Geology Final Exam Notes PDF

- Western Interior Seaway (separate occasions\ spanning the Mesozoic Era 250 to 65 million years\ ago) - Coprolites are fossilized animal dung. - Gastroliths are "stomach stones." These\ are actual rocks that some animals\ deliberately swallow, that stay in their\ stomach...

- Western Interior Seaway (separate occasions\ spanning the Mesozoic Era 250 to 65 million years\ ago) - Coprolites are fossilized animal dung. - Gastroliths are "stomach stones." These\ are actual rocks that some animals\ deliberately swallow, that stay in their\ stomachs and help to grind up and\ breakdown food - A cast forms when a body cavity\ fills with sediment, which\ hardens within it. The body is\ later destroyed, but the hard\ cast is left, preserving the shape\ of the body.\ υ A mold is the inverse of a cast.\ In this case a body is buried by\ sediment, which hardens around\ it. The body is later destroyed,\ but the hardened sediment\ leaves a vacant space (mold) in\ the shape of the body.\ υ Flattened molds are usually\ called impressions. - Flattened molds are called impressions - The Ediacara Fauna is around 600 million years old and represents our\ earliest fossils that are believed to be of multicellular animals. - One of the most important fossil deposits in the world is the Burgess\ Shale, which preserves a community of marine animals with tremendous\ diversity that was buried in a shallow sea and are now found in rocks at\ the top of the Canadian Rockies, due to plate tectonics. The entire\ community was buried in a rapid mudslide, preventing new oxygen from\ reaching decomposing microbes. - Pikaia (earliest known Chordate, the\ phylum to which humans belong, all having\ a nerve cord) - Amber is fossilized\ tree sap or resin. It\ represents an ancient\ forest.\ υ Amber preserves our\ best fossil record of\ ancient insects,\ which normally do\ not fossilize well\ (limited hard parts,\ like bones, teeth, and\ har - As mentioned previously,\ petrified wood represents an\ ancient forest buried by ash.\ Although turned to rock, the\ crystallization of silica in the\ plant cells preserves the original\ appearance of the wood,\ including tree rings - Trees growing in the tropics\ don't produce rings as growth is\ year-round. - Additional information from tree rings\ υ Each tree ring represents 1 year. If tree rings get closer\ together for a period of years, this indicates reduced\ growth, which could be a result of a cooling episode or\ a drought.\ υ If tree rings get much further apart, this indicates\ increased growth, indicating very favorable climactic\ conditions during that period, such as adequate rain,\ sunlight, and appropriate temperatures.\ υ The ash layers which preserve the petrified wood can\ be dated to the time of the volcanic eruption, giving us\ an approximate date of the tree's death. - Both the soft tissue of plants,\ like leaves and flowers, and\ the soft tissue of animals, are\ often preserved (when they\ are preserved at all, which is\ rare) by a process called\ carbonization. In this\ process, thin layers of\ residual carbon are\ preserved as the remnants of\ the soft tissue, but they\ maintain the basic shape and\ appearance of the original\ parts. - Stromatolites are mushroom-cloud shaped cemented sediment\ mounds that can reach 1 meter in height. They are formed by thin\ layers of sediment grains that are glued to each other by an\ organic "cement-like" substance.\ υ Fossil stromatolites are abundant and date back 3.5 billion years,\ making these our oldest fossils. - Modern stromatolites (and we assume, ancient fossil stromatolites\ as well) live mostly in shallow salty water. They are especially\ concentrated in hypersaline water which is too salty for other\ organisms that would graze on them - Hydraulic fracturing involves pumping very high-pressure fluid (water, sand,\ chemicals) into the ground to crack rocks blocking oil and gas reserves.\ × The sand is used to keep the cracks open and allow oil/gas to flow towards the\ well.\ × This has made natural gas, in particular, more affordable and accessible, and\ natural gas is more cost efficient, because it is a much cleaner fuel and produces\ more energy compared to the same volume of coal. It also doesn't produce the\ solid coal ash, which is a major toxic waste product derived from coal burning\ and requires proper disposal.\ × "Fracking" does contaminate the water that is used, which is typically disposed of\ by pumping into deep disposal wells, usually on site near the "fracking"\ operation. Occasionally, this water can move through underground fractures and\ contaminate groundwater supplies and lubricate ancient faults in the region and\ generate small or moderate earthquakes.\ × Although "fracking" has problems as described, using natural gas rather than\ coal is a better option (for a fossil fuel) in terms of climate change, because it\ generates less CO2 (50-60%) for the same amount of energy and is cleaner. - Chemicals in fracking fluid include oils that make the fluid slippery and\ better able to squeeze into cracks, acids to dissolve cement between grains,\ detergent to lower surface tension of water so it doesn't stick to grains,\ antifreeze to prevent fluid freezing, and biocides to kill bacteria that can clog\ pores. - Sedimentary Structures include ripples, cross beds, and mud cracks. If\ these can be observed in the sedimentary rock, they can give us further\ information about depositional environments. - Symmetrical ripples\ have a similar slope on\ either side of the ripple\ crest. This usually\ forms with an equal\ flow in 2 directions.\ × That type of flow is\ most indicative on a\ beach, as waves come\ up the beach and then\ retreat. - Asymmetrical ripples do not have\ the same slope on either side of the\ ripple crest, but instead appear to\ be leaning to one side.\ × This primarily represents a flow in\ one direction.\ × This can form in a river channel or\ as a wind deposit on a dune.\ × The disadvantage of ripples is that\ they are small, usually just a few\ centimeters high, and difficult to\ see in cross section. They are best\ seen if the top of the rock is\ exposed, which is less common. (It\ is usually covered by another rock,\ and only the cross section is\ visible.) In rare cases, giant ripples\ can be preserved. - Oxbow lakes are extreme\ meanders in a stream bed\ that get cut off during a rapid\ flow event, such as occurs\ due to heavy rain. The\ stream can create a new\ channel and cut across the\ exaggerated meander,\ leaving an isolated arc that\ now becomes a separate\ lake, since it is not longer\ part of the flow of the stream. - Dunes are wind-deposited and point bars are water-deposited. - n a warm swamp, the bottom waters of the swamp often become\ anoxic as the microbes which cause decomposition of dead organic\ material consume oxygen.\ × Since the water in a swamp is usually stagnant and doesn't circulate,\ oxygen in the surface waters often can't be mixed to the bottom\ waters.\ × With the absence of oxygen, decomposition is hindered, since O2 is\ used by these microbes for energy. (They can strip oxygen off of\ other molecules, but this is a slow process.)\ × With a slower rate of decomposition, organic matter can be buried\ before it is completely decayed.\ × Over millions of years, this ancient swamp vegetation can convert to\ various stages of coal. - Coal is by far the worst of\ these fuels, producing the\ most carbon dioxide. By\ comparison, burning natural\ gas produces 50-60% less\ CO2 compared to burning\ coal - Chert has the\ composition of\ silica (SiO2), usually\ quartz. It can\ represent a deep\ marine deposit of\ microscopic\ organisms called\ diatoms, which\ make their shells of\ silica. - Chert can form\ nodules within a\ previously deposited\ limestone formation,\ as microscopic\ quartz crystals\ slowly replace calcite\ crystals.\ υ This happens\ without the aid of\ organisms. - Agate is a form of\ chert which\ forms in\ concentric rings\ within a hollow\ space in the rock. - Chalk is a type of\ limestone made of\ microscopic shells,\ primarily\ coccolithophorids.\ It usually\ represents a deep\ marine deposit. - Coquina represents\ broken pieces of larger\ CaCO3 shells, visible\ without the aid of a\ microscope.\ υ This also represents a\ marine deposit, but\ closer to shore, since\ most larger organisms\ with CaCO3 shells live\ closer to land. - Travertine is a type of limestone that does NOT represent shells. It\ is not formed by organisms. Travertine precipitates when the water\ is supersaturated with Calcium and Carbonate ions. This can\ happen inside a limestone cave.\ υ After the cave forms due to dissolution of the original limestone by\ acidic groundwater and a subsequent drop in the water table, water\ dripping from the roof of the cave can slowly form stalactites, while\ water droplets landing on the floor of the cave can build stalagmites.\ υ When the stalagmites and stalactites meet, they can form a column. - Tufa is similar to\ travertine in that it is not\ formed by organisms but\ instead precipitates when\ water is supersaturated\ with Calcium and\ Carbonate ions, but tufa\ forms in hot springs or\ certain lakes.\ υ Mono Lake in California\ is famous for its tufa\ deposits, which have\ been exposed as the lake\ has been drained to\ supply Los Angeles with\ freshwater. - Sedimentary breccia is\ composed of angular\ fragments of all different\ sizes (very poorly sorted)\ but with mostly sharp\ edges.\ υ The angular fragments\ represent a short-distance\ of transport before\ deposition, since there was\ not enough time for\ extensive erosion to\ smooth away the sharp\ edges.\ υ Sedimentary breccia\ usually represents a mass\ wasting event, such as a\ landslide - In a conglomerate (like a\ breccia) there are fragments\ of all different sizes, so it is\ very poorly sorted. However,\ in a conglomerate, the\ fragments are mostly\ rounded with smooth edges.\ This represents long\ transport before deposition.\ υ Conglomerates can form in\ very high energy rivers,\ beaches, or as glacial\ deposits. If a glacial deposit,\ they are more likely to have a\ significant matrix of finer\ mud between the larger\ fragments (this is sometimes\ called diamictite). - New England is famous for its stone walls because farmers in the last 2 centuries\ needed to clear rocks left by the glaciers from their fields, so they dug them out and\ used them as property boundaries.\ υ Since most of these farms were abandoned by the early 1900's, many of the stone\ walls are now deep in secondary growth forests, that returned after the land was no\ longer farmed. - Six million years ago, the Mediterranean Sea completely\ evaporated over the course of about 600,000 years, when the\ narrow Strait of Gibraltar, which allows the Mediterranean Sea\ access to the Atlantic Ocean, closed off.\ υ The evaporation of the entire Mediterranean Sea created 1500\ meters of salt and lowered global salinity of the ocean by 6%.\ υ When water returned to the Mediterranean, it dissolved some of\ the surface salt on the seafloor, but the layers below were\ protected, and once the sea reached saturation with Na+ and Cl-\ ions, no more salt dissolved. - In hydrolysis, water\ reacts with minerals in\ rocks to convert them\ into new minerals. Some\ elements may be\ removed in solution, but\ not the entire mineral.\ Feldspar is very\ susceptible to chemical\ weathering and will\ usually convert to a clay\ mineral called kaolinite. - Oxidation causes iron-\ bearing minerals such\ as pyrite to essentially\ react with oxygen\ (usually in water) to\ form iron oxide\ minerals. This is what\ happens when "rust"\ forms. - Chemical weathering is dominant in warmer,\ wetter climates.\ Physical weathering is dominant in colder, drier\ climates.\ Both types of weathering can occur in any\ climate, however.\ The presence or absence of certain minerals\ can suggest climate at the time the parent rock\ was weathered to sediment. - For example, quartz and feldspar are the two\ most common minerals in the continental\ crust. Quartz is very resistant to chemical\ weathering, while feldspar is very susceptible\ to chemical weathering.\ A clastic sedimentary rock which contains a\ lot of feldspar formed from rocks which did\ not undergo a lot of chemical weathering, and\ the climate may have been colder/drier when\ the parent rock was weathered.\ A clastic sedimentary rock which is mostly\ quartz formed from rocks which likely\ underwent a lot of chemical weathering, and\ the climate may have been warmer/wetter\ when the parent rock was weathered - Clastic sedimentary rocks are\ primarily classified by the size\ of the fragments, the shape of\ the fragments, and the sorting\ of the fragments.\ υ Poorly sorted sediment/rock\ has fragments of all different\ sizes.\ υ Very well sorted sediment/rock\ has fragments that are all\ about the same size. - The shape of the fragments is a function of the extent of transport.\ Sediment which has not been transported during erosion for a very long\ distance will still have angles and sharp edges, while sediment which has\ been carried long distances before deposition will have smooth, rounded\ surfaces, as angles and edges were worn down. - Sandstone is usually very\ quartz-rich, indicating\ extensive chemical\ weathering of the parent rock.\ If it is felspar-rich, it indicates\ very little chemical\ weathering.\ - Sedimentary rocks are the rocks exposed at the\ surface over 75% of the Earth.\ This is not the case in places like New England, where\ most of the sedimentary rock was removed by glaciers\ during the most recent glacial stage of the Ice Ages,\ which peaked around 18,000 years ago.\ Glaciers removed the softer sedimentary rock layers\ and left the igneous and metamorphic rock layers\ beneath. - We use sedimentary rocks to provide information about past\ environments in the locations where the rocks are now found.\ These environments represent the environment which was\ present at the time that the sediment (which later formed the\ rock) was deposited.\ This often bears little resemblance to the environment\ presently in that locations, since over millions of years (the\ time it takes to form sedimentary rocks) environments change\ due to climate and sea level changes, plate tectonic movements\ across different latitudes, and elevation changes due to uplift\ and erosion. - Clastic sedimentary rocks are made from fragments of pre-\ existing rocks (any type) that have been weathered, eroded by\ wind, water, or ice, and deposited.\ Weathering can be physical or chemical.\ In physical weathering, the rock is broken into smaller pieces\ without a change in mineral composition.\ In chemical weathering, a change in composition takes place.\ Some minerals are removed completely, while others are\ changed into new minerals. Which type of weathering is\ dominant is often a reflection of the local climate when the\ original parent rock was weathered. - In frost wedging,\ water gets into\ cracks in the rock,\ and freezes. When\ water freezes to ice it\ expands by 9%. This\ can force the cracks\ wider until the rock\ breaks. - Abrasion is usually\ accompanied by\ erosion (which\ moves the sediment\ to a new location) but\ can be sufficient to\ cause the initial\ weathering of the\ parent rock,\ especially if done by\ glaciers. Abrasion is\ the scraping away of\ pieces from the\ original parent rock. - Exfoliation occurs\ when rocks expand\ as weight was\ removed by erosion,\ causing the rocks to\ crack and break off\ like the layers of an\ onion. - Half Dome in Yosemite is an exfoliation\ dome - Thermal expansion (and\ contraction) of the rock\ itself can occur when\ there is extreme\ temperature changes\ between night and day.\ This can weaken the\ rock enough to break.\ This is common in a desert - Near coastlines or in\ arid climates with\ salty groundwater,\ salt water can\ precipitate salt\ crystals within pore\ spaces in rock which\ can push apart\ grains and weaken - In dissolution, entire minerals,\ such as calcite or halite, can be\ dissolved and removed in\ solution, which may be\ precipitated in a new location - We are much better at predicting volcanic eruptions\ compared to earthquakes, which we cannot\ successfully predict at all.\ Volcanoes are generally consistent as to the type of\ eruption that we might expect, because we know the\ rock type that has melted to provide the magma. So\ we know which volcanoes have the highest risk of\ being explosive and which have a lower risk.\ Another pattern relates to the timing between\ eruptions. Usually, the greater the time between\ major eruptions, the larger the eruption will be.\ Relatively frequent eruptions do not allow a lot of\ time for new magma and gases to accumulate in the\ magma chamber. - Before a major eruption,\ earthquakes become more\ frequent and shallower in depth\ as magma approaches the\ surface.\ Note that the presence of\ earthquakes alone is not an\ indicator of an imminent volcanic\ eruption. There have been\ earthquakes under the\ Yellowstone region regularly\ since scientists started\ monitoring the area. - The largest single eruption that we have determined based on deposits\ actually came from a subduction zone volcano in Indonesia. (This is\ unusual, because subduction zone volcanoes, while typically explosive\ and deadly, are not normally the size of an eruption of a hot spot under\ a continent.)\ However, Mt. Toba in Indonesia erupted 73,000 years ago and was\ larger than any of the 3 known Yellowstone eruptions.\ It is believed by some that Mt. Toba caused a global winter of 6 to 10\ years, followed by a 1000-year cooling episode.\ It is believed that this caused a global ecological disaster, which\ included a significant decrease in the number of large mammals and\ primitive humans. However, this is hotly debated. - A caldera is a large crater\ which forms after a massive\ volcano has erupted and\ then collapsed in on itself\ after its magma chamber is\ emptied.\ Over time calderas will often\ be filled with rainwater and\ eventually sediment. - Italy has several active volcanoes\ and many dormant volcanoes.\ The 4 most active volcanoes are\ Vesuvius (Vesuvio on this map)\ and Phlegraean Fields (Campi\ Flegrei on map) both on the\ mainland, Etna on the island of\ Sicily, and Stromboli on the Aeolian\ Islands.\ Vesuvius/Phlegraean Fields are the\ greatest risks, as they are near one\ of the most densely populated\ volcanic regions in the world, with\ 3 million people potentially affected\ and 600,000 in the life-threatening\ danger zone. They are both close\ to Naples. - Hawaii is the most\ volcanically active\ region of the U.S. but it\ experiences primarily\ lava flows, and is\ seldom life-\ threatening.\ As mentioned\ previously, Mauna Loa,\ Kilauea (both on the\ Big Island) and Loihi\ (offshore) are the\ active volcanoes. - U.S. RISK ZONES: ALASKA\ Alaska has the most active\ explosive volcanoes in the\ U.S. but generally the region\ is sparsely populated.\ Most of the active volcanoes\ are on the Aleutian Islands.\ The largest eruption of the\ 20th century occurred in\ Alaska in 1912 at Novarupta.\ Volcanic eruptions in Alaska\ are more often a problem for\ planes passing over the area,\ that fly into an ash cloud\ accidentally. Ash can get\ into the engines and cause\ disruption. - The Cascade subduction zone\ is off of the coast of\ Washington, Oregon, and\ northern California, with active\ explosive volcanoes in all 3\ states.\ As is clear from the graph on\ the right, Mt. St. Helens is the\ most active volcano in the\ Cascades, having last erupted\ in 1980. Because of its\ frequency in eruptions,\ however, the individual\ eruptions, while explosive, are\ not large, on a global scale. - The most dangerous\ volcano in the Cascades is\ usually thought to be Mt.\ Rainier.\ Mt. Rainier is much larger\ than other Cascade\ volcanoes and has more\ glacial ice on its summit\ than all other Cascade\ volcanoes combined.\ The last major eruption\ was in 1894. - The Long Valley Caldera (which includes Mammoth Mountain and Mammoth\ Lakes) is in eastern California.\ This region is south and east of the Cascades, and significantly south and west\ of Yellowstone. The source of the volcanism is uncertain. While it is\ geographically closer to the Cascade subduction zone volcanoes, its last major\ eruption, 760,000 years ago, was more similar to Yellowstone in scale. (See\ previous slide for map and extent of ash deposits.) Its last eruption ejected 600\ cubic kilometers of material, which would put it between the 1.3 million and\ 630,000 year Yellowstone eruptions.\ It has had 20 smaller eruptions in the last 5000 years. All but 3 were explosive.\ Future eruptions are predicted to be small to moderate, based upon a seismic\ survey of the region beneath the caldera, which appears to be mostly smaller\ separate magma chambers at present. - In the 1980's, activity increased in the\ region around Mammoth Mountain.\ There was an increase in seismic\ activity, gas emissions from the ground\ (which killed stands of trees) and a\ slight uplift in the ground.\ Volcanologists raised the alert level,\ but the surrounding communities\ reacted with hostility. They were\ concerned that such an increase,\ although it did not indicate an\ imminent eruption, would negatively\ affect tourism in the region, since\ Mammoth Mountain is a popular ski\ destination. - In January of 2022, a volcano in\ the Tonga island chain\ (subduction zone) exploded with\ much of the explosion occurring\ underwater (similar to Krakatoa).\ The eruption was heard as far\ away as Fairbanks, Alaska, and\ tsunamis impacted coastlines\ around the Pacific, although there\ were only 5 fatalities (2 in Peru,\ across the ocean). It is believed\ that a landslide underwater\ allowed water to suddenly gain\ access to the magma chamber,\ triggering the explosion. - The second way that volcanoes can\ generate a tsunami does not require an\ explosive eruption.\ In this scenario, a very large volcanic\ island experiences a massive\ landslide, during which a significant\ portion of the island breaks off and\ slides into the sea.\ There is geologic evidence that this\ has occurred in the Canary Islands, off\ of Northwest Africa, which are hot spot\ volcanoes.\ It has been suggested that tsunamis\ generated from here could potentially\ reach the U.S. east coast. - Large explosive\ eruptions extrude\ enough ash and gases\ into the atmosphere to\ cause short-term global\ cooling episodes by\ reflecting sunlight away\ from the Earth's\ surface.\ In 1991, Mt. Pinatubo\ erupted in the\ Philippines. It caused a\ drop in global\ temperatures for about\ 2 years - This eruption in 1991 was the\ second largest volcanic eruption in\ the 20th century (after a remote\ Alaskan volcano in 1912).\ The eruption was predicted\ accurately and 20,000 people were\ safely evacuated.\ However, 800 people died outside of\ the evacuation zone when ash\ (weighted down with water, since the\ eruption occurred during a typhoon)\ collapsed roofs and killed the\ occupants - More significantly, a\ much larger eruption in\ Indonesia in 1815 at Mt.\ Tambora, which was the\ largest eruption in\ recorded human history,\ caused such an extreme\ drop in temperature that\ it caused a "year without\ a summer" in 1816 and\ cooling for several years\ afterwards.\ Although death tolls vary\ widely, it is estimated\ that around 90,000 died\ from famine and disease\ as a result of this\ cooling. Crops as far as\ North America were\ negatively impacted. - Lahars are volcanic mudflows. They are a mixture of water, ash, and\ rocks.\ The lahar moves several tens of kilometers per hour and has the\ consistency of liquid concrete.\ The water can come from either rainfall during or after the eruption, or\ melted snow and ice on the volcano.\ Even volcanoes in tropical regions can have permanent glaciers on\ their summits if they are high enough in elevation.\ Lahars can be created even weeks after an eruption when rain falls on\ ash deposits on steep slopes of a volcano. - n 1985, Nevado del Ruiz in\ Colombia erupted in the middle\ of the night and buried the\ entire town of Amero 60\ kilometers away in 5 meters of\ mud.\ Approximately 25,000 people\ were killed.\ While lahars are not as hot, nor\ travel as fast as pyroclastic\ flows, they can reach much\ greater distances, as so are a\ threat much further away from\ the volcano compared to a\ pyroclastic flow. - Both continent/ocean and ocean/ocean subduction zones are\ usually explosive - Part of the reason for the high\ silica content at continent/ocean\ subduction zones is that the\ rising magma melts some of the\ continental crust that it passes\ through before it reaches the\ surface. However, this doesn't\ explain explosivity at\ ocean/ocean subduction zones.\ That intermediate silica content is\ explained in two ways:\ Some sediment that is eroded\ from the continent covers the\ ocean crust, and some of this is\ subducted and melted.\ The subducted ocean crust is\ only partially melted, and the\ silica rich minerals melt first,\ increasing the silica content of\ the magma. - Lava flows can be associated\ with both explosive and non-\ explosive volcanic eruptions.\ They can be extensive and\ cause a lot of property\ damage but are normally not\ life-threatening.\ Once they extrude from the\ ground, they start to cool\ and slow down. People\ usually just move out of the\ way. - A pyroclastic flow is a hot cloud of gases and ash\ which is denser than air and rushes down the slopes\ of a volcano at speeds up to 600 kilometers per hour.\ A pyroclastic flow is extremely hot and incinerates\ anything in its path. In 1902, a pyroclastic flow\ destroyed the city of St. Pierre on the island of\ Martinique in the Caribbean. It was caused by the\ eruption of Mt. Pelee next to the city.\ The pyroclastic flow passed through the entire city in\ about 3 minutes, and killed all but 2 people, one in the\ town's jail, which was underground. He was found 4\ days after the eruption.\ The event was witnessed by boats offshore (St. Pierre\ was a coastal town.)\ Approximately 29,000 people were killed, the highest\ death toll from a volcanic eruption in the 20th century. - Ash falls can bury entire towns\ that are near the volcano and\ suffocate all of the residents.\ Further away from the volcano,\ ash can pile up on roofs and\ cause them to collapse, killing\ the residents inside.\ When Mt. Vesuvius erupted in 79\ AD, it completely buried the\ town of Pompeii under 6 meters\ of ash, and also covered 3 other\ towns.\ While about 1500 bodies have\ been excavated, up to 16,000\ people may have been killed\ (2000 in Pompeii) - At a continental rift, the hot spot is able to easily reach the surface through\ the many fissures in the continental crust. The magma does not melt much of\ the continent as it moves to the surface, so it remains low in silica and non-\ explosive.\ Hot spots under continental rifts produce enormous lava flows called Flood\ Basalts because they extend over massive areas.\ There are no active examples of this type of volcanism today, but the U.S.,\ India, Siberia, Brazil, and South Africa have examples of past flood basalts. - The Columbia River Flood Basalts cover\ significant parts of Washington, Oregon,\ and Idaho.\ They are not related to the volcanism\ associated with the Cascade Subduction\ zone.\ The Flood Basalts are 2 to 3 kilometers thick\ in some places.\ The majority of the eruptions occurred\ between 14 to 17 million years ago, but\ lasted until around 6 million years ago. - Under an ocean ridge, the hot spot provides a second source of\ magma in addition to shallow asthenosphere, which melts due to a\ reduction in pressure as the plates move apart.\ Because of this additional source of magma, these sections of the\ ocean ridge are able to extend above the surface of the water as\ islands, or clusters of islands.\ In general, they are not very explosive although they can produce a lot\ of ash in addition to lava. - Iceland is one of the\ most volcanically active\ regions in the world,\ and the largest single\ island that is part of the\ ocean ridge system. - We can predict the silica content of the magma if we are confident\ which rocks primarily melted to form the magma.\ Rocks in the mantle have the lowest silica (38-45%).\ Rocks from the ocean crust have low silica (45-52%).\ Rocks from ocean/ocean subduction zone volcanoes have intermediate\ levels of silica (52-66%).\ Rocks of the continental crust have the highest silica (66-76%).\ By knowing which rocks/minerals melted to form the magma, we can\ predict whether or not a volcanic eruption is expected to be explosive. - Ninety-five percent of all volcanic activity occurs on ocean ridges.\ These are generally fissure eruptions, with lava extruding from long fracture\ zones, rather than forming individual cones (although isolated cones on the\ flanks of the ridge can occur).\ These eruptions are usually not very explosive (although there are\ exceptions). - Hot spots are columns of magma that come from very deep in the\ mesosphere (lower mantle).\ Hot spots are relatively stationary. It is the plate above that moves.\ Hot spots can occur beneath oceanic or continental lithosphere and have\ very different eruption styles in each setting.\ Hot spots can also occasionally occur under divergent boundaries.\ Because the hot spot eruptions are often a reflection of the rock above them,\ we break hot spots into 4 subgroups. - If an oceanic tectonic plate moves across a hot spot, the column of magma\ forces its way easily through thin ocean crust to create a chain of volcanoes.\ These eruptions are extensive, but usually just low-viscosity lava flows.\ Only one region of the chain will have active volcanoes (the region over the\ hot spot). This may allow for more than one active volcano, but they will be\ close together.\ Older volcanoes which have moved off of the hot spot have lost their source\ of magma. They will become inactive and eventually erode and sink beneath\ the surface to form a seamount. - Mauna Loa, which is the\ largest volcano on Earth\ × Kilauea volcano\ × Loihi volcano, which is\ the newest Hawaiian\ volcano and has not yet\ reached the surface. It is\ offshore to the southeast\ of the Big Island