Volcanoes and Volcanic Deposits 2

Volcanoes and Volcanic Deposits 2 IN THIS LECTURE • Shield Volcanoes • Stratovolcanoes • Other Types of Volcanic Centres • Flood Basalt Provinces • Maar and Tuff Rings • Intermediate-silicic centres • Rhyolitic volcanoes • Submarine spreading ridges and seamounts • Intra- or subglacial volcanoes

Shield Volcanoes - Hawaiian • Hawaiian Shield Volcanoes • Summit calderas and major rift zones marked by spatter cones, spatter ramparts, collapse craters (pit craters), scoria cones and smaller superimposed monogenetic shields • Shape usually controlled by eruptions from the rift zones • Eruptions within the calderas occur slightly more frequently than on the rifts but the eruptions from the lateral rifts that give the shields their elongate form. • Calderas range from 5 to 20kms in diameter • Shields are built by lavas and minor pyroclastics as well as high level intrusives which may be present in the summit caldera walls. • Compositional differences occur as the shield volcano evolves changing from tholeiitic to progressively more alkalic • More explosive activity accompanies the eruptions of alkaline magmas. • Eruption frequency decreases with time

Hawaiian Volcanic Chain • The two most active shields on Hawaii are Kilauea and Mauna Loa. • Mauna Loa is the world’s largest active volcano • Rises nearly 9km from the pacific ocean floor to its summit of 4169m above sea level • Total volume of 40,000km3 • Combined growth rate of ~0.1 km3 per year indicates both Kilauea and Mauna Loa could have been built in less than 1 Ma • Large portion of the base of both volcanoes made up of pillow lava formed by subaqueous extrusions • Gravity sliding and slumping along normal faults is common on the flanks and occurs in response to oversteepening caused by addition of lava flows and intrusion of magma into the summit.

Mauna Loa Snow-covered Moku`aweoweo Caldera atop Mauna Loa shield volcano (Mauna Kea in background). The caldera is 3 x 5 km across, 183 m deep, and is estimated to have collapsed between 600-750 years ago. Several pit craters along the upper southwest rift zone of Mauna Loa (lower right) also formed by collapse of the ground. For more information on the world’s largest volcano visit http://hvo.wr.usgs.gov/maunaloa/

Shield Volcanoes - Icelandic • Icelandic shield volcanoes • Smaller – Ws < 15 km • Symmetrical • Almost entirely built up by effusive eruptions from a central summit vent • Summit crators usually < 1 km across and often have raised rims of spatter • Few radial fissures or lines of parasitic cones • Generally composed of large numbers of thin pahoehoe flows • Mostly monogenetic and usually constructed in less than 10 years.

Shield Volcanoes - Galapagos • There is a third type of shield volcano known as the Galapagos type. • Very similar to Hawaiian shield volcanoes but the shape of the upper summit is different • Gentle lower slopes that rise to steeper central slopes that flatten off around spectacular summit calderas. • Usually more alkaline than Hawaiian volcanoes Three-deminsional Space Shuttle Image of the Alcedo Shield Volcano, Galapagos -- The near circular caldera of the Alcedo shield volcano on the big island of Isabela is a feature common to many of the Galapagos shield volcanoes. The image, taken by the Space Shuttle Endeavor, covers an area of about 75 km by 60 km. The oblique view was constructed by overlaying a Spaceborne Radar Image on a digital elevation map. The vertical scale is exaggerated by a factor of 1.87.

Stratovolcanoes • Stratovolcanoes or composite volcanoes are the characteristic volcanic landform found at subducting plate margins • They represent the most abundant large volcano on the Earth’s surface • Stratovolcano morphology results from repeated eruptions of pyroclastics and relatively short lava flows from a central vent. • Volcaniclastic deposits (pyroclastic and epiclastic) are usually very important volumetrically and can make up more than 70% of the volcanic succession the rest being lavas. • At destructive plate margins, stratovolcanoes are built by eruptions of calc-alkaline magmas that are usually broadly andesitic or basaltic-andesite in composition. • Alkaline magmas generate stratovolcanoes which are on average larger than their calc-alkaline counterparts. • Average slopes on stratovolcanoes range from 15° to 33°. • Most active stratovolcanoes are less than 100,000 years old and have repose periods of up to 10,000 years

Stratovolcanoes Mount Mageik volcano viewed from the Valley of Ten Thousand Smokes, Katmai National Park and Preserve, Alaska. Mageik's broad summit consists of at least four separate structures built above different vents. Mount St. Helens is the youngest stratovolcano in the Cascades and the most active. Geologists have identified at least 35 layers of tephra erupted by the volcano in the past 3,500 years. This picture is prior to the 1980 eruption

Stratovolcanoes • Stratovolcanoes are composed of a wide variety of primary volcanic products • Various lava types from basaltic through to rhyodacitic • Pyroclastic flows • Welded air-fall tuffs • Ash deposits • Ignimbrite deposits • Pumice fall deposits • This variety of volcanic products arises because the generation, evolution and type of magma erupted from these volcanoes is complex and could represent magma chambers on different levels with complex conduits between them and replenishment by different batches of primary basaltic magma rising through the system. • The preservation of these primary volcanic products is complicated by the mass wastage and epiclastic processes that are common on the flanks of stratovolcanoes

Stratovolcanoes

Other Types of Volcanic Vents • Lava and tephra can erupt from vents other than these three main volcano types. A fissure eruption, for example, can generate huge volumes of basalt lava that make up continental flood basalts • Other types of volcanic edifices include • Flood basalts • Maars and tuff rings and cones • Rhyolitic volcanoes • Interediate or silicic multi-vent centres • Inter- or glacial volcanoes

Flood Basalts and their Source Vents • The source vents to flood basalts are not central or point-source volcanoes • They usually have high discharge rates up to 106 m3 per second • Flood basalts represent the largest single eruptive units known and usually have flowed great distances from their source. • Flood Basalts built up by repeated eruptions forming a vast lava plateau which may cover areas > 106 km with slopes generally less than 2-3° • Often closely associated with the initiation and early development of rifted margins • Dominantly tholeiitic but alkali basalts are also common • Many of the larger flows must have formed vast lava lakes that took many years to solidy as indicated by the well-developed massive columnar jointing preserved in many flood basalt provinces • Columnar jointing is often two-tiered related to cooling fronts propagating inwards from both the top and bottom of the lava flow.

Examples of Flood Basalts • Mid-Miocene Columbia River Plateau or Basalts • Occur in Washington, Oregon and Idaho • Deposited within 2-3 Ma • Cover 220,000 km2 and have an estimated volume of 195,000 km3 • Mid-Tertiary Ethiopian-Yemen plateau • Cretaceous Deccan Traps Northwestern India, 500,000 km2 and volume of more than 1 million km3 • Cretaceous Parana-Etendeka province of southern Brazil-Uruguay-Namibia • Jurassic Karoo in South Africa • Jurassic Ferrar in South America

‘Local’ Continental Flood Basalts

Maars and Tuff Rings and Cones • Volcanic craters that are usually monogenetic and produced by phreatomagmatic and phreatic eruptions • Second only to scoria cones in abundance • Maar is a general term for broad, low-rimmed volcanic craters that form when rising magma explosively interacts with ground water or surface-derived water below the original topographic surface and contain little or no juvenile magma • Tuff rings have craters that lie on or above the pre-eruption surface and form when rising magma interacts explosively with abundant water close to or at the ground surface and contain a higher proportion of juvenile magma. Tuff rings are usually basaltic but more acidic one are also common • Tuff cones differ from tuff rings by having smaller craters and larger height to width ratios and form in areas where surface water is located above the vent. • Maars, tuff cones and tuff rings consist of pyroclastic deposits of stratified and cross-stratified ash. • These types of volcanic centres often show a progression from phreatomagmatic to strombolian or hawaiian activity reflecting a decrease in the degree of magma-water interaction during eruption. • Duration of eruptions is thought to be fairly short from a few days to a few weeks

Maars and Tuff Rings and Cones Distinguishing characteristics of maar-type volcanoes

Urinrek Maars, Alaska Eruption column generated by phreatic and magmatic explosions rises from the larger east maar. Aerial view toward N of Ukinrek Maars, Alaska; Lake Becharof at top of photo. Water partially fills the eastern maar and completely covers a lava dome that was erupted in the 100-m deep crater during a 10-day eruption in 1977. Maar is about 300 m in diameter.

Rhyolitic Volcanoes • Rhyolitic volcanic centres are some of the largest volcanic landforms on the Earth’s surface • Usually polygenetic, multivent centres • Usually consist of multiple eruption points or volcanoes • Usually found in extensional tectonic regimes such as rifts, grabens and marginal basins. • Typically lack a topographically impressive cone cf stratovolcanoes • Sometimes form large broad volcano-tectonic depressions called inverse volcanoes of which Lake Taupo in NZ is the type example • Typically consist of a collection of low rhyolitic hills composed of rhyolite domes, coulees and pumice cones, rising from gently sloping ignimbrite sheets which may contain more than one ignimbrite sheet • Largest rhyolitic caldera known to exist is Lake Toba in Sumatra which has rim dimensions of 100 x 35 km. • Eruption rates are typically very low on the order of thousands of years and the period of repose may be quite long as much as one million years indicating that some rhyolitic volcanoes may have quite long lifespans. • Lake Taupo has been active for 0.6 Ma, while Yellowstone has been active for 2 Ma.

Rhyolitic Volcanoes • Ignimbrite forming eruptions are generally associated with major structural changes to the volcano • Caldera collapse occurs during or after the eruption, around a circular ring fracture formed above the drained or draining magma chamber. • Later volcanic activity is concentrated in this ring fracture. • Explosive phases precede the eruption of rhyolite domes and flows but rhyolite lavas do not travel far from the vent. • Rhyolitic volcanoes are thought to go through an evolutionary cycle with the following seven stages • Regional tumescence and generation of ring fractures • Ignimbrite eruptions • Caldera collapse • Pre-resurgence volcanism and intra-caldera sedimentation • Resurgent doming • Major ring-fracture volcanism and • Terminal fumarolic and hot spring activity.

Large Volume Rhyolite Lavas?? Felsic magmas will either (1) erupt explosively to produce extensive deposits of tephra, or (2) nonexplosively to produce degassed, viscous lava (domes, coulees, or obsidian flows) which advance only short distances from their vents. There has been a significant amount of controversy, therefore, over rare rhyolite lavas that appear to occur as large-volume flows (10-100 cubic kilometers). Most such flows occur near continental hotspots. The best known examples are those associated with (1) the Yellowstone hotspot track near the Idaho-Oregon border, and (2) the Ethiopian hotspot in northeastern Africa. These large-volume felsic volcanic rocks have outcrop, hand specimen, and thin section characteristics typical of lava flows. However, many volcanologists suspect that they are not lava flows at all, but rather rheomorphic ignimbrites. These are densely welded pyroclastic flows of pumice and ash, which were thick and hot enough to flow downslope and obliterate primary pyroclastic structures. They suggest that the original pumice and ash fragments have been streaked out like toffee strands so that the pyroclastic nature of the flow becomes unrecognizable. Is this the case with the large rhyolite lavas of the Lebombo Monocline?

Intermediate-silicic multi-vent centres • These types of volcanic centres are similar to Rhyolitic volcanoes but have lavas that are andesitic to dacitic in composition and often alkaline. • Normally involve a caldera and caldera collapse processes after explosive eruption activity • Often surrounded by large ignimbrite sheets similar to rhyolitic volcanoes

Intra- or subglacial volcanoes • The type locality for these eruptions is Iceland. • Compositionally all lavas types may occur including basaltic, andesitic, dacitic and rhyolitic. • Typically form steep sided ridges called Tindas or steep circular table mountains called tuyas • Basaltic subglacial volcanoes consist principally of masses of pillow lavas, palagonitised hyaloclastite breccias and sideromelane fragments. • Silicic eruptions beneath ice are likely to initially be explosive similar to subaerial silicic eruptions.

Volcanoes and Volcanic Deposits 2