课件-volcano_types_part1

Introduction to volcanoes, volcanic

eruptions, and volcanic landforms

What mainly controls eruptive style?

?Gas content of magma

?Viscosity of magma

Viscosity in magma

?High viscosity magma= high silica

content=more explosive

?Low viscosity magma= low silica

content=less explosive (effusive)

Why are viscosity and gas important? How would they affect volcanic hazards?

2. Eruptive style:

Explosive or effusive?

Felsic (high SiO2) = high viscosity

and more explosive (e.g. dacite)

Mafic (low SiO2) = low viscosity

and effusive (e.g. basalt)

Effusive eruption: advancing lava flow Explosive eruption:

pyroclastic flow Unzen Volcano, Japan, 1991

Where do volcanoes erupt—and how does this relate to plate tectonics?Types of volcanoes and volcanic landforms Shield volcano ex: Mauna Loa; Larch Mtn, OR

Composite volcano(stratovolcano) ex: Vesuvius,

Mount Rainier, Popo, Mount Fuji

Domes ex. Goat Mountain, Black Buttes & Mt. Shasta (CA)

Cinder cones and spatter cones ex: Paracutin, Puu O

(Kilauea), Sunset Crater (AZ)

Maar volcano ex: Battleground Lake, Ubehebe(CA)

Caldera ex: Yellowstone, Toba, Crater Lake

Flood basalt ex: CRBs17–15 Ma; Deccan Traps--India 65

Ma; Siberian Traps ~250 Ma)

Shield Volcano(typically

basaltic!)

Below: aerial view of Hawaii

Kupaianaha lava pond, Kilauea: photo by Pat Pringle, July 1987

Lower viscosity basaltic lava (mafic)

is ~45% to 54% silica

Effusive eruptions (less explosive)

Gentle relief + cinder cones.

Volcano = chiefly lava flows.

Shield Volcanoes

Pacific Plate moves over the Hawaiian hot spot; ages of the

Hawaiian Islands + geologic factors reflect plate movement—

older to the NW

2007 gps velocities ~ 7 cm/yr

Humongous landslides

from Hawaiian

volcanoes (dark gray =

islands). The landslide

deposits (stippled area)

are found on the sea

floor (note scale in km!)

So what? There is more to

see below the surface of the

ocean!

<= Tumac Mtn shield, NE of White Pass; view to East

Clayton (1983) estimated the age of Tumac Mtn at 30 to 20 ka.

ka = kiloannum

Basaltic volcanism in the Cascade Range

Larch Mtn shield, OR

Underwood Mtn shield, WA

Mount Adams composite volcano is constructed on top of a broad shield

Composite volcanoes

(aka stratovolcanoes)

?~55 –65 % silica (intermediate SiO 2) andesite, dacite ?Intermediate viscosity

?Moderately violent explosions ?~50/50 lavas/ fragmental deposits ?Thick lavas (to 20 or even 100 m)?Steep cones;can have domes ?

Lahars! (volcanic debris flows)

Where are Composite

volcanoes?

?Ring of Fire (Circumpacific belt)?Mediterranean belt

Glacial erosion increases relief and supplies ground-water => hydrothermal alteration and weakening

Mount Rainier from Glacier View Wilderness

Photo by Pat Pringle

Left : tectonic setting of Italian

volcanoes; right above : thickness of

deposits from AD 79 Vesuvius eruption

Below: isopach map of Vesuvius’AD 79 tephra

Composite volcano

Mount Hood

Mount Baker

May 18, 1980 Plinian eruption column Looking NNW

Photo by Bob Krimmel, USGS

Composite Volcano aka stratovolcano Mount St. Helens => changed the way we think about composite volcanoes!

Vesuvius, 1944

Monte Somma –remnant of pre-79 AD edifice

Crater walls of Mount St. Helens: note abundant fragmental debris!

Martha Sabel, USGS, 1982Incised canyon on the north flank of Mount

St. Helens near north end of 1980 Crater

floor; note great percentage of fragmental

debris! View looks NNE; photo by Pat

Pringle, September, 1982

Crater walls of Mount St.

Helens; photos by Pat Pringle; above

from south crater rim; right, from top of

Lava Dome.)

Dacite dome rock of Pine

Creek age (~2500 yr B.P.)

basaltic dike

Basalt of Castle

Creek age

(~1900 yr B.P.)

Summit dome dacite of Kalama age

(post AD 1479)