The lunar maria ( / ˈ m ær i . ə / MARR -ee-ə ; sg. mare / ˈ m ɑːr eɪ , - i / MAR -ay, MAR -ee ) are large, dark, basaltic plains on Earth 's Moon , formed by lava flowing into ancient impact basins. They were dubbed maria ( Latin for 'seas') by early astronomers who mistook them for actual seas . They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to the naked eye . The maria cover about 16% of the lunar surface, mostly on the side visible from Earth . The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus ('lake'), palus ('marsh'), and sinus ('bay'). The last three are smaller than maria, but have the same nature and characteristics.
20-575: Mare Insularum / ɪ n s ʊ ˈ l ɛər ə m / (Latin īnsulārum , the " sea of islands ") is a lunar mare located in the Insularum basin just south of the western Mare Imbrium . The basin material is of the Lower Imbrian epoch , with the mare material of the Upper Imbrian epoch. The mare is bordered by the craters Copernicus on the east, and Kepler on the west. Oceanus Procellarum joins
40-403: Is the near-total absence of water in any form in the lunar basalts. Lunar basalts do not contain hydrogen-bearing minerals like the amphiboles and phyllosilicates that are common in terrestrial basalts due to alteration or metamorphism. Tholeiitic The tholeiitic magma series ( / ˌ θ oʊ l i ˈ aɪ t ɪ k / ) is one of two main magma series in subalkaline igneous rocks ,
60-459: The Earth's mantle . Tholeiitic basalt constituting the oceanic crust is termed MORB: m id- o cean- r idge b asalt. Throughout the process of igneous differentiation , the oceanic crust acts to reduce the magma, producing the tholeiitic trend. In contrast, alkali basalts are not typical of ocean ridges, but are erupted on some oceanic islands and on continents, as also is tholeiitic basalt. Because
80-584: The Procellarum KREEP Terrane . While the enhancement in heat production within the Procellarum KREEP Terrane is most certainly related to the longevity and intensity of volcanism found there, the mechanism by which KREEP became concentrated within this region is not agreed upon. Using terrestrial classification schemes, all mare basalts are classified as tholeiitic , but specific subclassifications have been invented to further describe
100-411: The alkali corner as they cool. In the tholeiitic magma, magnesium-rich crystals are produced preferentially, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and simply moves towards the alkali corner as it loses iron and any remaining magnesium. With the calc-alkaline series, however, the precipitation of magnetite causes
120-468: The aluminium content, with tholeiitic basalts containing 12% to 16% Al 2 O 3 versus 16% to 20% Al 2 O 3 for calc-alkali basalts. Like all basalt, the rock type is dominated by olivine , clinopyroxene and plagioclase , with minor iron- titanium oxides. Orthopyroxene or pigeonite may also be present in tholeiitic basalt, and olivine, if present, may be rimmed by either of these calcium-poor pyroxenes. Tridymite or quartz may be present in
140-473: The far side are old, whereas the youngest flows are found within Oceanus Procellarum on the nearside. While many of the basalts either erupted within, or flowed into, low-lying impact basins, the largest expanse of volcanic units, Oceanus Procellarum, does not correspond to any known impact basin. There are many common misconceptions concerning the spatial distribution of mare basalts. The reason that
160-586: The final nomenclature, that of states of mind, was accepted, and do not follow this pattern. When Mare Moscoviense was discovered by the Luna 3 , and the name was proposed by the Soviet Union, it was only accepted by the International Astronomical Union with the justification that Moscow is a state of mind. The ages of the mare basalts have been determined both by direct radiometric dating and by
180-482: The fine-grained groundmass of tholeiitic basalt, and feldspathoids are absent. Tholeiitic rocks may have a fine, glassy groundmass , as may other types of basalt. Tholeiitic rocks are the most common igneous rocks in Earth's crust , produced by submarine volcanism at mid-ocean ridges and make up much of the ocean crust. Tholeiitic basaltic magmas are initially generated as partial melts of peridotite ( olivine and pyroxene ) produced by decompression melting of
200-447: The high-titanium concentrations are the least abundant. TiO 2 abundances can reach up to 15 wt.% for mare basalts, whereas most terrestrial basalts have abundances much less than 4 wt.%. A special group of lunar basalts is the KREEP basalts, which are abnormally rich in potassium (K), rare-earth elements (REE), and phosphorus (P). A major difference between terrestrial and lunar basalts
220-425: The iron content of the magma to remain more steady as it cools than with a tholeiitic magma. The difference between these two magma series can be seen on an AFM diagram, a ternary diagram showing the relative proportions of the oxides Na 2 O + K 2 O (A), FeO + Fe 2 O 3 (F), and MgO (M). As magmas cool, they precipitate out significantly more iron and magnesium than alkali, causing the magmas to move towards
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#1732898989730240-499: The iron-magnesium ratio to remain relatively constant, so the magma moves in a straight line towards the alkali corner on the AFM diagram. The AFM plot distinguishes the intermediate members of the tholeiitic and calc-alkali magma series quite well. However, the felsic end members of the two series are nearly indistinguishable, so granitic rocks are generally assigned to the calc-alkali magma series. The mafic end members may be distinguished by
260-588: The mare basalts are predominantly located on the near-side hemisphere of the Moon is still being debated by the scientific community. Based on data obtained from the Lunar Prospector mission, it appears that a large proportion of the Moon's inventory of heat producing elements (in the form of KREEP ) is located within the regions of Oceanus Procellarum and the Imbrium basin , a unique geochemical province now referred to as
280-872: The mare to the southwest. Copernicus is one of the most noticeable craters on the Moon. The rays from both Kepler and Copernicus protrude into the mare. It is located near the crater Fra Mauro , the site of the Apollo 14 landing. Sinus Aestuum forms a northeastern extension to the mare. The name was suggested by lunar geologist Don E. Wilhelms . Lunar mare The names of maria refer to sea features ( Mare Humorum , Mare Imbrium , Mare Insularum , Mare Nubium , Mare Spumans , Mare Undarum , Mare Vaporum , Oceanus Procellarum , Mare Frigoris ), sea attributes ( Mare Australe , Mare Orientale , Mare Cognitum , Mare Marginis ), or states of mind ( Mare Crisium , Mare Ingenii , Mare Serenitatis , Mare Tranquillitatis ). Mare Humboldtianum and Mare Smythii were established before
300-404: The oceanic crust. Almost all the basalt found on the Moon is tholeiitic basalt. Rocks in the tholeiitic magma series are classified as subalkaline (they contain less sodium than some other basalts) and are distinguished from rocks in the calc-alkaline magma series by the redox state of the magma they crystallized from (tholeiitic magmas are reduced; calc-alkaline magmas are oxidized ). When
320-406: The other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member. Rock types of the tholeiitic magma series include tholeiitic basalt , ferro-basalt, tholeiitic basaltic andesite , tholeiitic andesite , dacite and rhyolite . The variety of basalt in the series
340-400: The parent magmas of basalts crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals olivine and pyroxene , causing the iron content of tholeiitic magmas to increase as the melt is depleted of iron-poor crystals. However, a calc-alkaline magma is oxidized enough to precipitate significant amounts of the iron oxide magnetite , causing
360-533: The population of lunar basalts. Mare basalts are generally grouped into three series based on their major element chemistry: high-Ti basalts , low-Ti basalts , and very-low-Ti (VLT) basalts . While these groups were once thought to be distinct based on the Apollo samples, global remote sensing data from the Clementine mission now shows that there is a continuum of titanium concentrations between these end members, and that
380-519: The technique of crater counting . The radiometric ages range from about 3.16 to 4.2 billion years old (Ga), whereas the youngest ages determined from crater counting are about 1.2 Ga. Updated measurements of samples collected by the Chang’e-5 mission show that some lunar basalts could be as young as 2.03 billion years old. Nevertheless, the majority of mare basalts appear to have erupted between about 3 and 3.5 Ga. The few basaltic eruptions that occurred on
400-492: Was originally called tholeiite but the International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to that term. Tholeiitic rock types tend to be more enriched in iron and less enriched in aluminium than calc-alkaline rock types. They are thought to form in a less oxidized environment than calc-alkaline rocks. Tholeiitic basalt is formed at mid-ocean ridges and makes up much of
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