Lava flows come in three main types: basaltic, andesitic, and rhyolitic. Their silica content affects how they flow and look. Basaltic lavas are runny and spread out, while rhyolitic lavas are thick and form stubby domes.

Lava flow behavior is influenced by viscosity, eruption rate, and terrain. As lava cools, it forms a crust and eventually solidifies. Topography plays a big role in how lava flows, shaping the landscape and creating unique features over time.

Lava flow types

Composition-based classification

• Lava flows are classified into three main types based on their silica content:
  • Basaltic lavas have low silica content (45-52%)
  • Andesitic lavas have intermediate silica content (52-63%)
  • Rhyolitic lavas have high silica content (>63%)
• Silica content influences the lava's viscosity, which affects its flow behavior and morphology
  • Higher silica content results in higher viscosity and more viscous flow behavior

Morphology and flow characteristics

• Basaltic lava flows are characterized by low viscosity, allowing them to flow quickly and create thin, extensive sheets
  • Common basaltic lava flow morphologies include:
    • Pahoehoe (smooth, ropy surface)
    • Aa (rough, jagged surface)
• Andesitic lava flows have higher viscosity than basaltic flows
  • Results in thicker, shorter flows with blocky or rubbly surface morphologies
• Rhyolitic lava flows are highly viscous
  • Often form thick, stubby flows or domes with steep sides and rugged, blocky surfaces
• Lava flow morphology is also influenced by:
  • Eruption rate (lower rates promote more viscous, thicker flows)
  • Temperature (lower temperatures promote more viscous, thicker flows)
  • Gas content (lower gas content promotes more viscous, thicker flows)

Factors controlling lava flow

Viscosity and composition

• Lava flow velocity is primarily controlled by the lava's viscosity
  • Viscosity is a function of its composition, temperature, and crystallinity
  • Lower viscosity lavas (basaltic) flow faster than higher viscosity lavas (rhyolitic)
• The slope of the underlying terrain also affects lava flow velocity
  • Steeper slopes promote faster flow rates

Thickness and areal extent

• Lava flow thickness is influenced by:
  • Lava's viscosity (higher viscosity results in thicker flows)
  • Eruption rate (lower eruption rates result in thicker flows)
  • Cooling rate (faster cooling rates result in thicker flows)
• The areal extent of a lava flow depends on:
  • Volume of lava erupted (larger volumes lead to more extensive flows)
  • Velocity (higher velocities lead to more extensive flows)
  • Duration of the eruption (longer durations lead to more extensive flows)
• Topographic barriers, such as valleys or ridges, can confine or divert lava flows, affecting their areal extent and morphology

Lava flow cooling

Heat transfer processes

• Lava flows cool and solidify through a combination of heat transfer processes:
  • Conductive cooling: heat is transferred from the hot lava to the cooler atmosphere or underlying ground
    • Most effective at the lava flow's surface and base
  • Convective cooling: air or water circulates around the lava flow, removing heat
    • More efficient in lava flows with higher surface area-to-volume ratios (thin, extensive flows)
  • Radiative cooling: lava flow emits infrared radiation to the surrounding environment
    • Becomes more significant as the lava flow's surface temperature decreases

Solidification and crystallization

• As lava cools, it begins to crystallize, forming a solid crust on the flow's surface
  • The crust insulates the flow's interior, slowing down the cooling rate and allowing the lava to continue flowing beneath the surface
• The formation of lava tubes can further insulate the flow's interior
  • Enables lava to be transported over long distances with minimal heat loss
• The final stages of solidification involve the growth of crystals within the lava flow's interior
  • Leads to the development of the rock's final texture and mineralogy

Lava flow interaction with topography

Topographic control on flow behavior

• Lava flows are strongly influenced by the pre-existing topography
  • Topography can control their direction, velocity, and morphology
• Lava flows typically follow the path of least resistance
  • Often channeled into valleys or depressions in the landscape
  • Can result in the formation of elongated, narrow lava flows or the ponding of lava in topographic lows
• Steep slopes can increase lava flow velocity, leading to thinner, more extensive flows
• Gentle slopes or flat terrain can cause lava flows to slow down, resulting in thicker, less extensive flows

Modification of landscape

• Topographic barriers, such as ridges or hills, can divert or split lava flows
  • Causes them to change direction or form multiple lobes
• Lava flows can modify the topography by:
  • Filling in depressions
  • Creating lava plains
  • Building shield volcanoes or lava domes
• The interaction between lava flows and water bodies (rivers or lakes) can result in the formation of unique features
  • Pillow lavas or hyaloclastites due to rapid cooling and fragmentation of the lava
• Over time, repeated lava flows can significantly alter the landscape
  • Creates new landforms
  • Influences the development of drainage networks and ecosystems