Oceanic Oceanic Subduction

Fig. 2.11 Annual mean sea surface temperature. (Top) The total field. (Bottom) Departure of the local sea surface temperature at each location from the zonally average field. [Based on data from the U.K. Meteorological Office HadISST dataset. Courtesy of Todd P. Mitchell.]

32 The Earth System

2.1.2 The Cryosphere

The term cryo- (frozen) sphere refers to components of the Earth system comprised of water in its solid state, or in which frozen water is an essential component. The cryosphere contributes to the thermal inertia of the climate system; it contributes to the reflectivity or albedo of the Earth; by taking up and releasing fresh water in the polar regions, it influences oceanic thermohaline circulation; and it stores enough water to significantly the influence global sea level. The elements of the cryosphere are listed in Table 2.1 and all of them, with the exception of alpine glaciers, are represented in Fig. 2.12.

The continental ice sheets, dominated by Antarctica and Greenland, are the most massive elements of the cryosphere. The ice sheets are continually replenished by snowfall; they lose mass by sublimation, by the calving of icebergs, and, in summer, by runoff in streams and rivers along their periphery. The net mass balance (i.e., the balance between the mass sources and sinks) at any given time determines whether an ice sheet is growing or shrinking.

Over periods of tens of thousands of years and longer, annual layers of snow that fall in the relatively flat interior of the ice sheets are compressed by the accumulation of new snow on top of them. As the pressure increases, snow is transformed into ice. Due to the dome-like shape of the ice sheets and the plasticity of the ice itself, the compressed layers of ice gradually creep downhill toward the periphery of the ice sheet, causing the layer as a whole to spread out horizontally and (in accordance with the conservation of mass) to thin in the vertical dimension. Much of the flow toward the periphery tends to

Table 2.1 Surface area and mass of the various components of the cryosphere"

Cryospheric component

Area

Mass

Antarctic ice sheet

2.7

53

Greenland ice sheet

0.35

5

Alpine glaciers

0.1

0.2

Arctic sea ice (March)

3

0.04

Antarctic sea ice (September)

4

0.04

Seasonal snow cover

9

<0.01

Permafrost

5

1

a Surface area is expressed as percentage of the area of the surface of the Earth. Mass is expressed in units of 103 kg m~2 (numerically equivalent to meters of liquid water) averaged over the entire surface area of the Earth. For reference, the total surface area of the Earth and the area of the Earth covered by land are 5.12 and 1.45 X 1014 m2, respectively. [Courtesy of S. G. Warren.]

a Surface area is expressed as percentage of the area of the surface of the Earth. Mass is expressed in units of 103 kg m~2 (numerically equivalent to meters of liquid water) averaged over the entire surface area of the Earth. For reference, the total surface area of the Earth and the area of the Earth covered by land are 5.12 and 1.45 X 1014 m2, respectively. [Courtesy of S. G. Warren.]

Ice Sheet Northern Hemisphere

□ Permafrost (Continuous) ■Glaciers & Ice Sheets

□ Permaf rost (D isconti n uou s) □Perennial Sea Ice □Snow Extent [0Seasonal Sea Ice □Land ■ Ocean

Fig. 2.12 Elements of the northern hemisphere cryosphere. The equatorward edge of the snow cover corresponds to —50% coverage during the month of maximum snow extent. [Courtesy of Ignatius Rigor.] The inset at the upper left shows a NASA RADARSAT image highlighting these features.

□ Permafrost (Continuous) ■Glaciers & Ice Sheets

□ Permaf rost (D isconti n uou s) □Perennial Sea Ice □Snow Extent [0Seasonal Sea Ice □Land ■ Ocean

Fig. 2.12 Elements of the northern hemisphere cryosphere. The equatorward edge of the snow cover corresponds to —50% coverage during the month of maximum snow extent. [Courtesy of Ignatius Rigor.] The inset at the upper left shows a NASA RADARSAT image highlighting these features.

be concentrated in relatively narrow, fast-moving ice streams tens of kilometers in width (Fig. 2.13).

Along the divides of the ice sheets the movement is very slow and the layering of the ice is relatively undisturbed. In ice cores extracted from these regions, the age of the ice increases monotonically with depth to —100,000 years in the Greenland ice sheet and over 500,000 years in the Antarctic ice sheet. Analysis of air bubbles, dust, and chemical and biological tracers embedded within these ice cores is providing a wealth of information on the climate of the past few hundred thousand years, as discussed later in this chapter.

In many respects, alpine (i.e., mountain) glaciers behave like continental ice sheets, but they are much smaller in areal coverage and mass. Their fate is also determined by their mass balance. Parcels of ice within them flow continually downhill from an upper dome-like region where snow and ice accumulate toward their snouts where mass is lost continually due to melting. Because of their much smaller masses, glaciers respond much more quickly to climate change than continental ice sheets, and ice cycles through them much more rapidly. Some alpine glaciers also exhibit time-dependent behavior that is not climate

Parts Continental Glaciers

Fig. 2.13 Satellite image of the Antarctic ice sheet showing rate of creep of the ice (in m year-1) on a logarithmic scale. Dots show the locations of ice core sites. Vostok, the site of the ice core shown in Fig. 2.31, is indicated by the solid red dot. [Adapted with permission from Bamber, J. L., D. G. Vaughan and I. Joughin, "Widespread Complex Flow in the Interior of the Antarctic Ice Sheet," Science, 287, 1248-1250. Copyright 2000 AAAS. Courtesy of Ignatius Rigor.]

Fig. 2.13 Satellite image of the Antarctic ice sheet showing rate of creep of the ice (in m year-1) on a logarithmic scale. Dots show the locations of ice core sites. Vostok, the site of the ice core shown in Fig. 2.31, is indicated by the solid red dot. [Adapted with permission from Bamber, J. L., D. G. Vaughan and I. Joughin, "Widespread Complex Flow in the Interior of the Antarctic Ice Sheet," Science, 287, 1248-1250. Copyright 2000 AAAS. Courtesy of Ignatius Rigor.]

related: episodic surges of a few months' to a few years' duration interspersed with much longer periods of slow retreat.

Sea ice covers a larger area of the Earth's surface area than the continental ice sheets (Table 2.1) but, with typical thicknesses of only 1-3 m, is orders of magnitude less massive. The ice is not a continuous surface, but a fractal field comprised of ice floes (pieces) of various of shapes and sizes, as shown in Figs. 2.14 and 2.15. The individual floes are separated by patches of open water (called leads) that open and close as the ice pack moves, dragged by surface winds.

Seasonal limits of the northern hemisphere pack ice are shown in Fig. 2.12. During winter, ice covers not only the Arctic, but also much of the Bering Sea and the Sea of Okhotsk, but during the brief polar summer the ice retreats dramatically and large leads are sometimes observed, even in the vicinity of the North Pole. Antarctic pack ice also advances and retreats with the seasons.

The annual-mean sea ice motion, shown in Fig. 2.16, is dominated by the clockwise Beaufort Gyre to the north of Alaska and the transpolar drift stream from Siberia toward Greenland and Spitzbergen.5 Some ice floes remain in the Arctic for a decade or more, circulating around and around the Beaufort Gyre, whereas others spend just a year or two in the Arctic before they exit either through the Fram Strait between Greenland and Spitzbergen or through the Nares Strait into Baffin Bay along the west side of Greenland. Ice floes exiting the Arctic make a one-way trip into warmer waters, where they are joined by much thicker icebergs that break off the Greenland ice sheet.

New pack ice is formed during the cold season by the freezing of water in newly formed leads and in regions where offshore winds drag the pack ice away from the coastline, exposing open water. The new ice thickens rapidly at first and then more gradually as it begins to insulate the water beneath it from the subfreezing air above. Ice thicker than a meter is formed, not by a thickening of newly formed layer of

5 The existence of a transpolar drift stream was hypothesized by Nansen6 when he learned that debris from a shipwreck north of the Siberian coast had been recovered, years later, close to the southern tip of Greenland. Motivated by this idea, he resolved to sail a research ship as far east as possible off the coast of Siberia and allow it to be frozen into the pack ice in the expectation that it would be carried across the North Pole along the route suggested by Fig. 2.16. He supervised the design and construction of a research vessel, the Fram ("Forward"), with a hull strong enough to withstand the pressure of the ice. The remarkable voyage of the Fram, which began in summer of 1893 and lasted for 3 years, confirmed the existence of the transpolar drift stream and provided a wealth of scientific data.

6 Fridtjof Nansen (1861-1930). Norwegian scientist, polar explorer, statesman, and humanitarian. Educated as a zoologist. Led the first traverse of the Greenland ice cap on skis in 1888. The drift of his research vessel the Fram across the Arctic (1893-1896) was hailed as a major achievement in polar research and exploration. Midway through this voyage, Nansen turned over command of the Fram to Harald Sverdrup and set out with a companion on what proved to be a 132-day trek across the pack ice with dog-drawn sledges and kayaks, reaching 86 °N before adverse conditions forced them to turn southward.

Sacrificed his subsequent aspirations for Antarctic exploration to serve the needs of his country and to pursue humanitarian concerns. Was instrumental in peacefully resolving a political dispute between Norway and Sweden in 1905-1906 and negotiating a relaxation of an American trade embargo that threatened Norwegian food security during World War I. Awarded the Nobel Peace Prize in 1922 in recognition of his extensive efforts on behalf of war refugees and famine victims.

34 The Earth System

Victims ScienceGreenland East Coast

Fig. 2.15 Floes in pack ice streaming southward off the east coast of Greenland. The white area at the upper left is landfast ice that is attached to the coast, and the black channel adjacent to it is open water, where the mobile pack ice has become detached from the landfast ice. [NASA MODIS imagery.]

Fig. 2.15 Floes in pack ice streaming southward off the east coast of Greenland. The white area at the upper left is landfast ice that is attached to the coast, and the black channel adjacent to it is open water, where the mobile pack ice has become detached from the landfast ice. [NASA MODIS imagery.]

Fig. 2.14 Ice floes and leads in Antarctic pack ice. The lead in the foreground is 4-5 m across. The floe behind it consists of multi-year ice that may have originated as an iceberg; it is unusually thick, extending from ~15 m below to ~1 m above sea level. Most of the portion of the floe that extends above sea level is snow. At the time this picture was taken, the pack ice in the vicinity was under lateral pressure, as evidenced by the fact that a pressure ridge had recently developed less than 100 m away. [Photograph courtesy of Miles McPhee.]

Fig. 2.14 Ice floes and leads in Antarctic pack ice. The lead in the foreground is 4-5 m across. The floe behind it consists of multi-year ice that may have originated as an iceberg; it is unusually thick, extending from ~15 m below to ~1 m above sea level. Most of the portion of the floe that extends above sea level is snow. At the time this picture was taken, the pack ice in the vicinity was under lateral pressure, as evidenced by the fact that a pressure ridge had recently developed less than 100 m away. [Photograph courtesy of Miles McPhee.]

ice, but by mechanical processes involving collisions of ice floes. Pressure ridges up to 5 m in thickness are created when floes collide, and thickening occurs when part of one floe is pushed or rafted on top of another.

When sea water freezes, the ice that forms is composed entirely of fresh water. The concentrated salt water known as brine that is left behind mixes with the surrounding water, increasing its salinity. Brine rejection is instrumental in imparting enough negative buoyancy to parcels of water to enable them to break through the pycnocline and sink to the bottom. Hence, it is no accident that the sinking regions in the oceanic thermohaline circulation are in high latitudes, where sea water freezes.

Land snow cover occupies an even larger area of the northern hemisphere than sea ice and it varies much more widely from week to week and month to month than does sea ice. With the warming of the land surface during spring, the snow virtually disappears, except in the higher mountain ranges.

Permafrost embedded in soils profoundly influences terrestrial ecology and human activities over large areas of Siberia, Alaska, and northern Canada. If the atmosphere and the underlying land surface

Scales Atmospheric Motion Images
Fig. 2.16 Wintertime Arctic sea ice motion as inferred from the tracks of an array of buoys dropped on ice floes by aircraft. [Courtesy of Ignatius Rigor.]

Temperature (°C)

Temperature (°C)

Permafrost Temperature Profile

Fig. 2.17 Schematic vertical profile of summer and winter soil temperatures in a region of permafrost. The depth of the permafrost layer varies from as little as a few meters in zones of intermittent permafrost to as much as 1 km over the coldest regions of Siberia.

Fig. 2.17 Schematic vertical profile of summer and winter soil temperatures in a region of permafrost. The depth of the permafrost layer varies from as little as a few meters in zones of intermittent permafrost to as much as 1 km over the coldest regions of Siberia.

were in thermal equilibrium, the zones of continuous and intermittent permafrost in Fig. 2.12 would straddle the 0 °C isotherm in annual-mean surface air temperature. There is, in fact, a close correspondence between annual-mean surface air temperature and the limit of continuous permafrost, but the critical value of surface air temperature tends to be slightly above 0 °C due to the presence of snow cover, which insulates the land surface during the cold season, when it is losing heat.

Even in the zone of continuous permafrost, the topmost few meters of the soil thaw during summer in response to the downward diffusion of heat from the surface, as shown in Fig. 2.17. The upward diffusion of heat from the Earth's interior limits the vertical extent of the permafrost layer. Because the molecular diffusion of heat in soil is not an efficient heat transfer mechanism, hundreds of years are required for the permafrost layer to adjust to changes in the temperature of the overlying air.

2.1.3 The Terrestrial Biosphere

Much of the impact of climate upon animals and humans is through its role in regulating the condition and geographical distribution of forests, grasslands, tundra, and deserts, elements of the terrestrial (land) biosphere. A simple conceptual framework for relating climate (as represented by annual-mean temperature and precipitation) and vegetation type is shown in Fig. 2.18. The boundary between tundra and forest corresponds closely to the limit of the permafrost zone, which, as noted earlier, is determined by annual-mean temperature. The other boundaries in Fig. 2.18 are determined largely by the water requirements of plants. Plants utilize water both as raw material in producing chlorophyll and to keep cool on hot summer days, as described later. Forests require more water than grasslands, and grasslands, in turn, require more water than desert vegetation. The water demands of any specified type of vegetation increase with temperature.

Biomes are geographical regions with climates that favor distinctive combinations of plant and animal species. For example, tundra is the dominant form of vegetation in regions in which the mean temperature of the warmest month is <10 °C, and sparse, desert vegetation prevails in regions in which potential evaporation (proportional to the quantity of solar radiation reaching the ground) exceeds precipitation. The global distribution of biomes is determined by the insolation (i.e., the incident solar radiation) at the top of the atmosphere and by the climatic variables:

• annual-mean temperature,

• the annual and diurnal temperature ranges,

• annual-mean precipitation, and

• the seasonal distributions of precipitation and cloudiness.

n A

Grassland

Tundra

Annual mean temperature

Fig. 2.18 A conceptual framework for understanding how the preferred types of land vegetation over various parts of the globe depend on annual-mean temperature and precipitation.

Annual mean temperature

Fig. 2.18 A conceptual framework for understanding how the preferred types of land vegetation over various parts of the globe depend on annual-mean temperature and precipitation.

36 The Earth System

Which Continents Have Needleleaf Forests

Evergreen Needleleaf Forest Evergreen Broadleaf Forest Deciduous Needleleaf Forest Deciduous Broadleaf Forest Mixed Forest

Closed Shrublands Open Shrublands Woody Savannas Savannas Grasslands

Permanent Wetlands [ Croplands [

Urban and Built Up Cropland/Natural Vegetation Snow or Ice

Barren or Sparsely Vegetated Water

Evergreen Needleleaf Forest Evergreen Broadleaf Forest Deciduous Needleleaf Forest Deciduous Broadleaf Forest Mixed Forest

Closed Shrublands Open Shrublands Woody Savannas Savannas Grasslands

Permanent Wetlands [ Croplands [

Urban and Built Up Cropland/Natural Vegetation Snow or Ice

Barren or Sparsely Vegetated Water

Fig. 2.19 Global land cover characterization, as inferred from NASA AVHRR NDVI satellite imagery and ground-based data relating to ecological regions, soils, vegetation, land use, and land cover. [From USGS Land Processes DAAC.]

Insolation and climate at a given location, in turn, are determined by latitude, altitude, and position with reference to the land-sea configuration and terrain. The combined influence of altitude upon temperature (Fig. 1.9), terrain upon precipitation (Fig. 1.25), and local terrain slope upon the incident solar radiation (Exercise 4.16) gives rise to a variegated distribution of biomes in mountainous regions.

Several different systems exist for assigning bio-mes, each of which consists of a comprehensive set of criteria that are applied to the climate statistics for each geographical location.7 The "ground truth" for such classification schemes is the observed distribution of land cover, as inferred from ground-based measurements and high-resolution satellite imagery. An example is shown in Fig. 2.19.

The state of the terrestrial biosphere feeds back upon the climate through its effects on

• the hydrologic cycle: for example, during intervals of hot weather, plants control their temperatures by evapo-transpiration (i.e., by giving off water vapor through their leaves or needles). Energy derived from absorbed solar radiation that would otherwise contribute to heating the land surface is used instead to evaporate liquid water extracted from the soil by the roots of the plants. In this manner, the solar energy is transferred to the atmosphere without warming the land surface. Hence, on hot summer days, grass-covered surfaces tend to be cooler than paved surfaces and vegetated regions do not experience as high daily maximum temperatures as deserts and urban areas.

• the local albedo (the fraction of the incident solar radiation that is reflected, without being absorbed): for example, snow-covered tundra is more reflective, and therefore cooler during the daytime, than a snow-covered forest.

• the roughness of the land surface: wind speeds in the lowest few tens of meters above the ground tend to be higher over bare soil and tundra than over forested surfaces.

7 These systems are elaborations of a scheme developed by Koppen8 a century ago.

8 Wladimir Peter Koppen (1846-1940) German meteorologist, climatologist, and amateur botanist. His Ph.D. thesis (1870) explored the effect of temperature on plant growth. His climate classification scheme, which introduced the concept of biomes, was published in 1900. For many years, Koppen's work was better known to physical geographers than to atmospheric scientists, but in recent years it is becoming more widely appreciated as a conceptual basis for describing and modeling the interactions between the atmosphere and the terrestrial biosphere.

2.1.4 The Earth's Crust and Mantle

The current configuration of continents, oceans, and mountain ranges is a consequence of plate tectonics and continental drift.9 The Earth's crust and mantle also take part in chemical transformations that mediate the composition of the atmosphere on timescales of tens to hundreds of millions of years.

The Earth's crust is broken up into plates that float upon the denser and much thicker layer of porous but viscous material that makes up the Earth's mantle. Slow convection within the mantle moves the plates at speeds ranging up to a few centimeters per year (tens of kilometers per million years). Plates that lie above regions of upwelling in the mantle are spreading, whereas plates that lie above regions of downwelling in the mantle are being pushed together. Earthquakes tend to be concentrated along plate boundaries.

Oceanic plates are thinner, but slightly denser than continental plates so that when the two collide, the ocean plate is subducted (i.e., drawn under the continental plate) and incorporated into the Earth's mantle, as shown schematically in Fig. 2.20. Rocks in the subducted oceanic crust are subjected to increasingly higher temperatures and pressures as they descend, giving rise to physical and chemical transformations.

Collisions between plate boundaries are often associated with volcanic activity and with the uplift of mountain ranges. The highest of the Earth's mountain ranges, the Himalayas, was created by folding of the Earth's crust following the collision of the Indian and Asian plates, and it is still going on today. The Rockies, Cascades, and Sierra ranges in western North America have been created in a similar manner by the collision of the Pacific and North American plates. These features have all appeared within the past 100 million years.

Oceanic plates are continually being recycled. The Pacific plate is being subducted along much of the extent of its boundaries, while new oceanic crust is being formed along the mid-Atlantic ridge as magma

Oceanic Oceanic Subduction Steps

Fig. 2.20 Schematic showing subduction, sea floor spreading, and mountain building. [Adapted by permission of Pearson Education, Inc., Upper Saddle River, NJ. Edward J. Tarbuck, Frederick K. Lutgens and Dennis Tasa, Earth: An Introduction to Physical Geology, 8th Edition, © 2005, p. 426, Fig. 14.9.]

Fig. 2.20 Schematic showing subduction, sea floor spreading, and mountain building. [Adapted by permission of Pearson Education, Inc., Upper Saddle River, NJ. Edward J. Tarbuck, Frederick K. Lutgens and Dennis Tasa, Earth: An Introduction to Physical Geology, 8th Edition, © 2005, p. 426, Fig. 14.9.]

upwelling within the mantle rises to the surface, cools, and solidifies. As this newly formed crust diverges away from the mid-Atlantic ridge, the floor of the Atlantic Ocean is spreading, pushing other parts of the crust into the spaces formerly occupied by the subducted portions of the Pacific plate. As the Atlantic widens and the Pacific shrinks, the continents may be viewed as drifting away from the Atlantic sector on trajectories that will, in 100-200 million years, converge over what is now the mid-Pacific. A similar congregation of the continental plates is believed to have occurred about 200 million years ago, when they were clustered around the current position of Africa, forming a supercontinent called Pangaea (all Earth).

Some of the material incorporated into the mantle when plates are subducted contains volatile substances (i.e., substances that can exist in a gaseous form, such as water in hydrated minerals). As the temperature of these materials rises, pressure builds

9 The theory of continental drift was first proposed by Alfred Wegener10 in 1912 on the basis of the similarity between the shapes of coastlines, rock formations, and fossils on the two sides of the Atlantic. Wegener's radical reinterpretation of the processes that shaped the Earth was largely rejected by the geological community and did not become widely accepted until the 1960s, with the advent of geomagnetic evidence of sea-floor spreading.

10 Alfred Wegener (1880-1930). German meteorologist, professor at University of Graz. Began his career at the small University of Marburg. First to propose that ice particles play an important role in the growth of cloud droplets. Set endurance record for time a aloft in a hot air balloon (52 h) in 1906. Played a prominent role in the first expeditions to the interior of Greenland. Died on a relief mission on the Greenland icecap. The Alfred Wegener Institute in Bremerhaven is named in his honor. Son-in-law of Vladimir Koppen and co-authored a book with him.

38 The Earth System up beneath the Earth's crust, leading to volcanic eruptions. As will be explained later in this chapter, gases expelled in volcanic eruptions are the source of the Earth's present atmosphere, and they are continually renewing it.

2.1.5 Roles of Various Components of the Earth System in Climate

Atmospheric processes play the lead role in determining such fundamental properties of climate as the disposition of incoming solar radiation, temperatures at the Earth's surface, the spatial distribution of water in the terrestrial biosphere, and the distribution of nutrients in the euphotic zone of the ocean. However, other components of the Earth system are also influential. Were it not for the large storage of heat in the ocean mixed layer and cryosphere during summer, and the extraction of that same heat during the following winter, seasonal variations in temperature over the middle and high latitude continents would be much larger than observed and, were it not for the existence of widespread vegetation, summertime daily maximum temperatures in excess of 40 °C would be commonplace over the continents. The oceanic thermohaline circulation warms the Arctic and coastal regions of Europe by several degrees, while wind-driven upwelling keeps the equatorial eastern Pacific cool enough to render the Galapagos Islands a suitable habitat for penguins!

Plate tectonics shaped the current configuration of continents and topography, which, in turn, shapes many of the distinctive regional features of today's climate. The associated recycling of minerals through the Earth's upper mantle is believed to have played a role in regulating the concentration of atmospheric carbon dioxide, which exerts a strong influence upon the Earth's surface temperature.

These are but a few examples of how climate depends not only on atmospheric processes, but on processes involving other components of the Earth system. As explained in Section 10.3, interactions between the atmosphere and other components of the Earth system give rise to feedbacks that can either amplify or dampen the climatic response to an imposed external forcing of the climate system, such as a change in the luminosity of the sun or human-induced changes in atmospheric composition.

The next three sections of this chapter describe the exchanges and cycling of water, carbon, and oxygen among the various components of the Earth system.

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Responses

  • ari huovinen
    WHICH CONTINENTS HAVE NEEDLELEAF FORESTS?
    9 years ago
  • Veli-Matti
    Is the predominant igneous rock of the oceanic crust?
    9 years ago
  • alem
    How much of the earth was covered by ice sheets?
    9 years ago
  • elanor twofoot
    How the midatlantic ridge was formed?
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  • Caradoc
    What are the features at the mid atlantic ridge?
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  • Caitlyn
    How to draw a Sea Floor Spreading?
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  • aatifa michael
    What causes sea floor to descend back into the mantle?
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  • divo
    Where does the magma for a new oceanic crust?
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  • Alfie
    How new oceanic crust are formed?
    9 years ago
  • halfred
    What did Vladimir Koppen contibute to science?
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  • minna rosberg
    What does a mid ocean ridge look like and seafloor spreading look like?
    7 years ago

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