البراكين تكوينها وأنواعها ونشاطها

He knows lot about the science stuff, Professor Dave Explains. Throughout this geology series, we've been describing the Earth as dynamic place where huge plates of continental crust float atop plastic-like mantle, and the forces of wind, water, and gravity create unique sedimentary rocks. But now we're going to explore one of Earth's most dynamic features of all, volcanoes. We've all heard of volcanoes before, as these are fascinating and powerful structures, making them the subject of myth and legend over the millennia. Volcanoes are present on every continent, even Antarctica, and they are responsible for creating over 80% of Earth's surface above and below sea level. Geologists define volcano as any location where lava erupts onto the surface, either as subaqueous eruption on the ocean floor or subaerial eruption on land. The geologists who study volcanoes, who are called volcanologists, measure emitted volcanic gases, ground deformation, and seismic activity in order to predict volcanic behavior and understand the dynamic processes beneath Earth's crust. We talked about how volcanoes form when we were learning about igneous minerals. So let's start with quick review. Volcanoes form above areas where the mantle melts, either due to decompression melting or melting from the addition of water. Most of the active volcanism on Earth occurs at divergent plate boundaries, especially mid-ocean ridges. On land, volcanism occurs along subduction zones and above mantle plumes. Volcanoes form along curved line called volcanic arc at subduction zones, which corresponds to the area where the subducted slab releases its chemically bound water into the mantle. When the overriding plate is oceanic crust, the chain of volcanoes is called an island arc, and when the overriding plate is continental crust, it is called continental arc. Volcanoes that form above mantle plumes are called hotspots, which is the only type of volcano that can form outside of plate boundaries. Finally, volcanoes on continental crust tend to be explosive and erupt felsic lava, while volcanoes on oceanic crust are effusive, erupting mafic lava. Having reviewed volcanism's causes, let's now list the types of volcanoes. There are six main types that the geologists have identified on Earth. Cinder cones, composite volcanoes, shield volcanoes, large igneous provinces, seafloor volcanism, and kimberlite pipes. Before we elaborate on each type, let's talk bit more generally about volcanic structure and activity. Volcanoes are essentially just giant piles of interlayered ash and lava flows that erupt from the volcano's vent or vents. Volcanoes that are primarily made of unconsolidated material like ash are very weak and prone to collapse, whereas more solid volcanoes that are mainly lava flows are more structurally sound. There are two main types of lava, pahoehoe, which is less viscous, higher temperature lava that erupts from basaltic volcanoes, and a'a, which is more viscous, blocky lava that sort of avalanches its way down the volcano, collapsing under its own weight. In addition to lava, volcanoes can erupt solid material, generally called tephra, which can range from ash to large volcanic bombs, which are boulder-sized pieces of material that solidify in the air during eruption. Another important property of volcano is its explosivity. Explosive volcanoes erupt silica-rich felsic lava and have high amounts of dissolved gases, especially water, and effusive volcanoes erupt mafic lava at higher temperatures and have lesser amounts of dissolved gases. Explosive volcanoes tend to form along continental arcs, while effusive volcanoes are most common among mid-ocean ridges and seamounts, where they sit on oceanic crust. Now, back to the types of volcanoes. Scoria cones, sometimes called cinder cones, are the smallest type of volcano, standing between dozen and 1,000 ft tall. They are notorious for their high dissolved gas content and quick-growing, short-lived nature. Eruptions are powered by the pressure of their dissolved gases, which come out of solution during eruption and get frozen into the newly formed rock. This type of volcanic rock is said to be vesicular. Scoria cone eruptions are spattery and explosive due to their high gas content. When volcanoes erupt explosively, their ejected lava rapidly cools as it flies through the air, forming tiny particles of ash and glassy chunks of rocks called cinders. They are called cinders because they resemble ash from coal furnace. Though explosive in nature, scoria cones erupt basaltic lava. The volcano itself, which is mainly made of pieces of cinders piled up at the angle of repose, is more like pile of sediment than rocky mountain, and is therefore easily eroded. The formation of the volcano Parícutin nicely illustrates the rapid growth of cinder cones. On February 20th, 1943, Mexican farmer named Dionisio Pulido was tending to his field when suddenly the ground began to shake and soon swelled 2 upward, forming fissure that spewed sulfury smoke. By nightfall, lava was shooting from the fissure, and 50-m tall volcano had formed by morning, rising to 150 after week. The volcano stopped erupting in 1952, but not until forming scoria cone 424 ft tall and spreading lava over 26 square miles. In addition to destroying Pulido's farm, the volcano leveled the two nearest towns and displaced thousands. Composite or stratovolcanoes usually form at subduction zones, especially continental arcs. When most people imagine volcano in their minds, they are probably imagining stratovolcano. Stratovolcanoes can rise as tall as 8,000 ft with steep slopes up to 36°. The name stratovolcano arises from its structure, which is alternating layers or strata of lava flows and tephra with central vent. Though stratovolcanoes can produce lava flows, the lava they extrude usually just piles up atop the vent, forming giant blob of viscous magma called lava dome. Once the lava dome solidifies, it plugs up the volcano, stopping it from releasing its pressure. This, in combination with the high water content of their lava, leads to tendency for extremely explosive eruptions. Stratovolcano lava is viscous for two reasons. First, it is about 500° lower in temperature when compared with an effusive volcano like Hawaii, and second, it is rich in silica. When silica is component of melt, it polymerizes or forms networks of bonds, which makes it increasingly resistant to flow with increasing silica content. When an explosive eruption occurs, the pile of lava and tephra that is the volcano can be partially or entirely blasted away and incorporated into hot, rapidly moving mass of ash, rock, and gas called pyroclastic flow. After being blasted into the air by the eruption, the pyroclastic material begins to fall back down to Earth, where it flows along the ground and spreads out, reaching speeds of up to 700 km/h with temperatures of up to 1,000° Pyroclastic flows are the deadliest volcanic hazard and obliterate anything in their path. One of the most famous composite volcanoes is Mount St. Helens, which erupted on May 18th, 1980, in the deadliest volcanic event in American history. Instead of blowing its top like most volcanoes, Mount St. Helens exploded from the side, which directed the pyroclastic flow laterally, flattening an area 23 mi wide by 19 mi long. Large trees were snapped at the base of their trunks like twigs. 185 mi of highway were destroyed, and 57 people were killed. Shield volcanoes are some of the largest volcanoes on Earth, and they commonly form by mantle plume activity away from plate boundaries. They almost exclusively extrude runny basaltic lava from small vents and fissures that gradually build up to form volcanic dome with slopes less than 10°. The diameter of typical shield volcanoes can range from 3 to 4 mi with heights up to 2,000 ft. The Hawaiian Islands are some of the best examples of shield volcanoes. Mauna Loa, one of the Earth's most active volcanoes, is the largest, spanning 74 mi from north to south and rising over 28,000 ft above the ocean floor. Large igneous provinces are massive deposits of igneous rocks, where in some cases millions of cubic kilometers of lava have been erupted in very short amount of time, geologically speaking, with the bulk of the magmatism occurring in less than 1 million years in most cases. One type of LIP, called flood basalts or traps, are large igneous plateaus with step-like morphology and are associated with many of Earth's worst mass extinctions. For example, the Siberian Traps, which we discussed in the tutorial regarding Earth's history, are often cited as being the triggering event of the end-Permian mass extinction, which was the worst mass extinction of all time. In addition, another flood basalt called the Deccan Traps began erupting around the time of the end Cretaceous extinction event, which is typically attributed to an asteroid impact. Can this be coincidence or perhaps the asteroid impact somehow triggered this eruption? This is hotly debated topic among geologists. LIPs can form over both oceanic and continental crust and do not correlate with tectonic environments. So, what causes them? Due to the extremely high heat flow required to produce so much magma over such short period of time, it has recently been accepted that mantle plumes are the culprit. Mantle plumes sort of resemble lollipops and have bulbous head of swirling hot mantle that is supplied by smaller tail, which connects down to the core-mantle boundary. So, as the head of plume reaches the crust, it causes short period of intense volcanism that tapers off as its tail nears the surface. The Columbia River Basalt is well-studied flood basalt that began erupting around 17 million years ago in southeastern Oregon. It is composed of over 300 individual lava flows with around 10,000 years between major eruptions. The onset of volcanism was sudden and intense with 85% of the lava erupting over 1 million year period between 15 and 16 million years ago, which slowly tapered off to widely spaced and sporadic eruptions by 13 million years ago. Eruption of this LIP was probably related to the Yellowstone hotspot, which was located beneath southeastern Oregon during the onset of volcanism, though this too is highly debated topic. Seafloor volcanism occurs along mid-ocean ridges where basaltic lava erupts from large linear volcanoes or fissures. Fissure volcanoes also occur along areas that are rifting, such as Iceland and East Africa. The vast majority of the lava that has erupted on Earth over its history has come from fissure volcanoes along mid-ocean ridges. The intensity of seafloor volcanism also has large impact on ocean chemistry due to hydrothermal circulation along the ridge leaching magnesium from seawater. During times of rapid seafloor spreading, so much magnesium is removed from the ocean that different minerals begin to precipitate from seawater, which even affects the ability of marine organisms to build their shells. Most of the material erupted from volcanoes comes from the upper mantle and lower crust, but there are cases where deeper material can get dragged to the surface during an eruption. Kimberlite pipes are deep-seated magma conduits or pipes that draw up ultramafic magma from 150 to 450 km below the surface and exhibit violent explosive eruptions that rapidly carry mantle material to the surface. Kimberlites often contain diamonds and other high-pressure minerals like coesite, polymorph of quartz. They also have an unusual chemistry for ultramafic rocks, being rich in potassium, sodium, phosphorus, and carbon. In addition, kimberlites can contain fragments of rock and large crystals called xenoliths that provide geologists with their only physical samples of the lower asthenosphere. The most recent kimberlite eruption occurred 10,000 years ago in Tanzania. So, that covers the types of volcanoes and their formation. While volcanoes are fascinating and awe-inspiring, they can also be extremely deadly and dangerous. Sulfuric gas emissions, razor-sharp tephra, lung-filling ash, molten lava, pyroclastic flows, and fast-moving mudflows of volcanic ash called lahars cause widespread health effects and property destruction. However, with new developments in seismometers and satellite measurements of ground deformation, volcanologists can monitor the telltale signs of volcanic eruptions. Some of the most important signs of volcanic eruption are gas leaks, deformation or bulging around the volcano, increased seismic activity, and phreatic eruptions where steam is released from volcanic vents. Of course, most of us do not live near active volcanoes, but many people do, and these structures remain an important focus of geological research. Thanks for watching. Subscribe to my channel for more tutorials. Support me on Patreon so can keep making content, and as always, feel free to email me. [email protected]