Discuss the causes of volcanic eruption and describe the land-forms formed by deposition of its lava.

Causes of eruption

• Partial melting in the mantle or lower crust produces magma where heat, pressure drop, or volatiles (especially water in subduction zones) lower the melting point.
• At divergent plates and hotspots, decompression melting feeds basaltic magma; at convergent belts, wet melting feeds more viscous andesitic and rhyolitic magmas.
• Magma rises because it is less dense than surrounding rock; it stalls in chambers until gas exsolution, new magma injection, or roof failure raises pressure above the strength of the seal.
• High dissolved water and carbon dioxide make explosive eruptions when magma is viscous; low-viscosity basalt degasses more quietly as lava fountains and flows.
• Local triggers include earthquake shaking, glacier unloading, and landslide of a volcanic edifice, which uncork stored magma.

Landforms from lava deposition

• Flood-basalt plateaus, such as the Deccan Traps, form when successive low-viscosity flows bury huge areas in nearly flat sheets.
• Shield volcanoes (Mauna Loa type) are broad cones of many thin basaltic flows; composite or stratovolcanoes mix lava with pyroclasts, but the lava component still builds resistant ribs.
• Lava plains and valley flows follow pre-existing slopes; when they fill a river valley they may invert relief after surrounding rock is eroded.
• Pillow lava forms where basalt cools under water, producing sack-like lobes on the ocean floor or in subglacial settings.
• Spatter cones, lava domes of viscous lava, tumuli, lava tubes, and columnar jointed outcrops (as in some Deccan sections) are smaller depositional and cooling forms.
• Cinder cones are mainly pyroclastic, so they should not be listed as lava-deposition forms except where lava flows issue from their base.

Detail to keep

• Eruption cause is about melt and pressure; landform description here is about cooled lava, not ash plains or calderas from collapse.