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Earth's interior is layered by density, and the rocky mantle lies immediately below the crust, extending down to the core. The outer core and inner core are deeper still, and the atmosphere lies above the crust rather than beneath it.
Igneous rocks crystallize from magma or lava as it cools, so their texture records how quickly that cooling happened. Sedimentary rocks form from compacted sediment, and metamorphic rocks form when existing rock is altered by heat and pressure.
Condensation is the phase change from gas to liquid, and it is the step that builds cloud droplets from water vapor. Evaporation and transpiration add vapor to the air, while precipitation returns water to the surface.
A full cycle from new moon back to new moon takes about 29.5 days, a period called the synodic month. It is slightly longer than the Moon's orbital period because Earth moves around the Sun at the same time, so the Moon must travel a bit farther to line up again.
Earth turns once on its axis roughly every 24 hours, so each location rotates into and out of sunlight, producing day and night. Seasons come from axial tilt, Moon phases from the Moon's orbital geometry, and tides mainly from gravitational pulls.
Where two plates move apart, magma rises to fill the gap and cools into new oceanic crust, building a mid-ocean ridge. Trenches and folded mountains form where plates converge, and transform faults form where plates slide past each other.
Low-viscosity lava flows far before it solidifies, so many thin flows pile up into a broad, gently sloping shield volcano. Cinder cones and composite volcanoes form from stickier lava and explosively ejected rock fragments.
The focus is the actual point of rupture underground, and the epicenter is the location on the surface directly above it. Seismic waves spread outward from the focus, so the epicenter is generally where shaking is felt first and strongest.
Mechanical weathering breaks rock into smaller pieces without changing its chemical composition, and ice expansion in cracks is a classic example. Chemical weathering alters minerals instead, and erosion transports the broken material away.
The C horizon sits below the zones of strongest soil development and is made largely of partially weathered parent material with little organic content. The O, A, and B horizons are the organic-rich and leached layers that lie above it.
The troposphere holds nearly all of the atmosphere's water vapor and is where air masses, fronts, and most storms develop. The layers above are too dry and stable to support most weather.
Cirrus clouds form high in the cold upper troposphere, where water freezes into ice crystals that trail into wispy streaks. Stratus, cumulus, and nimbostratus are lower, thicker forms built mainly of liquid droplets.
At a warm front, advancing warm air is less dense and glides up over retreating cold air, producing broad layered clouds and steady precipitation. At a cold front the dense cold air pushes beneath the warm air instead, producing a steeper, narrower band of storms.
Earth's axis is tilted about 23.5 degrees, so as Earth orbits the Sun each hemisphere receives sunlight at a changing angle and for a changing length of time. Distance from the Sun cannot explain the cycle, because the two hemispheres experience opposite seasons at the same moment.
Near new and full moon, the Sun and Moon pull on Earth along nearly the same line, so their tidal effects add and produce the larger spring tides. When the two pulls are at right angles, as at the quarter phases, the effects partly cancel and neap tides result.
During a lunar eclipse Earth moves between the Sun and the Moon, so Earth's shadow falls across the Moon's surface. This geometry requires a full moon, when the Moon is on the opposite side of Earth from the Sun.
Mercury is the innermost planet, orbiting at an average distance of roughly 58 million kilometers from the Sun. Venus, Earth, and Mars orbit progressively farther out.
The asteroid belt circles the Sun between Mars and Jupiter, where Jupiter's strong gravity prevented the rocky bodies there from assembling into a planet. The region between Earth and Mars and the region beyond Jupiter contain no comparable belt.
The Sun's core is hot and dense enough for hydrogen nuclei to fuse into helium, releasing enormous amounts of energy. This is a nuclear process, not a chemical fire, and the Sun does not rely on fission of heavy elements.
Geologic time is nested so that each eon contains several eras, each era contains periods, and each period contains epochs. For example, the Phanerozoic Eon includes the Mesozoic Era, which includes the Cretaceous Period.
Index fossils come from organisms that were widespread but existed for only a short span of geologic time, so layers containing them can be correlated and assigned the same relative age. They establish relative ages rather than absolute ages in years.
Water percolating downward stops where all pore spaces are filled, and the top of that saturated zone is the water table. Above it lie the unsaturated zone and capillary fringe, while an aquiclude is a layer that blocks water movement.
Contour lines connect points of equal elevation, so where they crowd together the elevation changes rapidly over a short horizontal distance. Widely spaced lines indicate a gentle slope, and closed hachured loops mark depressions.
Wind is continuously replenished by solar heating of the atmosphere, so it is not used up on human timescales. Coal, petroleum, and natural gas are fossil fuels that formed over millions of years and are consumed far faster than they form.
Greenhouse gases such as carbon dioxide and water vapor absorb outgoing infrared radiation and re-emit it in all directions, some of it back toward the surface. This slows the loss of heat to space and raises surface temperatures.
Convergent boundaries occur where plates move toward each other. If an oceanic plate meets another plate, the denser oceanic slab usually sinks into the mantle, forming a subduction zone. This setting produces deep-ocean trenches, volcanic arcs, and deep-focus earthquakes.
Igneous rock forms when magma or lava cools and crystallizes. The cooling rate helps determine whether crystals are large or small. Sedimentary and metamorphic rocks form through different processes.
Erosion is the removal and transport of weathered material by agents such as water, wind, and ice. Deposition occurs when transported material is dropped, and compaction presses sediments together. Crystallization forms crystals from melt or solution.
The outer core is a liquid layer made mostly of iron and nickel. Its movements generate Earth's magnetic field. The inner core is solid, while the crust and mantle are made mainly of silicate rock.
Nitrogen makes up about 78 percent of the atmosphere by volume. Oxygen is the second most abundant gas at about 21 percent. Carbon dioxide and argon are present only in small amounts.
At new moon, the Moon lies roughly between Earth and the Sun, so its sunlit side faces away from Earth. The Moon is then difficult to see from Earth. Full moon occurs when Earth is between the Sun and the Moon.
Coal forms from plant material that accumulated in swampy environments and was compacted and altered over time. Limestone forms mainly from shells or chemical precipitation, sandstone from sand, and shale from clay and silt.
A seismograph detects ground motion and records it as a seismogram. The record shows the arrival times and strengths of different seismic waves. Barometers, anemometers, and hygrometers measure weather variables.
P waves are compressional waves and travel fastest through rock, so they arrive first. S waves are slower shear waves, and surface waves such as Love and Rayleigh waves arrive later. The time gap between arrivals helps locate an earthquake.
At a transform boundary, plates move horizontally in opposite directions. The crust is neither created nor destroyed on a large scale, though faults and earthquakes are common. Divergent boundaries involve separation, and convergent boundaries involve collision or subduction.
Vegetation protects soil by intercepting rain, slowing runoff, and holding particles with roots. Removing vegetation exposes soil to raindrop impact and faster water flow, which increases erosion. Mulch, grass, and terracing all tend to reduce erosion.
Metamorphic rocks form when existing rocks are changed by heat, pressure, or chemically active fluids without completely melting. The original rock's minerals may recrystallize or rearrange into new textures. Evaporation, cooling lava, and sediment deposition form other materials.
Cumulonimbus clouds build vertically through much of the troposphere and often produce thunder, lightning, heavy rain, and hail. Cirrostratus clouds are thin and high, altocumulus clouds are midlevel, and stratus clouds form low, flat layers.
A continental polar air mass forms over cold land and is therefore cold and dry. Maritime air masses pick up moisture over oceans, while tropical air masses form in warm regions. The combination of source latitude and surface type determines temperature and humidity.
Air moves from regions of higher pressure toward regions of lower pressure, producing wind. Earth's rotation affects wind direction through the Coriolis effect, but pressure differences are the primary cause of motion. Moon phases and tides do not drive ordinary surface winds.
As Earth rotates once about every 24 hours, different parts of the surface face the Sun, producing day and night. The seasons result mainly from Earth's axial tilt and revolution around the Sun. Moon phases result from the Moon's orbit around Earth.
Neap tides occur when the Sun and Moon are at right angles as seen from Earth. Their tidal bulges then partially cancel, producing smaller-than-usual tidal ranges. Spring tides occur when the Sun and Moon are aligned.
The Great Red Spot is a long-lived high-pressure storm in Jupiter's atmosphere. It is larger than Earth and has been observed for centuries. Mars, Saturn, and Neptune have different prominent atmospheric features.
An unconformity is a break or gap in the sequence of rock layers. It commonly records erosion or a period when no sediment was deposited. The layers above and below therefore do not represent continuous deposition.
An aquifer is a permeable body of rock or sediment that can hold and transmit groundwater. Wells are often drilled into aquifers to obtain water. An aquiclude restricts flow, while a watershed and a delta are surface features.
New oceanic crust forms at a mid-ocean ridge and records Earth's magnetic field as it cools. The resulting magnetic stripes are arranged in matching patterns on opposite sides of the ridge. This symmetry supports the idea that new seafloor spreads outward from the ridge.
The principle of cross-cutting relationships states that a feature cutting across rock layers must be younger than the layers it cuts. The fault had to form after the sedimentary layers were deposited. This principle allows geologists to establish relative ages.
Oceanic lithosphere is made of denser basaltic rock and is thinner than continental lithosphere. When the two meet at a convergent boundary, the denser oceanic slab tends to sink into the mantle. This process forms trenches and volcanic arcs.
A total solar eclipse occurs when the Moon passes directly between Earth and the Sun and its disk fully covers the Sun from the observer's location. The narrow path of totality experiences the total phase. Earth's shadow falling on the Moon describes a lunar eclipse.
The Coriolis effect arises because Earth rotates beneath moving air. In the Northern Hemisphere, moving air is deflected to the right, and in the Southern Hemisphere it is deflected to the left. Solar heating and pressure differences drive the air, while rotation changes its direction.