19) Eruptions of rhyolite lava tend to be more explosive than basaltic and andesitic lavas because _____. Pillow basalts attest to the oceanic origins of the rock layers, as they formed from lava flows that cooled on the ocean floor. In general, the more felsic the composition (richer in silica - resulting in an Intermediate composition), the larger the pillows, due to the increase in viscosity of the erupting lava. Pillow lavas are volumetrically the most abundant type because they are erupted at mid-ocean ridges and because they make up the submarine portion of seamounts and large … Ans: D Section: 4-4: Magma Differentiation 53. pillow basalt 10. The trace elements in basaltic rocks which most authors have unanimously found t o be among the most mobile during seawater weathering are Rb, B and Cs. Chunks of rocks that break off from country rocks and fall into magmas are called A) wall rocks. A) rhyolite lavas are less viscous B) rhyolite lavas contain more gases C) rhyolite lavas form larger volcanoes D) rhyolite lavas erupt at high temperature E) all of the above The great continental flood-basalt eruptions of the geological past are the largest eruptions of lava on Earth, with known volumes of individual lava flows exceeding 2000 cubic kilometres. Primary oceanic crust produced at spreading centers on Earth today is ∼6–7 km thick (), has an average bulk MgO content of 10–13 wt.%, and forms from small mantle melt fractions of 8–10% (Herzberg et al., 2010).The uppermost portion of the mafic crust (i.e. Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. Contact with the water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow. Pillow basalt is a type of Basaltin whichlava erupts underwater or flows into the sea. _____ composition lava is erupted at ocean ridges, whereas _____ composition lava is erupted on continents. Till, in geology, unsorted material deposited directly by glacial ice and showing no stratification.Till is sometimes called boulder clay because it is composed of clay, boulders of intermediate sizes, or a mixture of these. D) andesitic. MORB) is typically comprised of pillow basalts with an MgO content of ∼7 wt.% (White and Klein, 2014). Flood Basalts. This "pillow" texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Composition. Pillow lavas are commonly of basaltic composition, although pillows formed of komatiite, picrite, boninite, basaltic andesite, andesite, dacite or even rhyolite are known. There are three types of basalt lava flows: pillow, pahoehoe, and a'a.. Pillow lava. For comparison, the ongoing eruption of Kilauea volcano on Hawaii has produced just 1.5 cubic kilometres in 16 years! Sandstone and shale layers at Kenai Fjords are commonly metamorphosed and were so deformed during subduction and uplift that they are vertical in places. B) basaltic. This magma oozes out onto the sea floor to form pillow basalts (Figure 18.1), breccias (fragmented basaltic rock), and flows, interbedded in some cases with limestone or chert. When equal amounts of a basaltic and granitic magma mix, the resulting magma is A) mafic. The resulting blobs of lava are known as pillows, and they tend to form piles around a sea-floor lava vent (Figure 4.21). The rock fragments are usually angular and sharp rather than rounded, because they are deposited from the ice and have undergone little water transport. What is Basalt? B) … C) felsic. 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