Science 9 Reviewer - Unit 6: Modelling the Earth's Interior with Seismic Wave Data

Comprehensive vocabulary flashcards covering seismic wave types, triangulation methods, and the structural composition of the Earth's crust, mantle, and core as outlined in the Science 9 Unit 6 Reviewer.

Seismic waves

Vibrations that travel through the Earth, caused by earthquakes, volcanic activity, or artificial explosions, which provide indirect evidence about the composition and properties of the Earth's layers.

Body waves

Seismic waves that travel through the Earth's interior; they are the fastest seismic waves and can move through solid and liquid layers.

P-waves (primary waves)

The fastest seismic waves and the first to be recorded by seismographs; they move in a compressional (longitudinal) way through solids, liquids, and gases, causing only minor ground displacement.

S-waves (secondary waves)

Transverse or shear waves that move particles perpendicular to the wave direction; they are slower than P-waves, cause greater ground movement, and travel only through solids.

Seismograph

A device that detects and records seismic waves, utilizing a mass suspended on a spring.

Seismogram

The record made by a seismograph displaying wave arrival times and intensities, with arrivals ordered as: P-waves first, then S-waves, then surface waves.

Surface waves

Seismic waves that travel along the Earth's surface with lower frequencies, high amplitude, and slow speed, causing the most damage during an earthquake.

Love waves

Surface waves that move in a horizontal shearing motion, causing the ground to shift from side to side; they are faster than Rayleigh waves but slower than body waves.

Rayleigh waves

The slowest and most destructive seismic waves; surface waves that move in a rolling motion like ocean waves, producing both vertical and horizontal ground movement.

Epicenter

The location of an earthquake on the surface, determined by using the difference in arrival times of P- and S-waves.

Triangulation

A method that uses arrival-time data from at least three seismograph stations to draw distance circles on a map that intersect at a single point to locate an epicenter.

Three-station rule in triangulation

The requirement where one station provides a distance circle with infinite possibilities, two circles intersect at two points, and a third circle narrows the intersection down to a single point.

Reflection

A wave phenomenon where a seismic wave bounces back upon hitting a boundary between materials.

Refraction

A wave phenomenon where a seismic wave changes direction or bends because material properties change.

Speed of seismic waves

P-waves travel at $6\text{--}14\,\text{km/s}$, S-waves travel at $3.5\text{--}7.5\,\text{km/s}$, and surface waves are the slowest overall.

Earthquake magnitude

A measurement calculated from wave amplitude and duration; larger earthquakes have higher-amplitude waves and longer durations of shaking.

Crust

The thinnest and outermost solid layer of the Earth, accounting for less than $1\%$ of the Earth's volume, composed mainly of silicate minerals divided into tectonic plates.

Continental crust

The thicker ($30\text{ to }70\,\text{km}$) layer of crust underlying continents, composed mainly of granite; it is less dense, resists subduction, and can endure for billions of years.

Oceanic crust

The thinner ($5\text{ to }10\,\text{km}$) crust forming ocean basins, composed mainly of basalt; it is geologically younger (oldest sections are about $200\text{ million}$ years old) due to subduction recycling.

Mohorovičić Discontinuity (Moho)

The boundary between the crust and the mantle where seismic waves suddenly speed up; discovered in $1909$ by Croatian seismologist Andrija Mohorovičić.

Moho depth variations

The depth of the Moho boundary, which is shallower ($5\text{--}10\,\text{km}$) under oceanic crust and deeper ($30\text{--}70\,\text{km}$) under continental crust.

Isostasy

The gravitational balance between the Earth's crust and the mantle; like an iceberg, crust under mountains possesses deeper roots, resulting in thicker crust.

Cratons

Large, stable regions of the continental crust composed of old, complexly deformed Precambrian crystalline rocks known as shields.

Shields

The exposed crystalline rocks of a craton, which are typically covered by a thin veneer of sedimentary rocks called stable platforms.

Mountain belts

Elevated regions of uplifted and deformed rocks formed through orogeny (mountain-building events).

Ocean ridges

Mountain belts of newly formed oceanic crust, uplifted and faulted, located in the middle of oceans and standing about $2\,\text{km}$ high.

Deep ocean basins

Areas situated between continental margins and ocean ridges, containing features such as seamounts and abyssal plains.

Continental margins

Regions that separate continental crust from oceanic crust, featuring either passive margins or active margins with deep trenches and island arcs.

Seafloor spreading

The process where new oceanic crust is generated at mid-ocean ridges, pushed outward, and eventually subducted at deep-sea trenches.

Thickest and thinnest crust

Crust is thickest beneath large mountain ranges such as the Himalayas, and thinnest along mid-ocean ridges.

Granite

A less dense, silica-rich igneous rock that provides buoyancy to the continents.

Basalt

A dense, iron-rich volcanic rock that constitutes the primary composition of oceanic crust.

Seismic reflection (crustal study)

The technique of bouncing seismic waves off internal rock layers to identify compositional differences.

Seismic refraction (crustal study)

The technique of measuring the bending of seismic waves as they transition across materials to determine crustal thickness and structure.

Mantle

The thickest layer of the Earth, situated between the crust and the core; measuring about $2{,}900\,\text{km}$ thick and comprising about $84\%$ of Earth's volume.

Asthenosphere

The ductile, semi-fluid mantle layer lying just beneath the lithosphere that enables tectonic plates to move; derived from the Greek word asthenes, meaning weak.

Lithosphere

The rigid outer shell composed of the crust and uppermost mantle that is fractured into tectonic plates.

Gutenberg Discontinuity

The mantle-core boundary where seismic waves slow down or stop due to the presence of the liquid outer core.

Perovskite

The most abundant mineral present in the Earth's mantle.

Lower mantle

The solid layer beneath the asthenosphere that is significantly denser under immense pressure, facilitating faster seismic wave transmission.

Olivine

An upper mantle mineral that undergoes transformation into denser polymorphs (wadsleyite and ringwoodite) in the transition zone.

Transition zones

Mantle regions at depths of approximately $410\,\text{km}$ and $660\,\text{km}$ where minerals transform under extreme pressure, separating the upper and lower mantle.

Convection currents

Slow-moving thermal cycles of hotter material rising and cooler material sinking within the mantle, driving tectonic plate motions.

Mantle plumes

Columns of hot, upwelling mantle rock that ascend to produce localized volcanic hotspots, such as the Hawaiian Islands.

Hotspots

Anomalous regions of persistent volcanic activity fed by mantle plumes, exemplified by Hawaii.

Subduction zones

Regions where descending tectonic plates sink back into the mantle, recycling geological material.

Composition of the mantle

Composed primarily of silicate minerals rich in magnesium and iron.

Temperature of the mantle

Ranges from approximately $500^\circ\text{C}$ near the top to over $4{,}000^\circ\text{C}$ near the core-mantle boundary.

Seismic wave velocity indicators

Slower seismic speeds indicate hotter or less rigid materials, whereas faster seismic speeds indicate cooler or denser materials.

Core-mantle boundary (CMB)

A boundary located about $2{,}900\,\text{km}$ below the surface that influences mantle convection and the liquid outer core responsible for generating Earth's magnetic field.

Outer core

The liquid core layer made primarily of iron and nickel whose molten convective flow generates the Earth's geomagnetic field; P-waves slow down in it and S-waves cannot traverse it.

Inner core

The solid spherical center of the Earth made predominantly of iron and nickel, maintained in a solid state by immense confining pressure which elevates its melting point.

Growth of the inner core

The ongoing process by which the solid inner core expands at a rate of approximately $1\,\text{millimeter per year}$ as the outer core slowly cools and solidifies.

Shadow zone

A geographic band on the Earth's surface where direct S-waves cannot be detected following an earthquake because S-waves cannot propagate through the liquid outer core.

Planetary differentiation

An early Earth process occurring approximately $4.5\text{ billion}$ years ago where denser elements sank to form the core, while lighter silicate materials rose to form the mantle and crust.

P-wave velocity across core boundaries

P-waves slow down upon entering the liquid outer core due to lower rigidity, and their speed increases sharply upon entering the denser and more rigid inner core.

Composition of the core

Dominated by iron and nickel, supplemented by smaller quantities of lighter elements including sulfur, oxygen, and silicon.

Evidence for core composition

Derived from the high abundance of iron and nickel in the solar system, corroborated by structural and chemical studies of meteorites.

Depth of the core

The outer core extends to a depth of about $5{,}150\,\text{km}$, and the center of the inner core extends to approximately $6{,}371\,\text{km}$.

Outer core vs. Inner core wave transmission

In the liquid outer core, P-waves pass through at reduced speeds while S-waves are blocked entirely; in the solid inner core, both P-waves and S-waves are capable of propagating.