Earth Science Practice Test Flashcards

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Water Coverage

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About 71% of Earth’s surface is covered by water and the average ocean depth is about 2.3 miles. The deepest place is the Mariana Trench. These figures describe the global distribution and scale of Earth’s oceans.

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Mariana Trench

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The Mariana Trench is the deepest known part of the world’s oceans, located in the western Pacific. It reaches depths of about 11 kilometers and represents the greatest known oceanic depth. It is formed by an ocean-ocean convergent subduction zone.

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Geosphere

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The geosphere includes Earth’s solid parts: land, rocks, minerals, soils, and the planet’s interior structure. It encompasses processes like weathering, erosion, and volcanic activity. The geosphere interacts with other spheres to shape the solid Earth.

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Biosphere

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The biosphere comprises all living organisms and undecomposed organic matter on land and in the sea. Much of life exists in the ocean and relies on extracting nutrients and $CO_2$ from seawater. It includes ecosystems and the life-supporting interactions among organisms.

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Atmosphere

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The atmosphere is the gaseous envelope surrounding Earth that controls weather and climate. It is vital for life, regulating temperature and protecting against harmful radiation. Atmospheric composition and dynamics drive meteorological processes.

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Hydrosphere

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The hydrosphere includes all water on and beneath Earth’s surface and in the atmosphere: oceans, rivers, lakes, groundwater, rain, and snow. It is central to processes that make Earth habitable, such as the water cycle and nutrient transport. The hydrosphere interacts strongly with the biosphere and atmosphere.

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Cryosphere

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The cryosphere consists of Earth’s frozen components, including mountain glaciers, ice caps, and polar ice sheets. It stores large amounts of freshwater and influences sea level and climate. Seasonal and long-term changes in the cryosphere affect isostasy and global systems.

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Major Oceans

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The five named oceans are the Arctic, Atlantic, Pacific, Indian, and Southern (Antarctic) Oceans. Each ocean is defined by geographic boundaries and distinct oceanographic characteristics. These oceans together constitute the global ocean system.

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Eratosthenes Method

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Eratosthenes measured Earth’s circumference using shadow angles and the distance between two cities. He used the relation $\frac{360^\circ}{7.2^\circ}=\frac{x}{500\text{ miles}}$ to compute $x\approx25{,}000$ miles. This triangulation used geometry of the sphere and solar angles at solstice.

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Latitude

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Latitude measures angular distance north or south of the Equator, ranging from $0^\circ$ at the Equator to $90^\circ$ at the poles. Latitude lines are parallels that remain equally spaced and determine north–south position. One degree of latitude equals 60 nautical miles.

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Longitude

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Longitude measures angular distance east or west of the Prime Meridian (Greenwich), from $0^\circ$ to $180^\circ$. Longitude lines are meridians that converge at the poles, so the ground distance represented by one degree of longitude decreases with latitude. Longitude determines east–west position and relates to time.

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Degrees Conversion

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Degrees ($^\circ$) are subdivided into minutes (') and seconds ("). To convert $D^\circ M' S''$ to decimal degrees: first convert seconds to minutes and then minutes to degrees (e.g., $30''=0.5'$ and $23.5'=23.5/60=0.3917^\circ$). Decimal degrees are useful for computations and GPS input.

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Nautical Mile

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A nautical mile is defined as one minute of latitude and equals 1.85 km (about 1.15 statute miles). Therefore $1^\circ$ latitude = 60 nautical miles. Nautical miles link angular Earth coordinates to physical distances used in navigation.

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Polaris Latitude

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The altitude angle of Polaris (the North Star) above the horizon equals the observer’s latitude in the Northern Hemisphere. Measuring that angle with a sextant gives a direct determination of latitude. This method works because Polaris sits nearly above the North Pole.

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Longitude by Time

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Longitude can be computed from the time difference relative to GMT because Earth rotates $15^\circ$ per hour ($360^\circ/24\,$hr). Longitude $=\text{time difference (hours)}\times15^\circ/\text{hr}$. Being east of GMT means local time is earlier (positive longitude), and west means later (negative longitude).

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GPS

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GPS stands for Global Positioning System and uses a constellation of about 24 operational satellites to determine position. Receivers use signals from at least four satellites and trilateration (timing and distance) to compute latitude, longitude, and altitude. Satellites orbit at about 10,900 miles and provide global coverage.

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Spherical Evidence

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Three lines of evidence that Earth is spherical are: the round shadow Earth casts on the Moon during lunar eclipses, changing visible stars (e.g., Southern Cross visible only in the Southern Hemisphere), and the fact the Sun cannot be directly overhead everywhere at once. These observations are inconsistent with a flat Earth.

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Bathymetry

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Bathymetry is the measurement and mapping of seafloor depths relative to the ocean surface. Bathymetric maps use isobaths (contour lines of equal depth) to show seafloor shape and features. Bathymetry is analogous to topography for the seafloor.

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Topography

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Topography describes the height and shape of land surfaces above sea level. Topographic maps use contour lines to represent elevation and slope. Together with bathymetry, topography characterizes Earth’s surface in two and three dimensions.

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Isobath Spacing

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The spacing of isobaths on a bathymetric map indicates slope steepness: closely spaced isobaths mean steep slopes, while widely spaced isobaths mean gentle slopes. Interpreting spacing helps identify features like continental slopes, ridges, and abyssal plains. This principle mirrors topographic contour interpretation.

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Two-Way Travel Time

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Two-Way Travel Time (TWT) is the time for a sound pulse to travel from the source to the seafloor (or reflector) and back to the receiver. TWT is measured by echo-sounders and is used to calculate depth. The measured time includes the outbound and return legs of the sound path.

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Speed of Sound

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The typical speed of sound in seawater is about $1500\ \text{m/s}$ (varies with temperature, salinity, and pressure). Using sound speed and TWT allows calculation of depth. Variations in velocity must be accounted for precise depth estimates.

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Depth from TWT

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Depth is computed from sound speed and two-way travel time using the relation Depth $=v\times(t/2)$, where $v$ is sound speed and $t$ is TWT. For example, with $v\approx1500\ \text{m/s}$ and $t=4\,$s, Depth $=1500\times(4/2)=3000\,$m. This formula assumes a straight vertical path and uniform velocity.

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TWT Formula

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The TWT can be expressed from depth as $\text{TWT}=\dfrac{2d}{v}$, where $d$ is depth and $v$ is sound speed. Rearranging gives depth or velocity when the other quantities are known. This relation is fundamental to echo-sounding and seismic surveys.

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Exclusive Economic Zone

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The Exclusive Economic Zone (EEZ) extends 200 nautical miles offshore, granting a coastal state exclusive rights to explore and exploit marine resources. Territorial seas extend 12 nautical miles where a state can enforce laws. France has the largest EEZ due to many overseas territories distributed globally.

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Bathymetric Profiling

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Sketching a bathymetric map from spot depth measurements involves contouring isobaths and drawing a depth profile along a transect to show seafloor shape. Interpreting the resulting profile reveals features like shelves, slopes, and basins. This skill relies on converting point depths into continuous contours.

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Continental Shelf

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The continental shelf is the shallow, gently sloping submerged extension of a continent that ends at the shelf break. It is part of the continental margin and is typically rich in sediments and biological productivity. Shelves are important for fisheries and offshore resources.

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Continental Slope

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The continental slope is the steep descent from the shelf break down toward the deep ocean floor and marks the boundary between continental and oceanic crust. Slopes are characterized by steep gradients and can host submarine canyons. Sediment transport often occurs down the slope to the rise.

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Continental Rise

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The continental rise lies between the continental slope and abyssal plain and consists of sediments accumulated by turbidity currents and mass wasting. It forms a gentler gradient compared to the slope and merges into the abyssal plain. The rise records sediment flux from the continent.

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Abyssal Plain

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An abyssal plain is an extensive, flat region of the deep ocean floor formed by fine sediment deposition over oceanic crust. Abyssal plains are among the flattest places on Earth and cover large areas of the deep-sea basins. They are important sinks for pelagic and terrigenous sediments.

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Seismic Reflection

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Seismic reflection profiling uses sound (acoustic) pulses sent into the subsurface and records reflections from rock and sediment layers. It maps subsurface structure and stratigraphy, helping to locate sedimentary layers, faults, and potential resources. It is widely used in marine geology and hydrocarbon exploration.

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Density

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Density is mass per unit volume and is calculated as $p=\dfrac{m}{v}$. Units are typically kg/m^3 or g/cm^3. Density determines how materials layer inside Earth and influences buoyancy and isostatic equilibrium.

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Earth Interior

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Earth’s interior is layered by composition and physical state: a solid inner core, liquid outer core, viscous mantle, and solid crust. Knowledge of these layers comes from seismic wave behavior (e.g., S-wave shadow zones and P-wave speed changes). The core is iron-rich, the mantle is silicate rock, and the crust is chemically distinct.

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Lithosphere

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The lithosphere is the rigid outer shell of Earth composed of the crust and the uppermost mantle. It is broken into tectonic plates that move relative to one another. The lithosphere ‘‘floats’’ on the weaker, ductile asthenosphere below.

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Asthenosphere

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The asthenosphere is the hot, weak, ductile part of the upper mantle directly below the lithosphere. It flows slowly and allows tectonic plates to move on its surface. Convection in the asthenosphere contributes to plate motions.

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P Waves

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P waves are compressional body waves that travel fastest through Earth and can pass through solids and liquids in alternating push–pull motion. They arrive first on seismograms and their speed increases with rock density. P waves provide information about deep interior structure.

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S Waves

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S waves are shear body waves that travel slower than P waves and cannot propagate through liquids. Their absence in the outer core indicates the outer core is liquid. S waves help delineate solid versus liquid regions inside Earth.

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Surface Waves

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Surface waves (e.g., Rayleigh waves) travel along Earth’s surface, are slower than body waves, and usually cause the most earthquake damage. They decay with depth and have larger amplitudes than body waves at the surface. Surface waves dominate ground motion at long distances.

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Crust Composition

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Oceanic crust is primarily basaltic in composition and relatively thin and dense, whereas continental crust is largely granitic, thicker, and less dense. These compositional differences explain why continents stand higher than ocean basins. Continental crust also tends to be much older than oceanic crust.

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Crust Age

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Oceanic crust is relatively young (up to ~280 million years) because it is continuously created at mid-ocean ridges and recycled at subduction zones. Continental crust can be much older (up to >3.8 billion years) because it is less frequently recycled. This age contrast affects geology and tectonics.

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Isostatic Equilibrium

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Isostatic equilibrium occurs when lithospheric blocks are buoyantly balanced with the underlying mantle (no net vertical motion). Variations in crustal thickness and density produce different elevations that float at equilibrium. Isostasy governs mountain roots and basin subsidence.

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Glacial Rebound

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Glacial isostatic rebound is the uplift of crust formerly depressed by massive ice sheets once the ice is removed. Regions like central Canada are rising now because of post-glacial rebound, while peripheral regions (e.g., some U.S. east-coast areas) may experience relative sinking. This process restores isostatic balance over thousands of years.

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Radiometric Dating

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Radiometric dating uses known radioactive decay rates (half-lives) to determine the age of rocks or organic materials. For carbon-14 dating, the fraction of remaining $^{14}C$ compared to stable $^{12}C$ and the half-life yield age estimates. The method requires assumptions about initial concentrations and closed-system behavior.

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Convergent Boundary

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A convergent plate boundary occurs where two plates move toward each other and one may be subducted beneath the other. Types include ocean–ocean (island arcs and trenches), ocean–continent (trenches and continental volcanic arcs), and continent–continent (large mountain ranges). Convergent zones generate earthquakes, volcanism, and orogeny.

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Ocean–Ocean Convergence

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Ocean–ocean convergence involves two oceanic plates where one is subducted, forming a deep trench and an island-arc volcanic chain on the overriding plate (e.g., Japan). These zones produce strong earthquakes and explosive volcanism. They also form accretionary prisms from scraped sediments.

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Ocean–Continent Convergence

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Ocean–continent convergence occurs when an oceanic plate subducts beneath a continental plate, creating a trench offshore and volcanic mountain chains on the continent (e.g., the Andes). This setting produces earthquakes, mountain building, and arc volcanism. The subducted slab drives melting that fuels volcanism.

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Continent–Continent Convergence

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When two continental plates collide, neither is easily subducted, so the crust thickens and builds large mountain ranges (e.g., the Himalayas). This convergence produces intense crustal shortening, uplift, and frequent shallow earthquakes. Little volcanic activity occurs because no slab melts deeply under continents in this setting.

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Spreading Centers

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Spreading centers (mid-ocean ridges) are divergent plate boundaries where new oceanic crust is created by upwelling mantle and seafloor volcanism. They form ridge systems that mark plate separation and are associated with symmetric magnetic striping. Spreading rate varies by ridge (fast vs slow spreading).

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Seafloor Spreading Rate

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Spreading rate is calculated by dividing the distance of seafloor from the ridge by its age (rate = distance / age). Magnetic anomalies and rock ages provide the age; measured distances give the separation. Rates can be used to compute relative plate motions (e.g., combining half-spreading rates to get total plate separation).

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Oldest Oceanic Crust

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The oldest oceanic crust is found farthest from mid-ocean ridges, typically near continental margins; examples include the eastern coast of North America (>150 Ma). Youngest crust is at the ridge axis where new crust forms. Oceanic crust ages reflect continuous creation and subduction recycling.

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Spreading Rate Comparison

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The East Pacific Rise is a fast-spreading mid-ocean ridge, producing rapidly created oceanic crust, while the Mid-Atlantic Ridge is a slow-spreading ridge with wider axial valleys and slower creation rates. Fast spreading yields smoother topography and narrow ridge crests; slow spreading produces rougher seafloor and pronounced rift valleys.

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Hotspot

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A hotspot is a relatively stationary, anomalously hot mantle plume that produces volcanic activity independent of plate boundaries. As a plate moves over a hotspot, a linear chain of volcanoes (e.g., the Hawaiian Islands) forms, recording plate motion. Hotspots can produce islands, seamounts, and flood basalts.

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Transform Fault

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A transform fault is a plate boundary where two plates slide horizontally past each other. Transform faults commonly offset mid-ocean ridges and generate strike-slip earthquakes; the San Andreas Fault is a continental example. They do not create or destroy lithosphere on a net basis.

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Accretionary Prism

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An accretionary prism is a wedge-shaped body of deformed sediments scraped off a subducting plate and accreted to the overriding plate at an active continental margin. It forms near trenches and records sedimentary and tectonic deformation. Accretionary prisms influence fluid flow and chemical cycling at convergent margins.

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Plate Motion Drivers

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Plate motions are driven primarily by mantle convection, slab pull, and ridge push associated with the Earth’s loss of internal heat. Convective flow in the asthenosphere and density contrasts of sinking slabs provide the dominant forces. These mechanisms together produce plate tectonics observed at Earth’s surface.

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Active vs Passive Margin

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Active continental margins occur at plate boundaries (usually convergent) and feature trenches, volcanic arcs, earthquakes, and tectonic deformation. Passive margins lie within plate interiors, are tectonically quiet, and commonly have broad continental shelves and thick sediment accumulation (e.g., Atlantic coasts). The margin type controls seismic and volcanic hazards.