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Investigates how magma chemistry influences eruption styles and the resulting volcanic structures.
Imagine a liquid so thick it can shatter like glass, or so fluid it runs like water—this chemical difference determines whether a mountain peacefully grows or violently disappears in an instant.
The primary driver of volcanic behavior is viscosity (resistance to flow). This is largely determined by the amount of **silica ()** in the magma. Magma with high silica content () forms complex molecular chains that make it thick and 'sticky.' When gas bubbles try to expand in this viscous magma, they get trapped, building immense pressure. Conversely, basaltic magma has low silica content (), allowing gases to escape easily and the lava to flow like syrup. This leads to effusive eruptions rather than explosive ones.
Quick Check
Why does high silica content lead to more explosive eruptions?
Answer
High silica increases viscosity, which traps volcanic gases and allows pressure to build up until it reaches a breaking point.
The physical shape of a volcano is a direct result of its lava's properties. Shield Volcanoes are broad, low-profile domes built by fluid, basaltic lava (e.g., Mauna Loa). Stratovolcanoes (or composite volcanoes) are tall, conical peaks formed by alternating layers of viscous lava and pyroclastic debris (ash and rock). These are the most dangerous types. Finally, Cinder Cones are small, steep-sided mounds built from ejected rock fragments called scoria that accumulate around a single vent during a short-lived eruptive phase.
1. Observe the slope of Mauna Loa in Hawaii. The angle is usually less than . 2. Because the lava is basaltic, it can travel many kilometers before cooling. 3. This creates a 'shield' shape, resembling a warrior's shield lying flat on the ground.
Quick Check
Which volcano type is composed of alternating layers of lava and ash?
Answer
Stratovolcano (or Composite Volcano).
Not all magma reaches the surface. Intrusive (plutonic) features form when magma cools and solidifies within the Earth's crust. Batholiths are the largest, covering areas over . Smaller features are classified by their relationship to existing rock layers. Dykes are discordant, meaning they cut vertically across rock layers. Sills are concordant, meaning they squeeze horizontally between existing layers. If a sill pushes the overlying rock upward into a dome, it is called a laccolith.
Imagine a stack of horizontal sedimentary rocks. 1. A magma body that forces its way between two layers like a sandwich filling is a sill. 2. A magma body that cracks through those layers vertically like a wall is a dyke. 3. If erosion eventually removes the softer sedimentary rock, the harder igneous dyke may stand out as a 'wall' in the landscape.
In a geological survey, you find a granite dyke cutting through a batholith. 1. According to the Law of Cross-Cutting Relationships, the feature that cuts through another is the younger one. 2. Therefore, the batholith must have solidified first. 3. The dyke represents a later pulse of magmatic activity that found a fracture in the pre-existing batholith.
Which magma type is most likely to produce a violent, explosive eruption?
A horizontal layer of igneous rock that is injected between pre-existing sedimentary layers is called a:
Stratovolcanoes are primarily built from low-viscosity basaltic lava flows.
Review Tomorrow
In 24 hours, try to sketch a cross-section of a volcano including a magma chamber, a sill, and a dyke without looking at your notes.
Practice Activity
Research 'Shiprock' in New Mexico. Identify whether the main structure is a dyke, a sill, or a volcanic neck based on its appearance.