Earth's Formation and Structure

Earth's Uniqueness and Formation

• Earth is unique due to its hospitable atmosphere, abundance of water, diverse climates, and ability to support life.
• It formed from a swirling eddy of nebular material approximately 4.6 billion years ago.
• The Earth accreted as a solid body and then differentiated into a layered planet due to internal heating.
• Planetesimals gathered material to form Earth and other planets around 4.6 billion years ago.
• Earth orbits the Sun at an average distance of 150 million kilometers (93 million miles), making it the third planet.

Earth's Layered Structure

• Earth's layers include: crust, mantle, and core.
• The outermost layer, the crust, is divided into continental and oceanic portions.
• The crust and underlying solid upper mantle form the lithosphere, which overlies the asthenosphere.
• The asthenosphere is a plastic zone that flows slowly, underlain by the solid lower mantle.
• Earth's core consists of a liquid outer portion and a solid inner portion.
• The lithosphere is broken into tectonic plates that diverge, converge, and slide sideways.

Planetary Differentiation

• Early in Earth's history, heavier elements like iron and nickel sank to the center as the planet was molten, which formed the core.
• Lighter elements like silicon, oxygen, and magnesium rose, forming the mantle and crust.
• Planetary differentiation was driven by heat from radioactive decay and the planet's formation.
• As the planet cooled, material solidified, and layers formed based on density differences and the internal temperature and pressure conditions.
• Earth's internal structure (core, mantle, and crust) resulted from the segregation of materials in its molten phase, influenced by gravitational forces and material properties.

The Core

• The core is primarily composed of iron and nickel, which are dense metals and are pulled toward the center due to gravity.
• The inner core is solid due to extreme pressure, while the outer core is liquid due to slightly lower pressure.
• The core generates Earth's magnetic field through the movement of molten metals in the outer core, known as the geodynamo effect.
• The core has a density of 10-13 grams per cubic centimeter and occupies about 16% of Earth's total volume.
• Seismic data suggests a small, solid inner region and a larger, liquid outer portion, both mainly iron with some nickel.

The Mantle

• The mantle is composed of silicate minerals rich in magnesium and iron.
• It extends from about 35 kilometers beneath the surface to about 2,900 kilometers deep.
• The mantle is semi-solid, behaves like a thick fluid, and convects, driving plate tectonics.
• Mantle convection is driven by heat from the core; material rises toward the crust, while cooler material sinks.

The Crust

• The crust, made of lighter silicate minerals, is the outermost layer.
• It is divided into two types: oceanic and continental.
• Oceanic crust is composed of basalt, is denser, and is thinner (5–10 km thick).
• Continental crust is made of granite, is less dense, and is thicker (30–70 km thick).
• The crust is rigid and broken into tectonic plates, leading to earthquakes, volcanic activity, and mountain formation.

The Lithosphere

• Solid and rigid layer of the Earth, including the crust and uppermost part of the mantle.
• Its thickness varies, being thinner under oceans (5-10 km) and thicker under continents (up to 70-100 km).
• It does not bend easily, acting as Earth's "hard shell".
• Rocks with oxygen and silicon, known as silicates, are the most abundant elements in the lithosphere.

The Asthenosphere

• Defined as a layer of the Earth's mantle located just beneath the lithosphere, extending 100 km to 700 km beneath the surface.
• Semi-fluid, partially molten, and behaves like a very viscous material, deforming slowly over time.
• Less rigid than the lithosphere, facilitates the movement of the tectonic plates above.
• High temperature and pressure allow partial melting, causing the rock to behave plastically, enabling slow movement over geological time scales.

Atmosphere: Definition and Composition

• It is a blanket of gases enveloping Earth and retained by gravity.
• Consists of nitrogen, water vapor, oxygen, and carbon dioxide, which are atmospheric gases.
• Blocks most of the sun's ultraviolet radiation, conducts solar radiation and precipitation, and maintains an average surface temperature of about 15° Celsius (59° Fahrenheit).

Origins of the Atmosphere

• The atmosphere is closely linked to volcanic eruptions and outgassing.
• Earth's first atmosphere (4.6 billion years ago) was majorly composed of hydrogen (H2) and helium (He).
• Early atmosphere gases included water vapor, carbon dioxide, nitrogen, methane, and ammonia.
• The first atmosphere was lost quickly due to the Sun's solar wind.

Secondary Atmosphere

• Volcanic eruptions released gases such as water vapor, carbon dioxide, sulfur dioxide, methane, ammonia, and nitrogen from Earth's interior through the process of outgassing.
• Volcanic activity was extremely intense due to high internal heat and frequent asteroid impacts around 4.4 to 4.0 billion years ago.

Oxygen and the Atmosphere

• Around 2.4 billion years ago, the Great Oxygenation Event saw a rise in oxygen due to photosynthesis by early microbial life.
• Photosynthetic bacteria called cyanobacteria started producing oxygen through photosynthesis around 2.7 billion years ago.
• Earth's early atmosphere, composed mostly of carbon dioxide, water vapor, and nitrogen, evolved over time.
• Life on Earth began in the oceans, with the earliest known evidence of life dating back to around 3.5 billion years ago.

Ocean Formation

• Water vapor in the atmosphere condensed as the Earth cooled further and formed the Earth's early oceans.
• The Earth's surface became more stable, which allowed the development of a solid, water-covered crust.

Earth's Oxygenation

• Initially, oxygen production was low; any formed oxygen quickly reacted with surface minerals or dissolved in the oceans.
• The Great Oxygenation Event occurred around 2.4 billion years ago, leading to oxygen accumulating in the atmosphere.
• Anaerobic organisms began to go extinct and dramatic changes in Earth's ecology occurred.
• In the Carboniferous period, oxygen levels peaked around 300 million years ago, which supported giant insects and amphibians.
• Today, oxygen composes about 21% of Earth's atmosphere.

Banded Iron Formations(BIFs)

• Resulted from dissolved iron reacting with oxygen to form iron oxides
• The oxides then settled in layers onto the ocean floor
• Can be found in Western Australia, Canada, Brazil, and South Africa
• Formed between 3.8 billion and 1.8 billion years ago.
• As oxygen levels increased, nearly all dissolved iron was used up, and the formation stopped around 1.8 billion years ago.
• Consist of alternating bands of iron-rich and silica-rich minerals that occurred due to fluctuation production of oxygen in cycle.

The Hydrosphere

• Is composed of all the water on Earth, with about three-fourths of Earth covered in water.
• Freshwater accounts for less than three percent of the hydrosphere, frozen mostly in ice sheets and glaciers.
• It can be found underground in aquifers, as well as in rivers, lakes, and springs.
• Water circulates as vapor, condenses into clouds, and falls back as precipitation.
• The hydrosphere regulates Earth's temperature and climate.

Origins of the Hydrosphere

• Earth's water originated from internal (primordial) geological processes.
• During early Earth's history (~4.5 billion years ago), volcanic activity released water vapor, carbon dioxide, and other gases.
• Water also delivered ice from space via comets, asteroids, and meteorites during the Late Heavy Bombardment (~4.1-3.8 billion years ago).

Earth's Spheres

• Including the atmosphere, hydrosphere, and lithosphere, are are constantly interacting.
• Water helps determine abundance, diversity, and distribution of the other Earth's spheres.
• The biosphere affects the composition of atmospheric gases.
• Plate movements affect the size, shape, and distribution of ocean basins.
• These interactions make Earth a dynamic planet that has evolved and changed since its origin 4.6 billion years ago.