A lahar is a rapidly flowing dense mixture of rock debris and water originating from a volcano. Also known as ‘mudflows’ or ‘debris flows’, lahars resemble the behaviour of wet concrete as they flow. The source of the water may be a crater lake, heavy rain or snow. The loose ash and volcanic fragments are transformed into a dense fluid-rock mixture that rushes down the slopes of a volcano and into surrounding valleys. Lahars are destructive to everything in their path, and the threat from lahars may last for years after an eruption has ended

An abrupt movement of geological materials downhill in response to gravity.

Lateral blast
A lateral blast is a laterally directed volcanic explosion of rock fragments and gas that explodes outwards at high velocity from the side of a volcano. It can affect a 180° sector and extend up to

30 km outward from the volcano. Lateral blasts are not affected by topography and can develop without warning.

Magma which has reached the surface through a volcanic eruption. Refers to streams of liquefied rock that flow from a crater or fissure. It can also refer to cooled/solidified rock from such areas.

Lava Dome
See Dome.

Lava Flow
A lava flow is a hot stream of molten rock that travels down valleys on the slopes of a volcano. The distance lava flows travel depends on the viscosity (‘stickiness’) of the lava. If the lava is viscous (sticky), it cannot flow easily so it tends to form short thick lava flows or pile up around the vent to form a hill, or lava dome.

Leading wave
First arriving wave of a tsunami. In some cases, the leading wave produces an initial depression or drop in sea level, and in other cases, an elevation or rise in sea level. When a drop in sea level occurs, sea level recession is observed.

‘Live’ volcano
A volcano that is currently erupting or that scientists believe has the capacity to erupt again is sometimes referred to as ‘live’.

A process by which water-saturated sediment temporarily loses strength and acts as a fluid, like when you wiggle your toes in the wet sand near the water at the beach. This effect can be caused by earthquake shaking.

The lithosphere is the outer solid part of the earth, including the crust and uppermost mantle. The lithosphere is about 100 km thick, although its thickness is age dependent (older lithosphere is thicker).The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.

Love wave
A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

Local Tsunami
A tsunami from a nearby source for which its destructive effects are confined to coasts less than 1 hour tsunami travel time, or typically within about 200 km from its source. A local tsunami is usually generated by an earthquake, but can also be caused by a landslide or a pyroclastic flow from a volcanic eruption. Over history, 90% of tsunami casualties have been caused by local tsunamis.