Presentation on theme: "Chp 4: Volcanism Volcanism – the process whereby lava and its contained gasses, and pyroclastic materials are expelled upon the earth’s surface, or into."— Presentation transcript:
Chp 4: Volcanism Volcanism – the process whereby lava and its contained gasses, and pyroclastic materials are expelled upon the earth’s surface, or into the earth’s atmosphere. Volcano – a conical mountain formed around a vent where lava, pyroclastic materials, and gasses have erupted. One purpose of volcanoes is to help rid the interior of the earth of excess heat buildup. Volcanoes and Religion – Native Americans of the northwest tell of a titanic battle between the volcano gods Skel and Llao accounting for the huge volcanic eruptions that occurred ca. 6600 BP. In Hawaii, Pele is the goddess residing in the crater of Kilauea responsible for the eruptions and earthquakes there. Ancient Greeks believed that the god Pluto or Vulcan was responsible for eruptions there.
Chp 4: Volcanism Active Volcanoes – There are approximately 550 active volcanoes on earth today. (i.e. Mt. St. Helens, Mauna Loa, Kilauea, etc.) At any one time there are about 12 volcanoes erupting somewhere on earth. Dormant Volcanoes – There are numerous volcanoes that have erupted in the recent geologic past and probably will erupt again in the future. Mt. Vesuvius in Italy, Mt Pinatubo in the Philippines Extinct Volcanoes – These are volcanoes that have erupted in historic times but show no sign of erupting again.
Mount Vesuvius, Italy looking SE Fig. 4-CO, p.84
Fig. 4-1, p.88 Pompeii, Italy: -excavated city streets -casts of remains of people caught in eruption
Chp 4: Volcanism Volcanic Gasses – The gasses released from the magma as it moves upward. In sialic magma, expansion if restricted due to the high viscosity (high silica content) and gas pressure increases greatly causing explosions releasing ash and other pyroclastics. In mafic magmas, expansion occurs due to the low viscosity (low silica content) allowing gasses to expand and escape easily…creating a quieter eruption. Composition of volcanic gasses – 50 – 80% of all volcanic gasses are water vapor, with lesser amounts of CO2, N2, and sulfur gasses – sulfur dioxides and hydrogen sulfides. Very small amounts of CO, H, and Cl are released. The Blue Haze Famine – Iceland, 1783 – gasses (probably sulfur dioxide) escaped from the Laki Fissure, causing 75% of livestock to die. The gas caused the overall temperature to drop causing crop failures causing 25% of the human population to die. Cameroon, Africa, 1986 – A cloud of volcanic CO2 was emitted from under Lake Nyos (that sits atop a volcano) killing by asphyxiation ALL ANIMAL LIFE for miles around, including 1746 humans.
Volcanic gases can be very dangerous Fig. 4-3c, p.91
Chp 4: Volcanism Lava Flows I.Sialic (high viscosity flows) - tend to be thicker, “lobe-shaped flows with distinct margins. II.Mafic (low viscosity flows) – tend to be comparatively thinner, fluid flows that are widespread. III. Types of Lava Flows – (Hawaiian Terms) 1. Aa – a flow characterized by a surface consisting of rough, angular, jagged blocks and fragments. This is due to a higher silica content than: 2. Pahoehoe – a flow characterized by a smooth, ropy surface, almost like taffy. This is due to relatively lower silica content than Aa. Speed of Lava Flows Not very quick…the fastest low viscosity in Hawaii ever measured had a speed of 9.5 kilometers/hour. Faster speeds occur whenever the lava flow is insulated on all sides forming a lava tube or conduit. After the eruption and the lava tube drains, sometimes the roof of the tube collapses forming a skylight into the tube.
Fig. 4-5a, p.94 Pahoehoe lava- note the smooth surface
Fig. 4-5b, p.94 Pahoehoe lava- note the smooth surface, almost taffy-like
Chp 4: Volcanism IV.Pressure Ridges and Spatter Cones - Pressure Ridges – As the surface of a flow begins to solidify, pressure from the flow causes the surface to buckle, forming ripples of sorts called pressure ridges. Spatter Cones – Gasses escaping from the flow hurl globs of lava into the air. These globs fall back to the surface and stick together forming small, steep-sided spatter cones. These may rise several meters above the flow. V. Columnar Jointing - Common in flows that are relatively “thick” – several tens of meters to 100 meters thick. They form because of differential cooling of the flow – the outer surfaces “freeze” while the inside is still molten. This results in the vertical “splitting” of the flow into roughly pentagonal prism-like columns – hence “columnar jointing”. Examples include: Devil’s Postpile National Monument, California, Devil’s Tower, Wyoming.
Fig. 4-7a, p.95 ‘Columnar jointing’: Note the columns of apparently vertically stacked igneous rocks- hence the name…. As the lava cooled, vertical fractures formed and intersected to form 5 and 6 sided columns. 60 million year old lava flow at Giant’s Causeway Ireland
Fig. 4-7b, p.95 Columnar jointing’: Note the columns of apparently vertically stacked igneous rocks- hence the name….this time with a view from top. Staffa Is in Scotland- 85 km from previous picture, but same age basalts!!
Fig. 4-9, p.96 VI. Pillow Lava – Bulbous masses of basalt that resemble “pillows” result from mafic magma cooling under water. This is VERY common at the submarine spreading centers such as at the MOR (mid-oceanic ridge) Anywhere pillow lava is found on the surface of the earth indicates that there was mafic, basaltic lava that cooled underwater.
Chp 4: Volcanism VIII. Distinguishing Flows from Sills in Cross-section - Chilled Margin – The solidified outer edge of a lava flow or an intrusive body such as a sill. As a lava flow flows across the surface of the earth, the first to solidify is the “margin that touches the relatively cold ground. In a sill, the “chilled margin” encircles the intrusion since it “froze” at all contacts with the country rock. Gasses that are released in a flow tend to form “bubbles called “vesicles”. In a flow, these vesicles accumulate at the top of the solidification much like the foam of a beer poured into a glass. Vesicles usually are not as abundant in a sill since it cooled intrusively. Altered Country rock – Upon coming into contact with the surface of the ground, a flow usually alters the country rock on the bottom surface only, whereas a sill alters the country rock on all edges.
Chp 4: Volcanism Flood Basalts – Occur as high volume, low viscosity, mafic flows over a broad, flat area, many times resulting in “flood basalts’ up to 100 meters thick or greater. Columnar jointing is common. Lava “Plateaus” may be formed.
Fig. 4-18, p.110 Flood Basalts – Occur as high volume, low viscosity, mafic flows over a broad, flat area, many times resulting in “flood basalts’ up to 100 meters thick or greater. Columnar jointing is common. Lava “Plateaus” may be formed. Columbia R basalts Mt Pinatubo pyroclastic flows
Fig. 4-8, p.96 a.Lava fills a valley b.Adjacent layers erode much easier than lava, leaving lava flow now as a ridge. c. Black rock at top of this ridge is basalt flow that once filled valley.
Fig. 4-19, p.111 X. Anatomy of a Volcano - Supplying the volcano is a magma chamber deep beneath the surface vent where lava begins to pour forth.. Magma reaches the volcano via a volcanic pipe or conduit. The depression at the top of the volcano is the crater or main vent. Along the side, there may be smaller conduits forming side vents or side cones. Long slits may occur along the sides of the cone forming fissures or fissure eruptions.
Chp 4: Volcanism XI.Types of Volcanoes – 1. Shield Volcano - Characteristics: Usually mafic resulting in forming basaltic rocks Low viscosity of the magma due to low silica content Low slope angle of the cone – usually 6 – 12deg Many fissure eruptions, lava tubes, conduits, etc. Typical of the Hawaiian Islands or MOR volcanoes. 2. Cinder Cone - Characteristics: Quickly formed (sometimes overnight) Symmetrical cone with relatively steep sides Usually less than 300m high Composed entirely of pyroclastic materials (i.e. cinders and ash) Generally mafic to intermediate in composition Erodes away easily and quickly
Fig. 4-12, p.99 Shield volcano: Consist of many layers of thin basalt flows
Conical crater common at top of cinder cones Fig. 4-13b, p.102
Chp 4: Volcanism 3. Stratovolcano or Composite Cone - Characteristics: Formed by an alternating series of lava flows followed by pyroclastic flows. Usually intermediate to sialic magmas. Very explosive due to their high silica content and viscosity. i.e. Mt. St. Helens Many times after going extinct, the cone erodes away leaving the volcanic neck or volcanic pipe exposed. This may contain the rock Kimberlite associated with the formation of diamonds and other gems.
Fig. 4-16b, p.105 New magma uplift-lava dome- in center of volcano
Chp 4: Volcanism XII. Violent Magmas – Ash Flow Tuffs and Calderas Characteristics of Violent Magmas: a. “Dry”, granitic magmas b. less than 10% H20 Will not solidify before reaching the surface Volatiles separate from magma and form a frothy mixture of super hot gasses and magma Ensuing eruption is extremely violent “Nuee ardente” super hot gas and ash flow clouds are produced Many pyroclastics are produced – pumice, tuffaceous rocks, etc. Calderas are formed. This occurs after the eruption, whereby the top of the volcano falls back into itself creating a circular depression. These create very good mining districts for precious metals. i.e. Creede, Colorado.
Fig. 4-10b, p.94 Pyroclastic material blown into atmosphere at Pinatubo
Fig. 4-15a, p.105 Ash flows that buried village
Fig. 4-11, p.98 Caldera formation: explosion followed by collapse of center (Crater Lake, Ore)
Chp 4: Volcanism XIII. Hot Spots - Characteristics: Bodies of magma that have risen near the surface creating a localized zone of melting below the lithosphere. This results in surface eruptions over long periods of time. I.e. Hawaiian Islands, Yellowstone National Park The Hawaiian Island chain is composed of islands dating from: Hawaii (The Big Island) – 0.7 MYA to today Maui – 0.8 – 1.3 MYA Molokai – 1.3 – 1.8 MYA Oahu – 2.3 – 3.3 MYA Kauai – 3.8 – 5.6 MYA These islands are also listed as to size: Hawaii the largest, and Kauai the smallest. Hawaii is over the hot spot and is active. The other islands have either dormant or extinct volcanoes. The Pacific Plate has been slowly moving to the northwest where 5.6 MYA Kauai was over the hotspot and was active while the other islands had not even formed yet. There is a new volcanic seamount (an underwater volcanic mountain) called Loihi is forming and is 940 meters below the surface. It will someday take the place over the hotspot and become the largest island, while Hawaii erodes smaller as the other islands have done
Fig. 4-22, p.114 Hawaiian Islands: line of islands due to movement over Hotspot Volcanoes typically occur along Convergent Margins-where 2 plates collide-composite volcanoes are very common
Volcanoes can be characterized by their explosiveness Fig. 4-20, p.112
Fig. 4-21, p.113 Volcanology Today: monitor changes in shape of volcanoes, in order to determine likelihood of any eruptions.
Chp 4: Volcanism- Summary Volcanism – the process whereby lava and its contained gasses, and pyroclastic materials are expelled upon the earth’s surface, or into the earth’s atmosphere. Volcano – a conical mountain formed around a vent where lave, pyroclastic materials, and gasses have erupted. One purpose of volcanoes is to help rid the interior of the earth of excess heat buildup. Different types of Lava flows: 1. Aa: rough, blocky 2. Pahoehoe: smooth 3. Pressure ridges: buckling of lava flows into ridges (hard) 4. Columnar jointing: vertical stacking of columns 5. Pillow lavas: pillow like shape; submarine only
Chp 4: Volcanism- Summary 1.Chilled margins: edges of flow that cooled first 2.Vesicles: escaping gases as flow cools, large holes in rock Flood Basalts: high volume flows over surface of Earth, stack into’ Layers that can be 100m or more thick (India) 3 types of Volcanoes: -Cinder Cones: high relief, conical shape -Shield volcanoes: low relief, built up by flows and intrusions Stratovolcano or Composite: formed by an alternating series of lava flows followed by pyroclastic flows Ash Flows and Calderas: evidence for violent magmas (low H2O) -Ash flows: mixture of gases and magma that erupt into air -Caldera: remains of volcano that has erupted and blown top off
Chp 4: Volcanism- Summary Plate Tectonics and Volcanoes: Hot Spots: Bodies of magma that have risen near the surface Creating a localized zone of melting below the lithosphere. This results in surface eruptions over long periods of time. ‘Ring of Fire’ around Pacific: volcanoes related to edges of plates, where one plate is subducted (sinks beneath) another…oceanic beneath continental. Composite volcanoes frequently occur where plate tectonic boundaries- subduction zones especially-occur Monitor volcano shape changes in order to predict eruptions….!!