A pyroclastic flow consists of debris and air, but a lahar is a rapid, liquid flow. The abrupt release of accumulated gas pressure propels the lava upward, resulting in the formation of pyroclastic debris. Lahars are much more destructive than pyroclastics because they carry large amounts of rock and soil with them.
Lahars can travel hundreds of miles down valleys and across mountains, causing death and destruction wherever they go. They can wash away roads, contaminate water sources, and destroy houses and buildings. Although most lahars originate as volcanic eruptions, earthquakes can also trigger lahar formation. In addition, glacial activity can also cause lahars by releasing bubbles of gas trapped within the ice sheet.
When volcanoes erupt, some of the gases inside them build up pressure that eventually causes an explosion-like event called an eruption. If the volcano is steeply inclined, then part of the volcano may collapse due to the force of the exploding gas. This collapsed section of the volcano is called a pyroclastic flow deposit.
If the volcano is more horizontally oriented, then most of the volcano may not collapse, but rather a large amount of liquid rock known as lava is pushed out of the crater at high speed. This is called a lahar because it looks like a river of lava.
A pyroclastic flow is a thick, rapidly moving flow of hardened lava, volcanic ash, and hot gases. It is produced as a byproduct of some volcanic explosions. A pyroclastic flow is incredibly hot and will burn everything in its path. A heavy cloud of ash develops over the fast-moving torrent. This cloud may be several hundred feet high before it finally dissipates after few miles.
When lava flows onto a surface with no resistance such as snow or ice, then it forms a sheet flow. But if there are trees or other vegetation near the edge of the flow, they will be destroyed first. So instead of a smooth flow, you get a jagged one with small spires of rock called "tuff" mixed in. The speed of the flow depends on how much lava is being ejected from the volcano. If there is only a little bit, then the flow will be slow. But if a large part of the volcano collapses, then the flow can be very fast.
Volcanoes produce three main types of flows: lava flows, pyroclastic flows, and lahars (which are not really their own type of flow but rather a mixture of liquid water and volcanic debris). Lava flows and pyroclastic flows are both hard to stop once they start. But a lava flow can be stopped by building up a barrier of some kind e.g. a fence or wall.
Lahars are volcanic mudflows caused by the mixing of water (rain or melt water from glaciers) with ash. Lahars can form years after a volcanic explosion. Avalanches including hot volcanic gases, ash, and volcanic bombs are known as pyroclastic flows. Pyroclastic flows may reach speeds of exceeding 100 miles per hour on steep volcanoes. They can destroy everything in their path.
As with any question regarding volcanoes, knowledge of the specific volcano in question is required to understand exactly what type of eruption it has been observed to have. The distribution of lava flows, pyroclastic deposits, and surface manifestations of gas activity provide important information about past eruptions.
Lahars and pyroclastic flows are types of volcanic hazards. Both can be fatal if they move into populated areas or if they carry toxic gases that accumulate in small openings such as caves or mines. A lahar or pyroclastic flow that does not cause death or injury due to its suddenness is called a “non-violent” hazard.
A lahar or pyroclastic flow that causes deaths or injuries through some other mechanism is called a “violent” hazard. For example, a lahar that enters a village and carries away many people would be considered a violent hazard even if it did not kill anyone directly through the force of the flood.