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82-655: The subduction under the New Britain Trench to the north and the northern San Cristobal Trough to the east in the Woodlark Basin is unusual because the new crust is very young and light unlike most oceanic crust that gets subducted which is older and of higher density with higher sedimentary load. The Woodlark Basin is located between the Louisiade Archipelago off the southeastern coast of New Guinea , with Woodlark Island to its north and

164-447: A color index of 35 or greater. The physical properties of basalt result from its relatively low silica content and typically high iron and magnesium content. The average density of basalt is 2.9 g/cm , compared, for example, to granite ’s typical density of 2.7 g/cm . The viscosity of basaltic magma is relatively low—around 10 to 10 cP —similar to the viscosity of ketchup , but that is still several orders of magnitude higher than

246-423: A mantle wedge above the descending slab. The slab releases water vapor and other volatiles as it descends, which further lowers the melting point, further increasing the amount of decompression melting. Each tectonic setting produces basalt with its own distinctive characteristics. The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene . Olivine can also be

328-681: A transpressional regime. The overwhelming drag of the overriding Pacific plate caused the Solomon microplate to break away from the Indo-Australian plate, producing a mega shear zone between the two plates made up of at least two microplates. This shear zone has been named the Nubara Fault , Nubara faults or Nubara Transform Fault which is along the eastern part of the Woodlark Rise that separates at least two microplates, being

410-553: A Spanish expedition led by Luis Váez de Torres in 1606, that was part of the Fernandez de Quiros fleet which had sailed from South America in search of Australia. The Torres expedition visited various islands including Basilaki Island , which he named San Buenaventura in July 1606. It is possible that Malay and Chinese sailors also visited the islands earlier. More than a century later, in 1768, Louis Antoine de Bougainville visited

492-493: A basalt is diagnostic of how and where it erupted—for example, whether into the sea, in an explosive cinder eruption or as creeping pāhoehoe lava flows, the classic image of Hawaiian basalt eruptions. Basalt that erupts under open air (that is, subaerially ) forms three distinct types of lava or volcanic deposits: scoria ; ash or cinder ( breccia ); and lava flows. Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated , imparting

574-657: A distinct ecoregion , and are home to a number of endemic species, including several endemic trees (in genera Pandanus , Diospyros , and Hopea ), as well as five endemic frog species, two endemic lizard species, and five endemic bird species. The archipelago is divided into the Local Level Government (LLG) areas Bwanabwana Rural (western part, with Basilaki ), Louisiade Rural (central part, with Misima ), and Yaleyamba (eastern part, with Rossel and Vanatinai islands). The LLG areas are part of Samarai-Murua District of Milne Bay . The seat of

656-442: A distinctive pillow shape, through which the hot lava breaks to form another pillow. This "pillow" texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. The size of individual pillows varies from 10 cm up to several metres. When pāhoehoe lava enters

738-427: A flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns . These structures, or basalt prisms , are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on

820-430: A high content of augite or other dark-coloured pyroxene minerals, but can exhibit a wide range of shading. Some basalts are quite light-coloured due to a high content of plagioclase; these are sometimes described as leucobasalts . It can be difficult to distinguish between lighter-colored basalt and andesite , so field researchers commonly use a rule of thumb for this purpose, classifying it as basalt if it has

902-434: A key structural feature for crustal extension. In the Woodlark Basin the active low angle normal faults have a dips between 15°-35°, and are all located in the small range of 150.5° E to 152.5° E. This is the area in the Woodlark Basin where seafloor spreading makes the transition to continental rifting . However, some scientists do not believe that low angle normal faults exist actively in the world. They instead believe that

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984-621: A lightweight "frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidized iron from weathered iron-rich minerals such as pyroxene . ʻAʻā types of blocky cinder and breccia flows of thick, viscous basaltic lava are common in Hawaiʻi. Pāhoehoe is a highly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimes forms lava lakes . Lava tubes are common features of pāhoehoe eruptions. Basaltic tuff or pyroclastic rocks are less common than basaltic lava flows. Usually basalt

1066-623: A lower total content of alkali oxides than trachybasalt and most basanites and tephrites. Basalt generally has a composition of 45–52 wt% SiO 2 , 2–5 wt% total alkalis, 0.5–2.0 wt% TiO 2 , 5–14 wt% FeO and 14 wt% or more Al 2 O 3 . Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%. High-alumina basalts have aluminium contents of 17–19 wt% Al 2 O 3 ; boninites have magnesium (MgO) contents of up to 15 percent. Rare feldspathoid -rich mafic rocks, akin to alkali basalts, may have Na 2 O + K 2 O contents of 12% or more. The abundances of

1148-514: A maximum thickness of 1500 meters of sediment fill in the deepest section of the North Moresby graben, however most of the basin is covered in less than 1000 meters of sediment. The eastern basin volcanics near its spreading center are only covered by a thin sediment dusting composed of nanofossil-bearing clay. This is great for researchers because different features, such as faults and spreading centers, can easily be seen in satellite images of

1230-507: A phenocryst, and when present, may have rims of pigeonite. The groundmass contains interstitial quartz or tridymite or cristobalite . Olivine tholeiitic basalt has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass . Alkali basalts typically have mineral assemblages that lack orthopyroxene but contain olivine. Feldspar phenocrysts typically are labradorite to andesine in composition. Augite

1312-523: A significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite , ulvöspinel , and ilmenite . Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt. In tholeiitic basalt , pyroxene ( augite and orthopyroxene or pigeonite ) and calcium -rich plagioclase are common phenocryst minerals. Olivine may also be

1394-412: A spreading center is usually non-transform and that transform faults develop after seafloor spreading had begun. This issue took 40 years to resolve after the importance of transform faults to the theory of plate tectonics was first understood. In the region of the basin, towards its west, there has been unusually rapid emplacement, at greater than 1–2 cm/year (0.39–0.79 in/year) vertically, of

1476-544: Is a common rock on the surface of Mars . Molten basalt lava has a low viscosity due to its relatively low silica content (between 45% and 52%), resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying. Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometres and constitute the most voluminous of all volcanic formations. Basaltic magmas within Earth are thought to originate from

1558-602: Is also known that prior to 4 million years ago seafloor spreading in the Woodlark Basin was up to 240 km (150 mi) east of the Simbo Transform where it ends today. The basin is not currently being explored by any large companies seeking to find natural resources. Due to the extremely young age of the Woodlark basin (less than 5 million years old), the basin has not had enough time for significant amounts of natural resources, such as oil and gas, to form. Because

1640-401: Is also produced by some subglacial volcanic eruptions. Basalt is the most common volcanic rock type on Earth, making up over 90% of all volcanic rock on the planet. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below the ocean ridges . Basalt is also the principal volcanic rock in many oceanic islands , including

1722-414: Is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are more properly referred to either as diabase (also called dolerite) or—when they are more coarse-grained (having crystals over 2 mm across)—as gabbro . Diabase and gabbro are thus the hypabyssal and plutonic equivalents of basalt. During

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1804-512: Is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro . The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System . For example, the bulk of the plains of Venus , which cover ~80% of the surface, are basaltic; the lunar maria are plains of flood-basaltic lava flows ; and basalt

1886-545: Is classified as basalt when its QAPF fraction is composed of less than 10% feldspathoid and less than 20% quartz, and plagioclase makes up at least 65% of its feldspar content. This places basalt in the basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of under 52%. It is often not practical to determine the mineral composition of volcanic rocks, due to their very small grain size, in which case geologists instead classify

1968-761: Is considered a key to understanding plate tectonics , its compositions have been much studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform. For instance, compositions change with position along the Mid-Atlantic Ridge , and the compositions also define different ranges in different ocean basins. Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB). Isotope ratios of elements such as strontium , neodymium , lead , hafnium , and osmium in basalts have been much studied to learn about

2050-677: Is located 200 km southeast of New Guinea , stretching over more than 160 kilometres (99 mi) and spread over an ocean area of 26,000 square kilometres (10,000 sq mi) between the Solomon Sea to the north and the Coral Sea to the south. The aggregate land area of the islands is about 1,871 square kilometres (722 sq mi), with Vanatinai (Tagula) being the largest. Rogeia , Samarai and Sariba lie closest to New Guinea, while Misima , Vanatinai , and Rossel islands lie further east. The islands were discovered by

2132-545: Is no longer generally accepted. However refinements were being made up to less than a decade ago. Even so, the Nubara Transform Fault to the north of the basin is accommodating 0.93–0.95 cm/year (0.37–0.37 in/year) of movement, and the Australian plate is moving from the south between 2.09 cm/year (0.82 in/year) at the west and 4.02 cm/year (1.58 in/year) at the eastern side of

2214-531: Is rich in titanium compared to augite in tholeiitic basalt. Minerals such as alkali feldspar , leucite , nepheline , sodalite , phlogopite mica, and apatite may be present in the groundmass. Basalt has high liquidus and solidus temperatures—values at the Earth's surface are near or above 1200 °C (liquidus) and near or below 1000 °C (solidus); these values are higher than those of other common igneous rocks. The majority of tholeiitic basalts are formed at approximately 50–100 km depth within

2296-414: Is still rifting towards the edge of New Guinea . The Pocklington Rise and Plockington Trough separate the Woodlark Basin from the old subduction and spreading centers in the Australian plate from before 20 million years ago. A distinctive feature of the Woodlark Basin that scientists and researchers explored is the transition from continental rifting to sea floor spreading. The rifting at the western end of

2378-402: Is suppressed. Above this depth, submarine eruptions are often explosive, tending to produce pyroclastic rock rather than basalt flows. These eruptions, described as Surtseyan, are characterised by large quantities of steam and gas and the creation of large amounts of pumice . When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms

2460-440: Is too hot and fluid to build up sufficient pressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within the volcanic throat and buildup of volcanic gases . Hawaiʻi's Mauna Loa volcano erupted in this way in the 19th century, as did Mount Tarawera , New Zealand in its violent 1886 eruption. Maar volcanoes are typical of small basalt tuffs, formed by explosive eruption of basalt through

2542-455: Is ultimately derived from Late Latin basaltes , a misspelling of Latin basanites "very hard stone", which was imported from Ancient Greek βασανίτης ( basanites ), from βάσανος ( basanos , " touchstone "). The modern petrological term basalt , describing a particular composition of lava -derived rock, became standard because of its use by Georgius Agricola in 1546, in his work De Natura Fossilium . Agricola applied

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2624-539: The Clementine mission demonstrate that the lunar maria possess a continuum of titanium concentrations, and that the highest concentrations are the least abundant. Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism , lack of the oxidation typical of terrestrial basalts, and a complete lack of hydration . Most of the Moon 's basalts erupted between about 3 and 3.5 billion years ago, but

2706-491: The Euler poles for the region near the west coast of New Guinea between Port Moresby and Hood Point to its south. The most recent change in spreading direction of the Woodlark Basin spreading center occurred about 450,000 years ago with a slowing in rate of divergence about 200,000 years ago. The Woodlark Basin originally began to open as a sphenochasm with a pole near the tip of eastern Papua about 20 million years ago. This

2788-458: The Hadean , Archean , and early Proterozoic eons of Earth's history, the chemistry of erupted magmas was significantly different from what it is today, due to immature crustal and asthenosphere differentiation. The resulting ultramafic volcanic rocks, with silica (SiO 2 ) contents below 45% and high magnesium oxide (MgO) content, are usually classified as komatiites . The word "basalt"

2870-1228: The Paraná Traps in Brazil, the Siberian Traps in Russia , the Karoo flood basalt province in South Africa, and the Columbia River Plateau of Washington and Oregon . Basalt is also prevalent across extensive regions of the Eastern Galilee , Golan , and Bashan in Israel and Syria . Basalt also is common around volcanic arcs, specially those on thin crust . Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed. These are known as greenstone belts , because low-grade metamorphism of basalt produces chlorite , actinolite , epidote and other green minerals. As well as forming large parts of

2952-524: The lanthanide or rare-earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled. In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly . It arises because Eu can substitute for Ca in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form cations . Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are

3034-558: The upper mantle . The chemistry of basalts thus provides clues to processes deep in Earth's interior . Basalt is composed mostly of oxides of silicon, iron, magnesium, potassium, aluminum, titanium, and calcium. Geologists classify igneous rock by its mineral content whenever possible; the relative volume percentages of quartz (crystalline silica (SiO 2 )), alkali feldspar , plagioclase , and feldspathoid ( QAPF ) are particularly important. An aphanitic (fine-grained) igneous rock

3116-616: The Australian plate that in a current three plate model was assigned to the Solomon Plate. In the highlands due to the location of the systems eular pole there is a transition so that by the eastern end of the Papuan Peninsular the Australian plate is moving north towards the Trobriand plate by 1.57 cm/year (0.62 in/year). The western Woodlark plate triple junction with the Trobriand plate and Australian plate has

3198-463: The Earth's crust, basalt also occurs in other parts of the Solar System. Basalt commonly erupts on Io (the third largest moon of Jupiter ), and has also formed on the Moon , Mars , Venus , and the asteroid Vesta . The dark areas visible on Earth's moon , the lunar maria , are plains of flood basaltic lava flows. These rocks were sampled both by the crewed American Apollo program and

3280-647: The Louisiade Rural LLG is Bwagaoia on Misima Island, the population center of the archipelago. The seat of the Yaleyamba is changed every couple of years between Rambuso and Jingo (Rossell). The seat of the Bwanabwana was Samarai from the Louisiade archipelago, but was recently transferred to the mainland of Papua New Guinea to the city of Alotau . The Louisiade Archipelago consists of

3362-524: The Solomon Sea plate and the Woodlark plate. This means that the Nubara fault region is the only one of the four sides of the Solomon plate that is not being subducted currently. The passive rifting and seafloor spreading in the Woodlark Basin has produced the Woodlark plate in the northern part of the basin, as a separate microplate from the present Australian plate . The western end of the Woodlark Basin

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3444-716: The Solomons Trench-Woodlark Rift triple point gives underthrusting of the Solomons Plate beneath the North Solomon Trough in a northeasterly direction at about 11 centimeters per year. However the implications change slightly with later modelling. A recent best fit model suggests there is some continuing subduction at the Trobriand Trough which is at the southern aspect of the Solomon Plate. This model seems to account for

3526-506: The Woodlark Basin that separates the basin from the bordering Solomon Sea to the north. This spreading rift marks the southern boundary of the Solomon Plate, which is bounded by subduction zones in the north and east (the New Britain and northern Solomon's trenches (e.g. North Solomon Trough) respectively), and in the west by a combination strike-slip rifting (dip-slip) boundary in eastern Papua (New Guinea). A vector triangle solution near

3608-400: The Woodlark microplate tectonics carry much of this north-east movement if the reference point is a fixed Australian plate. Relative to the Woodlark microplate, Figure 2 shows each of the tectonic plates surrounding the Woodlark Basin along with the relative plate motion and direction that each plate is moving on a historic three plate model for the region, which apart from the movement vectors

3690-418: The Woodlark plate moving in an easterly direction at about 0.95 cm/year (0.37 in/year) and the Trobriand Trough is absorbing about 0.45 cm/year (0.18 in/year) of subduction. Definite subduction is current at the New Britain Trench, where Solomon Sea seafloor crust subducts to the north under New Britain and Bougainville in the new Britain subduction zone . The Australia Plate subducts to

3772-779: The basin due to the lack of sediment buildup. Basalts , basaltic andesite, andesite , and hydrothermal iron oxide has been recovered from the Franklin, Cheshire and Dobu Seamounts in the westernmost Woodlark area. From the Cheshire Seamount in the western Woodlark Basin samples had been intensely hydrothermally altered from precursor andesitic to rhyolitic composition with quartz growth, from over time magmatic processes, silicification , chloritization , formation of illite , sericitic alteration , replacement of plagioclase by albite , and sulfidation associated with concentration of precious metals and other minerals. In

3854-498: The basin has split the Woodlark Rise from the Pocklington Rise. The Woodlark Basin is one of the few places on Earth where scientists can actively study this transition. It is one of few ocean basins that has been completely systematically mapped, resulting in tectonic model refinement over the last 50 years. As a result of such understanding here, and elsewhere, it was known by 2009 that initial spreading offset faulting from

3936-405: The basin. Matters change for the size of the Solomon Plate, with the creation of a postulated still active Trobriand plate in a 4 plate, 7 triple junction, solution to the local complex tectonics. The northwestern part of the Trobriand plate in such a model, is moving to the south east towards Australia by 1.21 cm/year (0.48 in/year). The New Guinea Highlands host the plate junction with

4018-682: The bottom of the food chain are sulfide oxidizers but unlike some other vent communities there is evidence that they use both the Calvin–Benson–Bassham (CBB) cycle and the Reverse Krebs cycle alternatives. A particularly rich fauna has been characterised and other species found include: Louisiade Archipelago The Louisiade Archipelago is a string of ten larger volcanic islands frequently fringed by coral reefs, and 90 smaller coral islands in Papua New Guinea . It

4100-478: The characteristic igneous rocks formed at mid-ocean ridges. They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat REE patterns normalized to mantle or chondrite values. In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements. Because MORB basalt

4182-447: The crust, forming an apron of mixed basalt and wall rock breccia and a fan of basalt tuff further out from the volcano. Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites , quartz or calcite are frequently found. During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While

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4264-418: The deformation front lacks a flexed outer rise and bathymetric trench. The Woodlark Basin is one of very few places on the earth to have active low angle normal faults . A low angle normal fault is a normal fault that has a dip of less than thirty degrees (dip < 30°). This is a topic that is currently debated heavily, the low angle normal fault paradox . Low angle normal faults have been proposed to be

4346-575: The east and north beneath Guadalcanal and San Cristobal at the San Cristobal Trench. In both cases this results in normal faulting on the subducting plate, shallow thrust faulting beneath the forearc, and steeply dipping Wadati–Benioff zones extending to 200 km (120 mi) depth, with at the surface, active arc volcanoes. In between the Woodlark Basin is subducted northeast beneath the New Georgia Islands but here

4428-456: The east at the spreading center, tholeiitic basalt similar to mid-ocean ridge basalt (MORB) in composition is found with observation of cold pillow lava deposits. Where the Woodlark Basin is subducted northeast beneath the New Georgia Islands in the deformation associated with the northern San Cristobal Trough, the relative light plate made of recent oceanic type basalt stays shallow. It is presumably being melted on mantle contact and producing

4510-403: The eastern Woodlark Basin is older than the currently active spreading centres of the adjacent Lau and North Fiji basins, it may act as a biodiversity dispersion centre for modern hydrothermal vent fauna. Alviniconcha spp. a type of hot vent associated marine snail (gastropod) ecologically diversified 20 million years ago and has been found at Woodlark Basin hydrothermal vents. The bacteria at

4592-494: The evolution of the Earth's mantle . Isotopic ratios of noble gases , such as He / He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15–24 and more for ocean-island basalts thought to be derived from mantle plumes . Source rocks for the partial melts that produce basaltic magma probably include both peridotite and pyroxenite . The shape, structure and texture of

4674-469: The following island groups and islands: Basalt Basalt ( UK : / ˈ b æ s ɔː l t , - əl t / ; US : / b ə ˈ s ɔː l t , ˈ b eɪ s ɔː l t / ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low- viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon . More than 90% of all volcanic rock on Earth

4756-543: The islands and named them for Louis XV , the king of France . Visits were also paid by Admiral Bruni d'Entrecasteaux in 1793 and Captain Owen Stanley in 1849. The 1942 Battle of Coral Sea was fought nearby, after Japanese occupation in the same year. The Deboyne Islands were the site of the 1942 Raids on Deboyne . The islands have a moist tropical climate, and are largely covered with tropical moist broadleaf forests . The Louisiade Archipelago rain forests form

4838-740: The islands of Hawaiʻi , the Faroe Islands , and Réunion . The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is the rock most typical of large igneous provinces . These include continental flood basalts , the most voluminous basalts found on land. Examples of continental flood basalts included the Deccan Traps in India , the Chilcotin Group in British Columbia , Canada ,

4920-595: The low angle normal faults to date characterised are old normal faults that are no longer active and have been rotated to a different orientation. This would make a normal fault look like it was a low angle normal fault that is no longer active. The Woodlark Basin is actively spreading in a mostly north and south direction across its center, in an east–west orientation in a process that started about 3.6 million years ago. A large amount of sea floor has been formed as defined by magnetic studies. The spreading centers are offset by transform faults, creating five segments, this causes

5002-710: The mantle. Many alkali basalts may be formed at greater depths, perhaps as deep as 150–200 km. The origin of high-alumina basalt continues to be controversial, with disagreement over whether it is a primary melt or derived from other basalt types by fractionation. Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO 2 and the alkali oxides, i.e., Na 2 O + K 2 O , consistent with their TAS classification . Basalt contains more silica than picrobasalt and most basanites and tephrites but less than basaltic andesite . Basalt has

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5084-549: The many arc volcanoes of the western Solomon Islands. An exception may be the Savo Island volcano which is postulated to be related to a 200 km (120 mi) deep old Pacific plate slab with its associated deep earthquakes that underlies the southern portions of New Georgia Sound. The local tectonics are driven by the oblique convergence of the present Pacific and Australian plates at about 11 cm/year (4.3 in/year) near eastern Papua New Guinea. This produces one of

5166-411: The melt, and which are therefore the first to form solid crystals. Basalt often contains vesicles ; they are formed when dissolved gases bubble out of the magma as it decompresses during its approach to the surface; the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described as scoria . The term basalt

5248-540: The north-east North Solomon Trough, beyond the Solomon Islands from the Woodlark Basin, had southward subduction of the Pacific plate that finished 10 million years ago. The rifting resumed in the entire basin about 3 million years ago, based on magnetic anomaly data. Then about 1 million years ago, the spreading center in the western basin shifted to the Woodlark Rise. The Woodlark Rise is the northern section of

5330-461: The northern Solomon Islands . The historic Indo-Australian plate and the Pacific plate have had oblique convergence . This is when two tectonic plates come together and collide at an odd angle rather than head-on, as has been the case, from the time the basin was formed. An example of oblique convergence would be if a plate moving north collided with another plate moving east, which results in

5412-410: The oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method of crater counting , are estimated to have erupted only 1.2 billion years ago. From 1972 to 1985, five Venera and two VEGA landers successfully reached the surface of Venus and carried out geochemical measurements using X-ray fluorescence and gamma-ray analysis. These returned results consistent with

5494-475: The rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns. The character of submarine basalt eruptions is largely determined by depth of water, since increased pressure restricts the release of volatile gases and results in effusive eruptions. It has been estimated that at depths greater than 500 metres (1,600 ft), explosive activity associated with basaltic magma

5576-434: The relative lack of shallow earthquake activity except in the middle region of the Trobriand Trough. Historically an extra Trobriand plate moving independently of the Australian and Solomon plates must have existed, and it will still exist if there is present active subduction between it and the Solomon Plate. The Solomon microplate is moving slightly to the north-west at the moment, at 4.5 cm/year (1.8 in/year), so

5658-595: The robotic Russian Luna program , and are represented among the lunar meteorites . Lunar basalts differ from their Earth counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO. They also possess a wide range of titanium concentrations (present in the mineral ilmenite ), ranging from less than 1 wt% TiO 2 , to about 13 wt.%. Traditionally, lunar basalts have been classified according to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti. Nevertheless, global geochemical maps of titanium obtained from

5740-403: The rock at the landing sites being basalts, including both tholeiitic and highly alkaline basalts. The landers are thought to have landed on plains whose radar signature is that of basaltic lava flows. These constitute about 80% of the surface of Venus. Some locations show high reflectivity consistent with unweathered basalt, indicating basaltic volcanism within the last 2.5 million years. Basalt

5822-456: The rocks chemically, with particular emphasis on the total content of alkali metal oxides and silica ( TAS ); in that context, basalt is defined as volcanic rock with a content of between 45% and 52% silica and no more than 5% alkali metal oxides. This places basalt in the B field of the TAS diagram. Such a composition is described as mafic . Basalt is usually dark grey to black in colour, due to

5904-462: The sea it usually forms pillow basalts. However, when ʻaʻā enters the ocean it forms a littoral cone , a small cone-shaped accumulation of tuffaceous debris formed when the blocky ʻaʻā lava enters the water and explodes from built-up steam. The island of Surtsey in the Atlantic Ocean is a basalt volcano which breached the ocean surface in 1963. The initial phase of Surtsey's eruption

5986-527: The spreading center to be uneven across the basin. There are two larger transform faults called respectively the Moresby transform in the middle of the basin and the Simbo transform at the far eastern end of the spreading center. The rate of spreading in the eastern section of the basin at 6.7 cm/year (2.6 in/year) is significantly higher than the rate of spreading of 3.8 cm/year (1.5 in/year) on

6068-670: The term "basalt" to the volcanic black rock beneath the Bishop of Meissen's Stolpen castle , believing it to be the same as the "basaniten" described by Pliny the Elder in AD ;77 in Naturalis Historiae . On Earth, most basalt is formed by decompression melting of the mantle . The high pressure in the upper mantle (due to the weight of the overlying rock ) raises the melting point of mantle rock, so that almost all of

6150-535: The typical enrichment in gold or lead found in vents in back-arc settings (i.e. in contrast to the much older hydrothermal vents in the very western parts of the basin) and is consistent with basalt related hydrothermal fields along mid-oceanic ridges elsewhere. The Woodlark Basin is very young and only started spreading around 3 million years ago, so that there is very little sediment in the basin relative to most oceanic basins . These can have thousands of meters of sediment fill close to continental margins. The basin has

6232-580: The upper mantle is solid. However, mantle rock is ductile (the solid rock slowly deforms under high stress). When tectonic forces cause hot mantle rock to creep upwards, pressure on the ascending rock decreases, and this can lower its melting point enough for the rock to partially melt , producing basaltic magma. Decompression melting can occur in a variety of tectonic settings, including in continental rift zones, at mid-ocean ridges , above geological hotspots , and in back-arc basins . Basalt also forms in subduction zones , where mantle rock rises into

6314-434: The viscosity of water, which is about 1 cP). Basalt is often porphyritic , containing larger crystals ( phenocrysts ) that formed before the extrusion event that brought the magma to the surface, embedded in a finer-grained matrix . These phenocrysts are usually made of augite, olivine , or a calcium-rich plagioclase, which have the highest melting temperatures of any of the minerals that can typically crystallize from

6396-496: The western basin. This is only easily visible in satellite imaging due to the relatively small amount of sediment build up in the basin. Historically seafloor spreading to the west of the Moresby transform did not commence until 4 to 2 million years ago, after the arc volcanism that accompanied uplift of both the D'Entrecasteaux Islands gneissic domes and Suckling -Dayman massif of the Pupuan Peninsula of New Guinea. It

6478-618: The western side of the basin. Satellite imaging helped the visual identification that the spreading rates across the basin are uneven, before full global positioning studies. As the eastern side of the basin has opened up much faster than the western side, with the separation at the Moresby Transform Fault, there is a shallower seafloor to the West, and an axial graben to the East, that is about 500 m (1,600 ft) deeper than

6560-475: The youngest seven to five million year old rocks in metamorphic core complexes and gneissic domes. There are active hydrothermal vents in the eastern basin along the line of the seafloor spreading. A particularly active region was discovered in 2019 and is called the La Scala Vent Field at over 3.3 km (2.1 mi) depth. Geological samples from the dark smoking chimneys showed the lack of

6642-504: Was caused by left-lateral shear in the region induced by a change in the relative motion pole of the Indo-Australian plate and Pacific plate. The basin opened only a few degrees at this time, then stopped for a good period of time. The thickly sedimented troughs south of the Woodlark Basin (e.g. Pocklington Trough and Rennell Trough) are remnants of Paleogene northward subduction of the historic Indo-Australian plate. The distal to

6724-400: Was highly explosive, as the magma was quite fluid, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone. This has subsequently moved to a typical pāhoehoe-type behaviour. Volcanic glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions. Pillow basalt

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