by seismic waves are sensed by a device called a seismometer which records a record of the arrival time, duration and intensity of shaking on a recording called a seismograph. The first seismometers recorded shaking using a needle that traced a wiggly line on rotating paper, modern seismometers are digital and can also determine the velocity and acceleration of the ground.

• Earthquakes results in seismic waves that travel through the earth’s interior and along the earth’s surface.

  • BODY WAVES - travel through the subsurface of the earth in all directions. Their travel speed and orientation are influenced by the density and stiffness of the layers of rocks they pass through. Body waves are further categorized by the motion of shaking.

    • P Waves (Primary, Push) - represent a series of compression and expansion in the direction the wave travels. They travel quickly, arrive first and can travel through any medium. The speed of P waves is controlled by the material properties of the rocks including the bulk modulus, shear modulus, and density. In the crust, P Waves average 6-7 km/s.

    • S Waves (Secondary, Shear) - represent vibrational movement perpendicular to the direction of travel. They are slower and can only travel through solid material. Shear waves are further divided into their horizontal and vertical components. Shear wave speed is controlled by the shear modulus and density of the material, no change in volume occurs. Crustal velocities are slower 3.5-4 km/s. Shear waves can only travel through material that supports shear strain, ie solids with intermolecular forces holding the material together.

  • SURFACE WAVES - are like waves on the water, they travel across the surface of the earth starting from a point directly above the earthquake called the epicenter. These waves travel with low frequency and large amplitude and cause significant damage to structures. This is because all the energy of the wave is concentrated in a 2D plane at the surface-air interface. Earthquakes that occur very deep below the surface might not generate surface waves. Surface waves come in two forms:

    • Rayleigh waves - these waves are a combo of the longitudinal vibration of a P wave with the vertical component of an S wave causing the particles of the surface to move in an elliptical path in the vertical plane. These waves travel at about 92% of the S wave.

    • Love waves - these waves occur when the horizontal component of shear waves are trapped at the surface causing strong horizontal shaking.

• Because the different types of waves travel at different speeds and take different paths through the earth they take different amounts of time to travel from the epicenter through and around the earth. Comparison of their arrival times recorded at seismic stations indicate how far away the earthquake occurred. P Waves always arrive first followed by S waves followed by surface waves.

• Because the waves travel differently through different materials. Analysis of the speed and distribution of the different wave types gave the first indication that Earth had a solid core surrounded by a liquid core. Verification that earth has a solid core surrounded by a liquid layer occurred in 1936 by Danish seismologist Inge Lehmann by studying seismic waves traveling through the earth to seismic stations in New Zealand.

• Further analysis showed that waves were refracted by sharp changes in densities within the earth indicating there are several different zones within the earth. One of these density/velocity changes from slow surface material to deeper high velocity material is known as the Moho for Andrija Mohorovicic which occurs 5-10km below oceanic crust and 12.5-55km below continental crust.

• Recent research indicate seismic waves travel faster through the core from north to south than east to west. This indicates an alignment of iron crystals within the inner core.

Earthquakes tend to occur along plate boundaries, they are a result of a sudden release of energy caused by the movement of plates next to each other.

On a much smaller scale, GEOPHYSICISTS create small earthquake like explosion using dynamite, air blasts, or vibrating trucks to induce seismic waves into the earth over a targeted area while an array of “microphones” called geophones are laid out on the ground. They record the seismic waves that are reflect off of the layers of rock when there are changes in the bulk properties.

OTHER HUMAN GENERATED EARTHQUAKES

The energy pumped into the earth during fracking causes the ground to move and shake. That movement can be recorded and analyzed to help understand the characteristics of the earth along with some properties of the induced fractures left in the rock. This is called microseismic.

Some oil & gas activities, as they grow larger in scale, have caused earthquakes large enough to effect the people and structures in the area. Pumping disposal water into deep formations can cause the release of built up pressure resulting in earthquakes with a magnitude as high as 4-5

EARTHQUAKE SCALE

• Gravity measurements indicate the core is 10-12 times denser than water indicating a very dense metal under enormous pressure.