The Origins and Structure of the Earth

Vocabulary flashcards covering cosmological theories, solar system and lunar origins, early Earth atmospheric development, prebiotic theories, and the internal structure of the Earth.

Big Bang Theory

A scientific theory positing that the universe originated from an extremely hot and dense singularity approximately $13.8\,\text{billion years}$ ago and has been expanding ever since.

Steady State Theory

A cosmological model asserting that the universe has no beginning or end and maintains a constant average density as continuous creation of new matter balances its expansion.

Cosmic Microwave Background Radiation (CMB)

The faint radiation detected throughout the universe representing the thermal afterglow of the Big Bang, discovered by Penzias and Wilson in $1965$.

Redshift of Galaxies

The lengthening of light wavelengths from distant galaxies as they move away from us, observed by Edwin Hubble as evidence that the universe is expanding.

Big Bang Nucleosynthesis

The theoretical framework predicting the formation of light elements—primarily hydrogen, helium, and lithium—in the early universe.

Titius-Bode Law

An empirical rule noting that each planet in the solar system is located approximately twice as far from the Sun as its inner neighbor.

Ecliptic Plane

The geometric plane defined by Earth's orbit around the Sun, in which nearly all major planets in the solar system lie.

Capture Theory (Solar System)

A solar system formation hypothesis proposed by Michael Woolfson and Stuart Ross Taylor suggesting the Sun's gravity captured wandering rogue planets or passing stars.

Fission Theory (Solar System)

A hypothesis proposed by George Darwin positing that the rapidly rotating Sun ejected mass due to centrifugal forces, which formed into planets.

Stellar Collision Theory

A theory proposed by James Jeans and Harold Jeffreys suggesting the solar system formed from debris generated by a collision between two stars.

Tidal Theory

A model developed by James Jeans and Harold Jeffreys proposing that a passing star drew out long filaments of gas from the Sun that condensed into celestial bodies.

Disk Instability Model

A model explaining that dense regions within a protoplanetary disk can become gravitationally unstable and rapidly collapse to form giant gas planets directly.

Nebular Hypothesis

The widely accepted theory stating that the solar system formed $4.6\,\text{billion years}$ ago from the gravitational collapse and flattening of a giant rotating cloud of gas and dust.

Planetesimal Hypothesis

A refined version of the nebular hypothesis describing how small clumps accreted into planetesimals and protoplanets through collisions and gravitational growth.

Frost Line

The boundary distance in the early protoplanetary disk beyond which temperatures were low enough for volatile hydrogen compounds and ices to condense.

Giant Impact Hypothesis

The prevailing theory stating that the Moon formed when a Mars-sized celestial body named Theia collided with the proto-Earth $4.6\,\text{billion years}$ ago.

Theia

The hypothetical Mars-sized planet that struck proto-Earth in a massive collision, producing ejected material that coalesced to form the Moon.

Co-formation (Sister) Theory

A hypothesis proposing that Earth and the Moon formed together simultaneously from the same region of the protoplanetary disk.

Great Oxygenation Event (GOE)

An event occurring around $2.4\,\text{billion years}$ ago when photosynthesizing cyanobacteria significantly increased atmospheric oxygen levels.

Stromatolites

Layered sedimentary structures built by cyanobacterial mats trapping sediments, offering some of the oldest fossil evidence of life dating back over $3.5\,\text{billion years}$.

Banded Iron Formations (BIFs)

Layered rocks consisting of alternating iron oxides and silica formed between $3.8$ and $1.8\,\text{billion years}$ ago as dissolved oceanic iron reacted with biological oxygen.

Abiogenesis

The natural process by which life arose from non-living inorganic and organic chemical compounds on early Earth.

Miller-Urey Experiment

A $1953$ experiment simulating prebiotic Earth conditions with electrical sparks passed through water, methane, ammonia, and hydrogen, yielding amino acids.

Hydrothermal Vent Hypothesis

The hypothesis proposing that life originated at deep-sea hydrothermal vents, which provided heat, chemical energy, and mineral catalysis.

Panspermia Hypothesis

The hypothesis proposing that life or prebiotic organic molecules were delivered to Earth from space via comets, meteorites, or interstellar dust.

Late Heavy Bombardment (LHB)

A period of intense meteorite impacts between $4.1$ and $3.8\,\text{billion years}$ ago that heavily cratered inner solar system bodies and shaped early Earth.

Differentiation

The process early in Earth's history where a partially molten interior separated into layers based on density, causing heavy metals to sink to the core.

Continental Crust

The thicker, less dense outer layer of Earth's crust composed predominantly of granitic rocks rich in silica and aluminum.

Oceanic Crust

The thinner, denser section of Earth's crust composed primarily of basaltic rocks rich in iron and magnesium.

Asthenosphere

The ductile, semi-solid region of the upper mantle located beneath the lithosphere that deforms plastically and enables tectonic plate motion.

Lithosphere

The rigid outermost mechanical shell of Earth comprising the entire crust and the uppermost solid portion of the mantle.

Mohorovičić Discontinuity (Moho)

The seismic boundary separating Earth's crust from the mantle, marked by an abrupt increase in seismic wave velocity discovered by Andrija Mohorovičić in $1909$.

Gutenberg Discontinuity

The boundary separating Earth's lower mantle from the liquid outer core at a depth of about $2,900\,km$, characterized by the disappearance of S-waves.

Lehmann Discontinuity

The boundary between Earth's liquid outer core and solid inner core, discovered by Inge Lehmann in $1936$ via seismic wave reflections and refractions.

Magnetosphere

The region surrounding Earth governed by its magnetic field, generated by molten iron-nickel convection in the outer core, shielding the surface from solar wind.

P-waves (Primary Waves)

Compressional seismic body waves that travel fastest through Earth and can pass through both solid and liquid layers.

S-waves (Secondary Waves)

Shear seismic body waves that move material perpendicular to wave direction and can propagate only through solid media.