Continental plates move {plate tectonics}|, when olivine from upper mantle comes through rift in crust basalt, pushing plates apart. See Figure 1.
Plates can slide into each other, pushing one down and one up to make trenches and mountains. See Figure 2.
rates
Pushed plates move two centimeters per year. Sea-floor movement in Chile is 15 centimeters per year.
results
Upwelling at ocean ridges can make volcanoes with basalt lavas. Old rift valleys can fill with aulacogens.
evidence
Coal is in Antarctica. Similar fossils are on separated continents. All over world, iron in volcanic rocks aligns in many different directions, instead of only north and south. East South America and west Africa have similar coastlines.
Sea floor is spreading away from Mid-Atlantic Ocean Ridge. Basalt at Mid-Atlantic Ocean Ridge is younger than basalt near continents. Mid-Atlantic-Ocean Ridge basalt shows alternating iron-particle orientations every 700,000 years, when Earth magnetic field reversed. Sediment at Mid-Atlantic Ocean Ridge is less than at continent edges.
Pacific-Ocean floor has thicker sediments and is older than Atlantic-Ocean floor. Atlantic-Ocean floor is 200,000,000 years old. Atlantic-Ocean sediment averages only several thousand feet thick and in some places is much thinner. If ocean floor had not changed for 200,000,000 years, sediment would be several miles thick.
Six major, and many minor, crust pieces {continental plate} {crustal plate}| float on upper mantle.
Continental plates move when olivine from upper mantle comes through basalt crack {rift}|, pushing plates apart. Pushed plates move one inch per year. Atlantic-Ocean middle has rift north to south that rises above sea level at Iceland, Azores, and Ascension Islands. Southeast South Pacific Ocean and central Indian Ocean have rifts.
Old rift valleys can fill with sediment {aulacogens}.
Along rift, lava makes mountain ridge, with valley down middle {sea floor spreading}|.
Continent granite {shield, continent}| can be at surface.
Rock layers can have shape like upside-down V {anticline}.
Rock layers can bend up or down {monocline} {flexure}.
Rock layers can have shape like V {syncline}.
Slips along rock faults cause movements {earthquake}|. Slow plate movements and collisions lead to sudden shifts of one plate against the other. Earthquakes can be several miles deep or even in mantle under ocean trenches. After earthquake, Earth vibrates at low frequency for several days. About 20 major earthquakes and 10^6 minor ones happen each year. Major earthquakes have been in China 1976, Tokyo 1923, San Francisco 1906, Lisbon 1755, Calcutta 1737, and China 1556.
Rock has big cracks {joint, rock}.
Earthquakes can be along rock fractures {fault}|.
Instruments {seismograph}| can measure Earth movements.
Seismographs can use logarithmic scales {Richter scale}|, from 1 up. Largest earthquake was 8.5.
Earthquakes {silent earthquake} can be slow and quiet. Perhaps, water percolation from rain or from trapped water in rocks causes them.
Earthquake shaking can cause loose wet sandy soil to become like quicksand {soil liquefaction}|.
Earthquakes under ocean can make fast waves, which then slow near shore and bunch to make towering waves {tsunami}|.
Earthquake shocks can travel through crust as slow surface waves or through Earth interior as very fast primary waves {P wave}.
Earthquake shocks can travel through solids as fast secondary shock waves {S wave}.
Both horst and graben processes form mountains {orogeny}.
Mountain building processes can make large lowered masses {graben}|, as in Death Valley USA, Red-Sea basin, and East-Africa rift valleys.
Mountain building processes can make large raised masses {horst}|, as in Sierra Nevada Mountains and Alps Mountains.
Upper-mantle convection currents rise near surface at 20 locations {plume, mantle} {hot spot, mantle}|. Plumes have 300,000 meters diameter. Plumes in crustal-plate middle can send alkali-rich basalt lava up to surface to form volcanoes, as in Hawaiian Islands.
Colliding plates can move straight into each other {pressure ridge} to make mountains, with no overriding. Alternatively, one plate can slide over other one, forming both mountains and ocean trenches.
Along rift, lava makes mountain range {ridge, mountain}|, with valley down middle.
At plate sides opposite from rifts, plates slide under other plates {subduction, plate}|. Plates meet {subduction zone}, and one plate goes up and the other goes down, at 45-degree angles. Plates can go 700 kilometers into mantle. Subduction is at North-America and South-America west coasts, at Asia east coast, and from Spain and north Africa to Italy, to Greece, to Turkey, to India, to Burma, to Celebes.
Ocean crust and underlying mantle {ophiolite} can uplift onto continent.
Colliding plates can make especially deep and steep ocean floor {trench}|.
Continents move on upper mantle {continental drift}|. Upper-mantle asthenosphere and possibly all mantle has stable constant one-inch-per-year convection currents, caused by heat. Currents provide energy to move continents. Continents have been drifting for last 2,000,000,000 years. Six major and many minor crustal plates float on upper mantle.
Plate movements make crust slide, fold, and fault {diastrophism}.
Increased fluid pressure, changed electrical resistivity, decreased Earth natural electric currents, increased deep-well-water radon content, changed seismic-wave travel time, and seismicity affect crustal-plate movements {dilatancy}. Dilatancy models earth movements as inelastic swelling. Steady stress increase splits crust, allowing water flow. If water flows in slower than cracks open, crust splitting slows. Then water under pressure quickly fills crack, causing sudden slip. Changes from compression waves to shear waves cause seismic-wave travel-time changes.
Continents are 10% lighter than crust, and crust is 10% lighter than upper mantle, so continents float on crust, which floats on upper mantle {isostasy}.
Plate movements {tectonic process}| make crust slide, fold, and fault in diastrophism.
Magma comes from mantle, 2 to 100 miles down, to surface {volcano}| through crust fissures. Magma then cools and hardens. Most of Earth water vapor and gases came from volcano eruptions.
types
Thick magma has more gas, is red hot, erupts explosively, and makes steep mountains. Thin magma has little gas, is white hot, and makes wide mountains.
examples
Famous volcanoes are Mount Vesuvius in Italy, which buried Pompeii [79]. Krakatoa in Indonesia exploded island [1869]. Mount Etna in Italy caused enormous avalanche and undersea mudslide [-6000] and started a huge tsunami: it is still active. Mauna Loa in Hawaii is active.
Volcanoes can erupt {volcanism} where plates collide, making andesite lava.
Magma can spread to make stock rock masses, which can be thousands of square miles wide {batholith}.
Volcano tops have craters {caldera}|.
Magma can flow into vertical rock fissures and cool and harden {dike, magma}.
Cylindrical columns {kimberlite pipe} from mantle to crust can have 300-meter diameter.
Magma {lava}| can reach surface.
Molten igneous rock {magma}|, mixed with gas and water vapor, comes from mantle, 2 to 100 miles down, to surface through crust fissures.
Magma can make underground pools {sill, magma}.
Magma can spread to make large rock masses {stock, rock}.
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Date Modified: 2022.0225