Scientific revolutions Science does not grow by accretion Science is a discontinuous process, directed along certain paths by ruling paradigms Paradigm: how a community uses common assumptions or guiding principles to organize knowledge Communities abandon paradigms when they no longer organize knowledge effectively New ruling paradigms must ‘gain the consent of the relevant community’ Science moves ahead by these paradigm shifts

Special earth sciences perspectives Many earth scientists use an observation-inference methodology rather than one of several experimental methodologies Observe and describe objectively features or processes Interpret features in light of the processes inferred to operate (or have operated) to generate them (draw upon principles from biology, chemistry, or physics) ‘Test’ inferences by making ‘predictions’ Compare ‘predictions’ with observations

Methodology leads to the tenet ‘the present is the key to the past’ Observe geologic processes that occur on earth today, and determine how those processes affect rocks at the present time Assume that analogous processes operated at different times in the past, i.e. throughout geologic time Infer, by comparing modern and ancient rocks, which processes were responsible for forming ancient rocks

Marine geology and the earth system

Global topography Highest elevation - Mount Everest ~29,000 ft above sea level ~5.5 miles above sea level ~8.85 km above sea level Greatest depth - Marianas/Philippine trench ~38,000 ft below sea level ~7.5 miles below sea level ~11 km below sea level Total range = ~13 miles or ~20 km

Radius of earth = 6371 km (3982 mi) 20 km (13 mi) is a large distance to us. Relative to earth’s radius, however, 20 km is small. Radius of earth = 6371 km (3982 mi) Diameter of earth ≈ 12,700 km 20 km /6400 km ≈ 0.003 20 km ≈ 0.3% Earth’s radius

What is the distribution of elevations on earth What is the distribution of elevations on earth? Is land area distributed ‘normally’ between the two extreme elevations? No - elevation is distribution is bimodal One common elevation corresponds to the elevation of continents Mean elevation of continents is 840 m above sea level Second common elevation corresponds to the elevation of sea floors Mean depth of ocean floors 3-4 km below sea level

Greatest changes in average elevation occur between 1 and 3 km below sea level The best place to locate the transition from continent to ocean is, then, somewhere between 1 and 3 km below sea level We place the ends of continents at 1 to 3 km below sea level

Continents Six continents at present time Continents possess a wide variety of rock types Sedimentary rocks Igneous rocks, either extrusive or intrusive Metamorphic rocks

Continents Six continents at present time North America, South America, Eurasia, Africa, Australia, & Antarctica Continents possess a wide variety of rock types Some rocks originated as rock debris (detritus) or sediment; geologists call these rocks sedimentary rocks Some rocks originated by freezing molten silicate liquids; geologists call these rocks either extrusive or intrusive igneous rocks Some rocks originated as sedimentary or igneous rocks, but were changed when the ambient temperatures or pressures changed; geologists call these rocks metamorphic rocks

Continents have rocks of all ages, ranging from those formed yesterday to those formed about 4 Ga ago 1 Ma = 1 million years 1000 Ma = 1 billion years = 1 Ga It is on the continents that we find the most extensive record of earth history

Rocks of different origins and ages occur in three fundamentally different geological provinces Mountain belts Linear belts of high elevation Arcuate in plan view Contain thick accumulations of sedimentary rocks; strata are faulted and folded Contain extrusive or intrusive igneous rocks Rocks often metamorphosed in characteristic ways

Rocks of different origins and ages occur in three fundamentally different geological provinces Mountain belts Long, linear, and smoothly arcuate provinces characterized by generally high elevation Have thick accumulations of sedimentary rocks of marine origin Have extensive suites of intrusive igneous rocks and volcanic activity; volcanoes are often explosive Sedimentary rocks are deformed by faults and folds which accomplish crustal shortening Sedimentary and igneous rocks are often metamorphosed as well as deformed Cratons or shield areas Rift systems

Rocks of different origins and ages occur in three fundamentally different geological provinces Mountain belts Cratons or shield areas Broad regions of low elevation & low relief near centers of continents Core of cratons usually deformed & metamorphosed rocks older than 1 Ga old Often have a veneer of younger sedimentary rocks, called platform deposits

Rocks of different origins and ages occur in three fundamentally different geological provinces Mountain belts Cratons or shield areas Are broad regions of generally low elevation and low relief, generally found near the centers of continents Surface topography resembles a shield; thus the name Rocks in cratons are usually deformed metamorphic rocks older than 1 billion years old = eroded roots of ancient mountain belts Cratons sometimes covered by veneers of younger sedimentary rocks, called platform deposits (many platform sequences contain rocks younger than 600 Ma) Rift systems