Volcanic rocks are usually fine-grained or aphanitic to glass in texture. Photomicrograph of a volcanic lithic fragment ( sand grain) upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter. It has been estimated that volcanic rocks cover about 8% of the Earth's current land surface. On land, they are very common at plate boundaries and in flood basalt provinces. Volcanic rocks are among the most common rock types on Earth's surface, particularly in the oceans. Volcanic rocks and sediment that form from magma erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. Ignimbrite, a volcanic rock deposited by pyroclastic flows
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