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Earth Science Concepts

Major Topics Covered in Earth Science

Amity Region #5 High School, Woodbridge, CT


Earth's Origin and Dimensions

According to the protoplanet hypothesis, the solar system began as a rotating cloud of gas and dust that condensed into the sun, planets and moons; the oceans formed from steam released from volcanoes. Earth's inner and outer cores, the mantle and the crust, including the lighter continents, formed as a result of differentiation in density. Scientists give a date of 4.6 billion years for the origin of Earth.

Because Earth is an oblate spheroid (flattened at poles, thicker around equator), objects weigh more at the poles, less at the equator. Weight is dependent on an object's distance to the center of Earth. Eratosthenes measured the circumference of Earth by determining the relationship between the distance and angle between two places -on Earth's surface. Earth's average density is 5.5 g/cm3, but its surface density is only 2.8 g/cm3, so geologists theorize that Earth's core is much denser and made of nickel and iron.



Mercator maps show lines of longitude and latitude crossing at right angles, but land masses are distorted in the upper latitudes.  Latitude lines go east-west, just like the equator, but measure distances north or south. Longitude is based on observations of the sun and go north-south, but measure distances east or west.

Topographic maps are the most detailed of maps and show the features of land through the use of contour lines. The closer isolines (contour-isobar-isotherm) are, the steeper the slope or gradient.  Contour lines point upstream as they cross rivers. Geologic maps indicate rock types; hydrologic maps show the presence of surface and groundwater.


Earth’s Place in Space

Telescopes collect electromagnetic energy from high-energy gamma rays to low energy radio waves.  The visible light is part of the electromagnetic spectrum. As wavelength increases the frequency decreases.  Higher the frequency the warmer the object, because more frequency means more energy.  All the energies of the electromagnetic spectrum travel at the same speed (speed of light) in waves.

Most telescopes are more effective when placed in space, away from Earth’s atmosphere that reduces the resolution of signals.

The Doppler Effect (Red Shift) of visible light can be used determine the relative motion of stars and galaxies away from the viewer. Astronomers use the H-R Diagram to relate the luminosity and temperature of stars and to group stars by their relative ages. The current hypothesis is the universe formed from a single event known as the “Big Bang” about 13-15 billion years ago.  Stars formed from condensing clouds of gas and dust that began to glow as they become protostars, but became true stars only after hydrogen fusion began to occur in the star's core. Once the all hydrogen in the core of a star has changed to helium, the star becomes a red giant or supergiant. In old age, massive stars explode into supernovas, and become neutron stars, pulsars, or black holes; less massive stars become white dwarfs. The sun is a main sequence star, and is expected to become a red giant, then a white dwarf before dying. 
Stars can give off variable light because they are binaries that eclipse each other, because they are novas or supernovas, or because they are pulsating stars, such as Cepheid variables, which change energy output in a regular, cyclical manner.


Kepler's Laws of Planetary Motion state 1) that planets travel in ellipses with the Sun at one focus; 2) that planets sweep equal areas in equal times (closer a planet is to the sun [perihelion] the higher it's velocity); and 3) that the period squared of a planet is equal to the cube of its distance from the Sun.

Newton's Law of Universal Gravitation states that the gravitational force between two objects is directly proportional to the mass of the objects and inversely proportional to the distance squared between the objects.

Earth is one of nine planets that revolve around the Sun, a star. Most of the planets have moons revolving around them. Asteroids (too small to be considered planets) and comets (with highly eccentric orbits) are also members of the solar system. The inner planets, also known as the terrestrial planets have a rocky crust, denser mantle layer, and very dense core, average density above water, smaller in size, fewer moons.  The outer planets, also known as the Jovian Planets, are huge compared to Terrestrial Planets, gaseous, much less dense, colder, rings and have more moons.  There is debate about whether Pluto is a planet.  The solar system is a member of the Milky Way Galaxy, a spiral galaxy.

The Earth revolves counterclockwise around the Sun in 365-1/4 days.  During its revolution the Earth is closest to the Sun in January. Earth’s axis is tilted 23-1/2o to the ecliptic plane. The revolution of Earth on its tilted axis produces years with seasonal variations. The Equinoxes are March 21st and September 23rd, when the Sun’s direct rays are over the Equator; and the Solstices are June 21st and December 21st. The rotation of Earth (spin on its axis) produces night and day. Foucault's pendulum and the coriolis effect prove the earth rotates.

The period from one full moon to the next is 29.5 days, but the time it takes for the moon to fully revolve around the Earth is 27.3 days.  The moon has phases because of the angle at which we view its surface (Remember though: half is always lit). Lunar eclipse only occurs during full moon and is visible to more of the Earth than a solar eclipse.  Solar eclipse only occurs during new moon.  During a total solar eclipse, the dark part of moon's shadow called the umbra reaches a small part of Earth's surface. During a lunar eclipse, Earth's shadow covers the Moon.

Tides are caused by the gravitational pull of the Moon and the Sun on Earth. Spring tides have a high range from high tide to low tide because the Earth, Moon, and Sun are lined up. When the Moon and Sun are pulling on Earth at 90° angles, neap tides occur. Spring tides only occur during new and full moons, neap tides occur at quarter moons.


Earth's Lithosphere

Minerals are naturally occurring, inorganic solids with definite crystalline chemical compositions. Only eight elements make up 98.5% of the minerals that make up the rocks of Earth's crust. Oxygen is the most common element, followed by silicon.  Minerals can be identified by physical tests such as hardness, color, luster, specific gravity, cleavage/fracture and crystal shape. The most common minerals, silicate minerals, have the silica tetrahedron as a building block.  Minerals combine to make rocks.

The rock cycle produces sedimentary, igneous, and metamorphic rocks through the processes of erosion, burial, lithification, and plate tectonics. Igneous rocks form at the surface from lava and inside the crust from cooling magma.  Sedimentary rocks form from weathered or deposited fragments which are buried and lithified under pressure. Agents of erosion, such as running water, can sort sediments by size. Any kind of rock can alter to metamorphic if it undergoes extreme temperature and pressure changes.


Natural Resources, Mineral Resources and Energy Production

Renewable resources, such as wood and water (through the water cycle) can be replaced by nature. Nonrenewable resources, such as iron or petroleum cannot be replaced. Mineral resources refers to the total amount of a mineral, whereas mineral reserves refers to the amount that can be mined profitably. Fossil fuels, such as petroleum and coal, are formed from ancient organisms; there are limited supplies of petroleum and natural gas worldwide. Large quantities of  oil are stored in oil shale and tar sands, but presently they cannot be extracted profitably. Wind power, solar power, water power, and geothermal power are energy sources that do not use fuel, but which have other limiting characteristics. Energy wastes and their by-products, such as acid rain, toxic wastes and nuclear waste are environmental and political problems of our society.


Plate Tectonics

According to the theory of plate tectonics, Earth's crust is made of rigid plates that cover Earth's surface and that move due to convection currents in the underlying asthenosphere. Ocean crust forms as plates diverge at mid-ocean ridges. Earthquakes occur in California where the Pacific Plate and the North American Plate slide past each other. Converging plates form faulted and folded mountain systems where two continental plates collide; converging plates form trenches and volcanic mountain chains where an ocean plate subducts another ocean plate or a continental plate. Alfred Wegener laid the foundation for the plate tectonic theory when he studied the similar shapes of adjoining continents as well as the correlation of fossils and rock types. Today additional evidence, such as magnetic polarity reversals, heat flow patterns, and sediment age patterns at diverging boundaries add credence to the plate tectonic theory.

An original continent (craton) can grow when land masses (terranes) from a neighboring plate collide with and become attached to the craton.


Igneous Activity: Volcanic and Plutonic

Molten rock below the surface is called magma; when magma reaches the surface, gases

escape which result in a change in composition; the liquid is then called lava. Magmas are initially mafic (because of the high concentration of iron and magnesium in the asthenosphere and ocean crust), but when mafic magma works its way through a continent it becomes felsic as a result of incorporating continental materials, especially silicates and water. Felsic magma contains more dissolved gases and produces violent eruptions when it reaches the surface. Ash, lapilli, bombs, and blocks are examples of different sizes of solid fragments of lava.

Volcanoes occur at mid-ocean ridges (Mid-Atlantic Ridge; Eldfell, Iceland), hot spots (Hawaiian Islands, Iceland) and near the boundary of subduction zones (Cascades, Andes). Trenches occur on the boundaries of subduction zones.

Magma that does not make it to the surface cools to become plutons such as sills (magma squeezed between sedimentary layers), dikes (magma cutting across rock layers), laccoliths mushroom-shaped intrusions that can produce dome mountains) and volcanic necks



An earthquake is a release of energy due to pressure changes in Earth's crust and are usually related to plate movement. The focus is the place inside Earth where the earthquake begins; the epicenter is the spot on the surface immediately above the focus. P-waves (compressional) and S-waves (shear) travel inside Earth. S-waves cannot travel through liquids and have been the source of evidence that Earth's outer core is liquid. The boundaries between the crust (Moho), mantle, and cores have been determined by studying earthquake waves. L-waves travel on Earth's surface like ripples and cause surface damage.  P-waves are the fastest waves, followed by S-waves.

Seismographs record all three types of earthquake waves and can be used to determine the distance to an earthquake from the seismograph due to different arrival times of P and S waves. With three seismographs, at a minimum, one can also determine the epicenter of the earthquake and its magnitude. The Mercalli Scale, the Richter Scale and seismic moment are all ways of describing the magnitude or intensity of an earthquake. Seismic moment gives the most accurate assessment of the total energy given off by a particular earthquake, but is difficult to measure.

Earthquakes usually occur on a subduction zone (deep: Alaskan quake), sliding boundary (shallow: San Andreas Fault), or mid-plate zone (deep: New Madrid, MO). Shallow earthquakes do not travel for long distances because the waves are absorbed by loose earth materials.


Mountains and Plateaus: Structural Geology

Collision mountains are the result of reverse (compressional) faulting; in a reverse fault the hanging wall is raised. Reverse faults can produce plateaus when a large block of crust is thrust upward in a basically horizontal position. Compression can also produce folded mountains if the compression occurs deep enough and in a rock type that tends to plasticize. The upfold of folded rock strata (hill shape) is an anticline; the downfold (valley shape) is a syncline. When faulting occurs due to tension, a center portion sinks (normal fault) and a graben like Death Valley can form. Strike-slip faulting (horizontal movement such as occurs on a sliding boundary) does not produce mountains. Dome mountains are nearly circular folded mountains produced by either a laccolith or by upward tectonic pressure without the involvement of magma.


Connecticut Geology

Connecticut formed during several compressional mountain building periods in which the Western and Eastern Uplands developed as terranes collided with the North American craton thus forming Pangaea. Connecticut's Central Valley is a graben which formed as Pangaea began to break up from tension that produced faulting, tilting and sinking. Connecticut's surface has been altered by the recent glaciation. Connecticut's thin soil is transported glacial soil.


Earth History

Paleontologists date fossils and rock in relative and absolute time. Relative time is determined using the laws of Superposition (Oldest layer is on bottom), the Law of Cross-Cutting Relationships (Intrusions are younger than rocks they intrude) and ]Law of Included Fragments (Fragments are older than rocks in which they are found). Gaps in the rock record due to erosion are unconformities. Tree rings and varves (seasonal sediment in lakes) can be used to date ages of trees and lakes. The half-lives of several radioactive elements can be used to date fossils and rocks. In one half-life, half of a radioactive element will have decayed to a stable element. Carbon-14 is used to date relatively young organic remains-up to 100,00 years old. Uranium, rubidium, and potassium isotopes are used to date rock of various ages.

The geological time scale subdivides the four eras of Earth's history into periods that are subdivided into epochs. The first and largest segment of geologic time is called the Pre-Cambrian. There was very little life until the beginning of the second era, the Paleozoic, during which most organisms lived in the sea. Dinosaurs evolved in the Mesozoic Era, also known as the Age of Reptiles. The Cenozoic Era is commonly called the Age of Mammals. We live in the Recent Epoch of the Quaternary Period of the Cenozoic Era. Divisions on the time scale are determined by the appearances and disappearances of species of plants and animals.


Weathering and Soils

Weathering is the wearing down of rock into fragments; erosion is removal of weathered materials by glaciers, running water, wind and waves. Weathering is mechanical if the weathered products are chemically the same as the parent rock, weathering is chemical if the weathered product is chemically different from the parent rock. Different weathering rates result from differences in climate and rock composition. Of the common minerals quartz is the most resistant to weathering. Soil is the end result of weathering; mature soil consists of -three horizons: topsoil, which contain organic material (humus), subsoil which contains clay and soluble minerals leached from the topsoil, and weathered bedrock. Soils can be transported from another area by streams, glaciers or wind or they may reside immediately above the parent rock from which they formed. Soil may be depleted if crops are not rotated. Soils in dry areas with high evaporation can become salinized which reduces crop productivity. Various agricultural practices, like contour farming and terracing can reduce soil erosion.


Water Cycle and Groundwater

Most of the 3% of Earth's fresh water (97% is saltwater.) is frozen in glaciers; much of the rest is stored underground as groundwater. Water budgets relate the amount of water that precipitates or is stored in the ground in a particular area to how much is lost to evaporation or runs off as surplus. Water is continually recycled by the processes of evaporation, condensation (Clouds are water droplets or crystals-not vapor.), precipitation, percolation and runoff. The capacity for air to hold water vapor is dependent on its temperature, so relative humidity changes from morning to night. The water table is the surface of stored water in the ground.

Permeable rock is rock that can transmit water. In sorted round particles (clay, sand, pebbles, cobbles) of different sizes, the percent porosity remains the same regardless of particle size. Permeability (rate of flow) increases as particle size increases. The amount of capillary water retained increases with smaller particle size because of increased surface area.

Except in areas of volcanism, groundwater is cold. Below 40 meters the temperature gradually warms with depth from Earth's 'interior heat.

Karst topography results in areas where the bedrock is limestone because acid rain slowly dissolves limestone that is made of the mineral calcite. Features of karst topography are: sinkholes, caves, lost rivers, and underground drainage.


Streams: Running Water

River systems begin as young mountain streams which have a high gradient, high velocity, clear water, straight path, waterfalls, rapids, and eventually canyons and alluvial fans. Erosion occurs mostly on the stream bed until the gradient becomes more gentle; then lateral erosion becomes more prominent and the stream slows down, begins to form meanders (curves), sandbars, cutoffs, ox-bow lakes and deltas. In a meander, erosion occurs on the outside curve and deposition occurs on the inside curve. Divides (highest ridges) divide continents into the drainage basins that produce river systems such as the Mississippi River System.



Glaciers are large bodies of ice that flow under their own weight. Valley glaciers form on mountain tops and flow down mountain valleys producing cirques, hanging valleys, horns, aretes, and moraines. Continental glaciers are large enough to actually cover mountains; they produce features such as kettle lakes, eskers (winding mounds from floor of glacial stream), drumlins (canoe-shaped hills of till), and moraines (such as the end moraine of Long Island). Till is unsorted material dumped by a glacier; outwash is sorted material deposited by streams inside the glacier. Canada and the northern part of the United States was covered by a continental glacier that receded 12,000 years ago.


Wind as an Erosive and Depositional Force

Loess is a silt-sized, wind deposited sediment. Most water waves result from wind; some water waves erode shorelines producing features such as sea cliffs and stacks; other waves deposit sand to produce depositional beaches. Waves are described by their length, height, period and speed. A tsunami is a wave produced by an earthquake on the sea floor.