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[Photo: Volcano Profiles]
Some are small, others gigantic. Some have steep-sided cones, others gentle slopes. Some erupt explosively, others don't.

Size depends on the volcano’s lifespan and magma output.

Shape depends on how sticky or viscous its magma is. Viscous lavas build steep cones. Fluid lavas build broad, gently sloping cones.

Explosivity depends on how much gas (mainly water and carbon dioxide) the magmas contain and how easily the gas can escape.





Flood Basalt Plateaus

[Photo]
Photo: Basaltic Lava  
  Basaltic Lava  

In a few dozen places on Earth, huge volumes of fluid basaltic lavas have erupted from fissures, blanketing up to 750,000 sq km (465,000 sq mi). Repeated eruptions quickly built thick plateaus. Some huge outpourings of flood basalt are the same age as biological mass extinctions. Did such eruptions change the history of life on Earth?


Flood Basalt: Columbia Plateau
Location: United States
(Washington, Oregon)
Thickness: Up to 4.2 km
Area Covered: 164,000 km2
Volume: 175,000 km3
Slope: Less than 2°
Lifespan of main activity: 2-3 million years





Shield Volcano

[Photo]
Photo: Basaltic Lava  
  Basaltic Lava  

Repeated eruptions of fluid lava build shield volcanoes, named for the resemblance of their broad slopes to a warrior's shield. Lava can erupt either from the summit or from fissures on the flanks. Large shield volcanoes typically form over oceanic hot spots.


Shield Volcano: Mauna Loa
Location: United States (Hawaii)
Height above sea floor: 9 km
Length at sea level: 130 km
Volume of cone: 65,000-80,000 km3
Slope: 3-5°
Total lifespan: 600,000 years





Caldera

[Photo]
Photo: Non-welded Ryolite Tuft  
  Non-welded
Rhyolite Tuft
 
Photo: Welded Ryolite Tuft  
  Welded Rhyolite Tuft  

When a large eruption rapidly empties a magma chamber, the rocks of the chamber's roof can collapse into the void below, forming a broad circular depression, or caldera.
Caldera-forming eruptions also produce towering ash columns and ground-hugging avalanches of hot ash and gas, called pyroclastic flows.


Caldera: Long Valley
Location: United States (California)
Present caldera wall height: 250-1,000 m
Amount of caldera subsidence: 2-3 km
Volume of erupted magma: 600 km3
Caldera diameter: 32 km x 17 km
Total lifespan: about 2 million years





Cinder Cone

[Photo]
Photo: Spindle Bomb  
  Spindle Bomb  
Photo: Basaltic Cinders  
  Basaltic Cinders  

Like a fireworks display, cinder cones are short-lived but spectacular. Propelled by expanding gases, clots of incandescent magma and finer ash shoot upward and harden in the air. Falling particles pile up around the vent, building a small, steep-sided cone. Lava flows can erupt simultaneously from vents near the cone base. Cinder cones are found at hot spots, subduction zones (where one plate descends beneath another), and continental rift zones.


Cinder Cone: Parícutín
Location: México
Height: 424 m
Maximum diameter: 900 m
Volume of cone: 0.08 km3
Slope: 30-34°
Total lifespan: 9 years





Lava Dome

[Photo]
Photo: Breadcrust Block  
  Breadcrust Block  

Like toothpaste squeezed from a giant tube, viscous lava piles up around a vent to form a lava dome. Often, lava domes are created by a single eruption. Typically, they form within — or on the flanks of — a stratovolcano. Hot blocks that spall off lava domes can develop a quenched glassy skin, while expansion of gases continues within. This can lead to a cracked "breadcrust" texture.


Lava Dome: Mount St. Helens
Location: United States (Washington)
Height: 270 m
Diameter: 1000 m
Volume: 0.07 km3
Slope: 30-37°
Total lifespan: 6 years





Stratovolcano

[Photo]
Photo: Andesite Lava  
  Andesite Lava  

Stratovolcanoes grow from alternating eruptions of lava and tephra (ash, cinders, and pumice). Both pile up around the vent. Together, they produce a steep cone built of different layers, or strata thus the name, stratovolcano. Mount Rainier towers above the Cascade Range near Seattle, Washington. It is one of about a dozen stratovolcanoes overlying a subduction zone, where one plate descends beneath other.


Stratovolcano: Mount Rainier
Location: United States (Washington)
Height above surroundings: 2.3 km
Diameter: 8 km
Volume of cone: 86 km3
Slope: 20-35°
Total lifespan: about 500,000 years



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