Earth's Structure and Spheres

• The text is about resources from the Earth, its structure, and the determination of Earth's age.

Structure of Earth and its Spheres

• Earth is unique among the planets and moons of the solar system, as it has land, oceans, air, and life.
• These components are called the geosphere, hydrosphere, atmosphere, and biosphere, respectively.
• The unique combination of these components on Earth has been termed as an example of the Goldilocks effect.
• The Earth is 'just right' in terms of size, temperature and other physical conditions for the co-existence of a hydrosphere, atmosphere, geosphere and biosphere.

Forming Earth's Three Inorganic Spheres

• The three inorganic spheres of Earth—geosphere, hydrosphere, and atmosphere—developed early in Earth's history.
• Their existence, structure, and composition are a consequence of the formation of the solar system from a cloud of interstellar gas and dust.
• This dust formed the Sun, planets, moons, asteroids, comets, meteors, and other materials and consisted mostly of hydrogen and helium.
• Metallic cores and rocky mantles in planets and moons are from carbon, silicon, oxygen, iron and other elements in the dust.
• Planetessimal Accretion theory has been used to explain the solar system formation using the data from probes sent to explore planets and comets since 1960.

The Planetessimal Accretion Theory

• The Planetessimal Accretion theory describes the formation of the solar system, including Earth, from a solar nebula about 4.56 billion years ago.
• A supernova explosion may have sent a shock wave through the nebula, causing the material to concentrate.
• Most material condensed into the Sun, and the rest began to orbit the center.
• The orbiting dust collected into clumps due to gravity, forming asteroids and planetessimals.
• Larger planetessimals attracted each other, collided, and grew into small planets, marking the accretion phase about 4.5 billion years ago.
• These collisions converted gravitational potential energy into heat, melting the material.

The Accretion Phase

• By the end of the accretion process, the eight large planets of our solar system were formed.
• Some remaining planetessimals were captured as moons, while others were ejected to the outer solar system.
• Toward the end, the planets cooled, radiating heat into space.
• On Earth, denser metals sank to form the core, and less dense silicates formed the mantle by about 4.4 billion years ago.
• Icy comets and planetessimals mixed with silicates in the mantle, releasing water into the forming atmospheres.

Geosphere Composition

• Between 4.4 and 4 billion years ago, Earth's surface cooled, forming the first rocky crust and solidifying the mantle.
• About 4 billion years ago the crust stabilized, oceans formed, and early life emerged around 3.8 billion years ago.
• The geosphere includes all material below Earth's solid surface, consisting of the rocky crust, mantle, and metallic core.
• Until 1936, it was thought to be solid, but Lehmann's investigations revealed a liquid outer core.
• The geosphere contains 99.977% of Earth's mass, with an estimated total mass of 5.9722 × 1021 tonnes.
• The crust varies in thickness from 10km to 100km, and the mantle-core boundary is 2900km below Earth's surface.
• Core thickness is 3500km.
• Temperature, pressure, and seismic wave velocities change through these layers.
• The average density of Earth is 5510kgm¯³ (5.51gcm³).

Elemental Composition

• The average elemental composition of Earth: iron (Fe) is 32.1%, oxygen (O) is 30.1%, silicon (Si) is 15.1%, and magnesium (Mg) is 13.9%.
• In the core: iron constitutes 85.5%, in the mantle: oxygen constitutes 44.8%, in the crust: oxygen constitutes 46.6%

Earth's Core Details

• Earth's spherical core is about 3500 km thick and primarily composed of an iron metallic alloy, with nickel, silicon, and sulfur.
• The inner core started solidifying about one billion years ago and is surrounded by a liquid outer core which contains 99% of Earth's iron.
• Convection currents in the liquid core generate Earth's magnetic field.
• The inner core's surface temperature is estimated to be 5000°C; the outer core's surface temperature is about 3700°C.

Earth's Mantle and Crust Details

• The mantle is a 2900km thick shell of crystalline material surrounding the core, composed mainly of magnesium, silicon, and oxygen, with varying silicates and oxides.
• Mantle temperatures range from 500°C at the top to 3750°C at the base.
• The crust, is the outermost layer of the geosphere, ranging from 10–100km thick, and is composed of silicon and oxygen with other silicates.
• Crust temperature ranges from 500°C at the base to 15°C at the top.

Earth's Atmosphere

• Earth's atmosphere is its outer gas layer, composed of nitrogen (78%), oxygen (21%), argon (0.9%) and carbon dioxide (0.04%).
• Other gases like neon, helium, methane, and sulfur dioxide are present in small amounts.
• Water vapor varies, from almost none in cold air to 4% by weight in hot surface air.
• The atmosphere has a mass of about 5.15 × 1018kg.

Hydrosphere

• The hydrosphere includes water in rivers, lakes, oceans, ice sheets, the atmosphere, and subsurface groundwater.
• Earth's total water mass is about 1.4 × 1021 kg, or 0.023% of Earth's mass.
• Most water is in the oceans (96.5%), with 1.74% in ice sheets and glaciers, and 1.69% as groundwater.
• Oceanic water contains about 3.72 × 10¹ºkg of dissolved salt (NaCl).
• Water is essential for Earth's internal and surface processes, plate tectonics, and as a medium for the chemistry of life.

Seismic Waves to Probe Earth's Interior

• Earth's internal structure is inferred from mathematical models depicting seismic wave movement.
• Compositional characteristics are estimated based on average planet density, crust and mantle composition, and solar system element abundance.
• Seismometers measure seismic waves from earthquakes and explosions.
• Seismic wave movements depend on the composition, density, and physical state of subsurface layers.
• Seismic waves refract or reflect at boundaries between materials of different density, detected using underground nuclear bomb tests.

Types of Seismic Waves

• Earthquakes occur when crustal blocks slide past each other along a fault plane.
• Energy from these vibrations is transmitted as seismic waves, causing ground shake.
• Body waves include P-waves (primary) and S-waves (secondary).
• P-waves travel faster and are compressional through solids, liquids, and gases; S-waves are shear waves, transmitting through solids only.
• The time difference in arrival between P- and S-waves determines the distance to the earthquake.

Identifying Boundaries with Seismic Waves

• Since the late 19th century, geologists and engineers have used seismic waves to study earthquakes.
• In 1909, Andrija Mohorovičić identified the crust-mantle boundary (Moho) based on P-wave arrival times.
• Mohorovičić plotted seismic wave arrival times against distance and determined slower P-waves traveling through the crust and faster waves refracted into the mantle.
• The depth to the Moho is determined by the distance of the crossover point in the direct and refracted wave lines.
• Laboratory measurements matched P-wave velocity in the lower layer with that of peridotites, rocks requiring high temperatures and pressures to form.

The Core-Mantle Boundary

• Most seismic waves curve as they travel through Earth due to gradual density increases.
• Refraction occurs when waves cross a boundary, and waves travelling perpendicularly continue unaffected.
• Refraction causes the seismic wave's curvature with depth. Another phenomenon is the reflection of sonic waves off of a boundary creating symmetrical formations. This is useful for identifying subterranean geological structures given the density and strength of these formations. Different materials will also impact the reflection patterns based on angle, too.

Seismic Wave Shadow Zones

• S-waves do not travel through liquids, while P-waves do.
• They cause areas where no P- or S-waves arrive, called seismic wave shadow zones.
• The S-wave shadow zone shape relates directly to the core size because S-waves cannot travel through the liquid core.
• In 1936, Inge Lehmann identified weak P-waves within the P-wave shadow zone, which led to the confirmation of the existence of an inner core.

Methods to Identify Earth's Inner Layers

• Testing with seismometers confirm how sonic waves are reflected off Earth's inner core, with wave tests from US nuclear bombs indicating the same. This data helps define size and material composition in relation to velocity during travel. Combining metallic elements of a mix on iron, with some nickel, silica and sulfur to product the measurable P-Velocity.

Identifying the Base and Location of the Lithosphere

• The lithosphere is Earth's brittle outermost shell, comprising the crust and stiff part of the mantle.
• Oceanic is basaltic material. continental crust comprises granites, metamorphic and sedimentary rocks.
• The seismic tomography method is used to map lithospheric plate boundaries and thickness, similar to ultrasound/CT scans in medicine.
• P- and S-wave velocity variations create three-dimensional images of Earth's interior with velocity anomalies depicting the lithospheric plates.

Age of Earth: Initial Information

• Earth's surface is being weathered and converted.
• Earliest life and age likely will not be found.
• A minimum of 4.4 billion years old from radiometric dating.
• Use Moon rock too to infer it's formation.

Meteorite Evidence

• Chondrites were accreted to form the first planetessimals.
• The Earth is between 4.4 and 4.5 billion years of age.

Isotopes

• Earth materials contain similar isotope make up.
• Isotopes are used to measure the date.

Meteorites

• Classified into stony, iron and stony iron with sub classifications.
• Iron meteorites show that the Earth's core is made from Iron.

Dating Earth's Layers

• Use of different decay amounts for different dating ranges. Oldest materials will be the base age. This helps give Earth formation age.
• Zircon Crystal the oldest, with similar element make-up.