Volcanoes: Composite Cone Volcanoes

• Volcanoes are openings in a planet's surface that allow molten rock (magma) to reach the surface, ejecting lava, rock fragments, and hot vapor and gases.
• Volcanoes are differentiated based on size, composition, and eruptive style.

Types of Volcanoes

• Composite
• Shield
• Cinder Cone
• Lava Domes

Composite Cone Volcanoes

• Also called stratovolcanoes
• Cone-shaped volcanoes containing layers of ash, lava, and rock fragments.
• They are the largest volcanoes, reaching heights up to approximately 2500 m, and are typically steep-sloped.
• Formed by the eruption of different materials deposited on a slope over thousands of years.
• Eruptive Process
  • A conduit system taps into a magma reservoir deep within the Earth.
  • Molten rock moves up toward vents at the surface.
  • Pressure builds up, causing a vent or the central crater to burst.
  • Lava, ash, or pyroclastic flow erupts, leaving a distinct layer of material.
  • Pyroclastic flow: a dense collection of ash, rock fragments, and hot gases ejected explosively, rushing down the slope.
• Highly viscous lava, typically felsic, with high-to-intermediate silica levels, leads to explosive eruptions.
• Silica Content of Lava
  • Rhyolite: 60-77%
  • Andesite: 50-60%
• Examples
  • Mount Saint Helens
    • Located in Washington State, US
    • Last erupted in May 1980
    • Scientists expect more eruptions in the future
  • Mount Vesuvius
    • Located near Pompeii, Italy
    • Destroyed Pompeii in 79 A.D.

Shield Cone Volcanoes

• Made almost entirely of fluid lava flows, with gentle slopes.
• Diameters can be up to 6 km, and heights of 600 m.
• Have a shield or dome shape.
• Formation and Eruption:
  • Less explosive eruptions than composite volcanoes due to less viscous and more fluid lavas.
  • Wider base and gentler slope.
  • Eruptive Process:
    • Lava exits vents at the center or along rift zones.
    • Low-viscosity lava flows overlap and spread across great distances.
    • Lava cools in thin sheets.
• Low-viscosity lavas are typically mafic, with low silica content (45-55%), becoming explosive only if mixed with water or other volatiles.
• Basalt is typical mafic lava
• Examples:
  • Hawaiian Islands, including Mauna Loa and Mount Kilauea.

Cinder Cone Volcanoes

• Extremely steep-sided volcanoes made from congealed lava exiting a single vent.
• Most have a bowl-shaped crater.
• Rarely reach more than 400 m tall.
• Simplest type of volcano
• Formation and Eruption:
  • Often monogenetic, experiencing only one eruption that may last for several years.
  • Eruptions are typically fairly weak.
  • Eruptive Process:
    • Magma approaches the Earth's surface with large amounts of dissolved gas.
    • Magma breaks through the surface, releasing trapped gases.
    • Gases expand, causing an explosive eruption and launching lava into the air.
    • Lava cools and falls back as cinders or glassy rock fragments.
    • Cinders fall near the vent, creating the cone.
• Lavas are typically basalts or andesites.
• Examples:
  • Paricutin, Mexico, formed in 1943 in a farmer's field.
  • Cinder cones are thought to be typical on the Moon and Mars.

Lava Dome Volcanoes

• Created by viscous lavas that cannot flow far from the vent.
• Vary in shapes and sizes.
• Not generally as impressive as other types of volcanoes.
• Types of Lava Domes:
  • Low Lava Dome
  • Peleean
  • Coulees
  • Upheaved Plug
• Commonly occur on the sides or near the craters of large composite volcanoes.
• Formation and Eruption:
  • Formation varies, created by a single event or built over time by multiple eruptions.
  • Can grow by in-filling, with material added inside, pushing the top out. Formation time varies, from hours to over 100 years.
  • Eruptive Process:
    • A small mass of viscous magma rises in the vent.
    • Non-explosive lavas exit the vent and settle nearby.
    • Lavas cool and build up around the volcano, giving it a dome shape.
• More viscous lavas, typically felsic, ranging from basaltic andesites to rhyolites.
• Examples:
  • After the 1980 eruption at Mount Saint Helens
    • Lassen Peak
    • Mono domes in California
  • Lassen Peak is the largest lava dome on Earth, measuring around 970 m tall.

Volcanoes are openings that allow molten rock to reach the surface. Composite cone volcanoes, also called stratovolcanoes, are cone-shaped and contain layers of ash, lava, and rock fragments. They are formed by the eruption of different materials deposited over thousands of years.