Geology of the Hawaiian Islands

Transcription

1 Geology of the Hawaiian Islands Class February 2004 Any Questions? Styles of Eruptions and Volcanic Hazards Styles of volcanic eruptions Some volcanoes may erupt only once (Diamond Head) Other volcanoes erupt many times (Kilauea) Some eruptions gentle (Kilauea) Others very violent and explosive (Mt. St. Helens, Mt. Pinatubo) Explosive volcanoes found in subduction zones Explosiveness is a function of magma viscosity Magmas that generate the explosive volcanoes more viscous than the non- explosive magmas Hawaiian magmas Not very explosive Not very viscous Eruptive styles Classified by level of eruption explosiveness Flood eruptions Least explosive eruptions Very fluid basalt erupted in very large quantities Lavas erupted over large areas Columbia Plateau More than 420,000 cubic km of lava 1

2 Hawaiian-type eruptions Hawaiian-type eruptions The next least explosive kind of eruption Fluid magmas with small amounts of gas Eruptions relatively gentle Hawaiian-type eruptions Build shield volcanoes More explosive eruptions Build composite cones Fragmentation (grain size) Surtseyan Vulcanian Strombolian Peléan phreatoplinian Plinian Intensity (dispersal) Modified from Julia Sable Styles of explosive volcanism Strombolian eruptions Named for volcanic island off coast of Italy Characterized by less fluid lavas Moderate explosive activity Tephra common Lots of ash blankets country side Basalt and andesite common Large steep-sided sided composite volcanoes built by repeated Strombolian eruptions Mt. Shasta, California A a flows 2

3 Vulcanian eruptions Characterized by more viscous lavas Stubbly flows are common Lots of ash Andesite or dacite most common Arenal Volcano, Costa Rica Peléan eruptions Named for 1902 eruptions of Mount Pelée on Martinique in the Caribbean Mt. Pelé eruption Almost 30,000 people were killed instantly Peléan eruptions Lavas highly viscous, very explosive Explosive eruption of highly gas-charged lava leads to nuée ardente (glowing avalanche) Nuées ardentes formed during 1968 eruption of Mt. Mayon Ash cloud is 30,000 ft high Plinian = most powerful eruption Named for Roman naturalist was killed in explosion of Mt. Vesuvius in 79 A.D Very destructive Accompanied by major collapse 3

4 Plinian Eruption Mt. St. Helens is good example Plinian = most powerful eruption Crater Lake in Oregon formed by similar explosive eruption Ash from this volcano (Mt. Mazama) found all over the Pacific Northwest Plinian = most powerful eruption Greatest Plinian eruption of modern times: 1813 explosion of Tambora volcano (eastern Indonesia) 1883 explosion of Krakatau (Sunda Strait between Java and Sumatra) Explosion heard in northern Australian, 2400 km away Other Recent Eruptions Pinatubo = Vulcanian-type eruption Mt. Unzen (Japan,1991) -- generated a number of lava domes that collapsed, sending Nuées ardentes down the side of the volcano, so it can be classified as a Peléan eruption Fragmentation (grain size) Surtseyan Vulcanian Strombolian Peléan Intensity (dispersal) phreatoplinian Plinian Styles of explosive volcanism Volcanic Hazards Lava Flows Volcanic Hazards Falling tephra Falling tephra in Yakima, Washington during Mt. St. Helens eruption 4

5 Volcanic Hazards Glowing avalanches Mudflows Toxic volcanic gases Hazards in Hawaii Lava flows Note: The next few slides are courtesy of G&G graduate student Chris Gregg Lava Flow Hazard Zone Map of Hawaii Historic Eruptions Mauna Loa: 33 flows since reached ocean in Kona Hualalai: 3 flows since ~ reached ocean in Kona High effusion rates: 3-12 x 10 6 m 3 day -1 Steep slopes > 6 %: Hualalai: >50 % Mauna Loa: 35 % Kilauea: <5 % Fast transit times: < 24 hours 1877 Lava Flows From Hualalai and Mauna Loa That Have Affected Kailua-Kona Kona Resorts Airport Subdivision MAUNA LOA s Radial Vents (33) and Rift Zones Historic radial vent eruptions: 1843, 1859, 1877, 1935 Typical Mauna Loa Eruptions: Summit followed by fissure eruptions Hualalai s last eruptions (c ) 1801) 5

6 What Controls the Flow Paths of Lava Flows? Topography What Controls the Speed of Lava Flows? Slope angle, surface roughness, eruption rate, lava type (viscosity) 1950 Eruption: Flow velocities km/hr Worst volcanic hazards often occur after major eruptions Ash covers sides of volcano Ash becomes unstable during heavy rain Ash mud flow 10 years after Pinatubo eruption, mudflows were still killing people Volcanic gases Very dangerous CO 2, CO, SO 2, H 2 S, HCl and HF Hawaiian volcanoes Do not usually emit enough gas to harm people Vog (Even reaches O`ahu with Kona wind) Avoiding volcanic hazards Predict them Volcanologists getting good at predicting eruptions of dangerous volcanoes Mt. St. Helens, Unzen and Pinatubo But, missed 1993 Mayon eruption Predictions Mostly made by monitoring: Seismic activity Ground tilting As magma moves into shallow levels under volcano: Increase in earthquake activity usually occurs Volcano also inflates 6

7 Questions? Exam Review First Exam Next Tuesday (February 17 th th ) Will be less than 1 hour long Will cover all material so far including Homework and Field Trip Reading: p. 1-38, , Nebular Hypothesis of Solar System Formation Timeline for the Sun, Earth, and Moon Plate Tectonics Unifying Theory in Geology What led to the theory? Early geologists saw the similarities between the coasts of Africa and South America. 7

8 There is evidence that huge glaciers occupied large parts of the southern continents about 250 million years ago. Coastlines match and fossil and rocks continue if the continents are fit together Plate Tectonics Only accepted in the last 30 years WHY??? What did we learn? Plate Tectonics How do we know about the Earth s interior? Meteorites and study of earthquake waves (seismology) Meteorites have struck the Earth in the past. Many meteorites are made of iron and nickel. We think this represents material of planets similar to Earth. Earth s interior (core) is probably also composed of iron and nickel. Seismic (earthquake) waves travel through the Earth. They tell us about the internal structure of the Earth. LOWER MANTLE The Earth has several layers Note that the crust under the continents is thicker (45 km) than under the oceans (8 km) The Earth s Major Plates 8

9 Water boiling in a pan on your stove is an example of convection Convection Convection within the Earth: The Lithosphere moves away from the spreading centers and sinks back into the Asthenosphere at Trenches (also known as Subduction Zones) Divergent (Spreading) Transform Three Types of Plate Boundaries Convergent (Subduction Zone) Benioff- Wadati Zone Benioff-Wadati Zones define the interface between plates at Subduction Zones Breakup of Pangaea Our Islands were formed by a Hot Spot Note the major bend in the Hawaii Emperor ridge The trail of the Hawaiian Hot Spot goes all the way to the far northwest Pacific 9

10 Volcanoes Craters are bowl- or funnel-shaped depressions Styles of volcanic eruptions Some volcanoes may erupt only once (Diamond Head) Some erupt many times (Kilauea) Styles of volcanic eruptions Some volcanoes are very gentle during eruptions Styles of volcanic eruptions Some are very violent and explosive Explosiveness of a volcano is a function of the fluidity or viscosity of the magma. The magmas that generate explosive volcanoes are much more viscous than the non- explosive magmas. Viscosity is controlled by: Chemical composition of the magma Magma temperature The amount of gas that it contains The higher the content of silica (SiO 2 ), the more viscous the magma. 10

11 Hawaiian magmas: Not very explosvie Low silica content Where does the magma come from? Where does the magma reside? From the Hot Spot in the Astenosphere In magma chambers 2-5 km below the volcanoes Definitions Rocks that crystallize from magma are Igneous rocks Volcanic rocks are erupted at the surface Plutonic rocks form when magma chambers crystallize in place Differences Plutonic rocks have large crystals because they cool slowly Volcanic rocks have small crystals because they cool quickly This is called a pyroclastic eruption pyro=fire clastic=fragments Cinder cones and spatter cones are built by fragments falling around the vents. Ash Cones and Tuff Cones form when hot magma erupts into cold sea water This is an eruption of the volcano Surtsey off the coast of Iceland Resulting tuff cone is asymmetric due to ash particles being blown by the wind 11

12 Hawaiian Lavas Two Kinds: Pahoehoe `A`a Hawaiian Lavas Pahoehoe Smooth Surface Ropey character Hawaiian Lavas `A`a Rough, rubbly surface Rubble at top Solid interior Rubble at base (clinker) Crosssection of an `a`a flow Pahoehoe vs. `A`a No real difference in chemical composition Viscosity is the main difference: `A`a comes from more viscous lava Pillow Lava When lava flows into the water, its surface cools very rapidly But the interior is still very hot and fluid. So the lava flows out in small tubes. The cooled remnants look like pillows. 12

13 Lava Tubes Formed when upper surface of a lava river solidifies, but interior is still hot and molten Thurston Lava Tube Shield Volcanoes Hawaiian volcanoes classic examples Broad, gentle slopes cover large areas. Typical Shield Volcano Composite Volcanoes Different than shield volcanoes Composites have much steeper slopes due to the more viscous lava Composites have more interbedded pyroclastic material. Composite Volcano The process of magma filling the magma chamber is called intrusion It is usually accompanied by inflation of the summit region Rift eruptions Magma moves out to the flank. 13

14 Rift eruptions Summit area deflates as magma moves out to the flank. Material coming into the rift zones that solidifies below the surface is called a dike. Caldera = circular area near the top of a volcano that has dropped down a few tens of meters to form a sunken crater Mauna Loa s summit caldera: Mokuaweoweo Pit craters Caldera vs. Pit Crater Graben linear region that has dropped down between parallel faults When hot lava flows into the ocean, it is blasted into small bits that collect in bays to form black sand beaches. Wai`anae Volcano Ko`olau Volcano 14

15 Round Top Sugar Loaf Lava Flow Ko`olau Basalt Ka`imuki Lava Shield Rocks are classified into 3 major categories: Igneous Metamorphic Sedimentary Igneous rocks Solidified from molten magma Granite Sedimentary Rock Rocks Aggregates of minerals Minerals Naturally occurring inorganic solids Possess a definite internal structure and a specific chemical composition Atoms Building blocks of minerals 15

16 Silicon (Si) + Oxygen (O) Most common arrangement of atoms in igneous minerals Produces a tetrahedron with Si in the middle Tetrahedral arrangement of SiO 4 Extrusive (volcanic) rocks Erupted at the earth's surface Exposed to surface temperatures Cool very rapidly from > 1000 C to less than 100 C Intrusive (plutonic) rocks Do not reach the surface Solidify in the crust Phenocrysts Large crystals form if magma stays in chamber long enough Bowen s Reaction Series Hawaiian volcanoes Produce several kinds of mafic rocks Most common is basalt Formed of Ca-plagioclase and pyroxene Igneous rocks arranged into three general series: Tholeiitic Alkalic Nephelinitic Tholeiitic rocks Relatively poor in Si Poor in alkalies (Na + K) 16

17 Poor in Si Rich in alkalies Alkalic rocks Nephelentic rocks Even more alkalic than alkalic rocks No plagioclase nepheline instead This is what the Sugar Loaf flow is made of Xenoliths (foreign rocks) Fragments of rocks within host rock Example: ultramafic fragments in basalt What do they tell us? Life Stages of Hawaiian Volcanoes Hawaiian volcanoes evolve thru a number of characteristics during their lifetimes. 3 major stages that Hawaiian volcanoes go through Shield-building Post-caldera ~~~~Erosion~~~~ Rejuventated New Volcanism Example: Oahu (Ko`olauKo`olau Volcano) Evolved through the submarine and subaerial shield-building stages and then became dormant. Erosion for about 2 my Styles of volcanic eruptions Some volcanoes may erupt only once (Diamond Head) Other volcanoes erupt many times (Kilauea) Some eruptions gentle (Kilauea) Others very violent and explosive (Mt. St. Helens, Mt. Pinatubo) Explosive volcanoes found in subduction zones Explosiveness is a function of magma viscosity Magmas that generate the explosive volcanoes more viscous than the non- explosive magmas Hawaiian magmas Not very explosive Not very viscous 17

18 Eruptive styles Classified by level of eruption explosiveness Flood eruptions Least explosive eruptions Very fluid basalt erupted in very large quantities Lavas erupted over large areas Hawaiian-type eruptions Hawaiian-type eruptions The next least explosive kind of eruption Fluid magmas with small amounts of gas Eruptions relatively gentle Hawaiian-type eruptions Build shield volcanoes More explosive eruptions Build composite cones Fragmentation (grain size) Surtseyan Vulcanian Strombolian Peléan phreatoplinian Plinian Intensity (dispersal) Modified from Julia Sable Styles of explosive volcanism Nuées ardentes formed during 1968 eruption of Mt. Mayon Ash cloud is 30,000 ft high Volcanic Hazards Lava Flows 18

19 Volcanic Hazards Falling tephra Volcanic Hazards Glowing avalanches Mudflows Toxic volcanic gases Lava Flow Hazard Zone Map of Hawaii Historic Eruptions Mauna Loa: 33 flows since reached ocean in Kona Hualalai: 3 flows since ~ reached ocean in Kona High effusion rates: 3-12 x 10 6 m 3 day -1 Steep slopes > 6 %: Hualalai: >50 % Mauna Loa: 35 % Kilauea: <5 % Fast transit times: < 24 hours Worst volcanic hazards often occur after major eruptions Ash covers sides of volcano Ash becomes unstable during heavy rain Ash mud flow 10 years after Pinatubo eruption, mudflows were still killing people Volcanic gases Very dangerous CO 2, CO, SO 2, H 2 S, HCl and HF Hawaiian volcanoes Do not usually emit enough gas to harm people Vog (Even reaches O`ahu with Kona wind) Avoiding volcanic hazards Predict them Volcanologists getting good at predicting eruptions of dangerous volcanoes Mt. St. Helens, Unzen and Pinatubo But, missed 1993 Mayon eruption Predictions Mostly made by monitoring: Seismic activity Ground tilting As magma moves into shallow levels under volcano: Increase in earthquake activity usually occurs Volcano also inflates 19

20 Earth Systems Today CD As a supplement for exam study, work through sections on: Volcanism Atoms & Crystals Rocks & Rock Cycle For Exam If can t get into this room, meet in front of POST Building (Ewa( side; between POST and Sakamaki) If you have a medical problem, car trouble, etc., you MUST let me know BEFORE the exam via or phone call! Questions?? 20