Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in an upward direction. The summit area typically contains a surprisingly small summit crater. Stratovolcanos typically suffer explosive eruptions. Besides Mount Meru, examples of stratovolcanoes include Mt. St. Helens, Mt. Cotopaxi, Mt. Pinatubo, Mt. Rainier, Mt. Fuji and many others.
Stratovolcanoes are usually 50/50 pyroclastic material and lava, and because these two are layered on top of each other, stratovolcanos are also commonly called as composite volcanoes.
Ash layers tend to be thin but widespread. Each eruption, regardless of what it produces, is fed from the magma chamber by a vent. Most startovolcanos have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone.
The lava at stratovolcanoes occasionally forms a lava flow whose surface is broken into rough angular fragments, but more commonly it barely flows at all, preferring to pile up in the vent to form volcanic domes. Some stratovolcanoes are formed as collection of domes piled up on each other. Stratovolcanoes are commonly found along subduction-related volcanic arcs, and the magma supply rates to stratovolcanoes are lower.
Formation of the Ash Cone of Mt Meru and its associated lava dome and flows in the caldera is younger than 7,800 years. The main cone of the volcano has a caldera and a large breach on its eastern side. The last eruption of Mt Meru took place at Ash Cone on October 26th, 1910
The eruptive history of most stratovolcanoes is delineated by highly explosive Plinian eruptions. These dangerous eruptions are often associated with deadly pyroclastic flows composed of hot volcanic fragments and toxic gases that advance down slopes at hurricane-force speeds. Stratovolcanoes are polygenetic; however, they differ from shield volcanoes in that they erupt infrequently, with typical repose intervals of hundreds of years between eruptions. This is often caused by cooler and differentiated magma compositions. Most active stratovolcanoes worldwide appear to be < 100,000 years old, although some may be more than 1 million years old.
Although they are not as explosive as large silicic caldera complexes, stratovolcanoes have caused by far the most casualties of any type of volcano. There are several reasons for that: as said before most of the volcanos in the world are stratovolcanos and also there are lots of people living close to them. Additionally, stratovolcanoes have typically steep piles of ash, lava, and domes that are often shaken by earthquakes, oversteepened by intruding blobs of magma and they have domes that are often rained heavily on. This makes the likelihood of landslides, avalanches, and mudflows all very high. Occasionally entire flanks of stratovolcanoes collapse, in a process that has been termed "sector collapse". The most famous example of this is Mt. St. Helens, the north flank of which failed during the first stages of the big 1980 eruption. Mt. St. Helens was certainly not the only volcano to have suffered an eruption such as this, however. Two other recent examples are Bezymianny (Kamchatka) in 1955-56, and Unzen (Japan) in 1792. The 1792 Unzen sector collapse dumped a flank of the volcano into a shallow inland sea, generating devastating tsunami that killed almost 15,000 people along the nearby coastlines.
Another very common and deadly hazard at most stratovolcanoes is called a Lahar. Lahar is an Indonesian word for a mudflow, and most geologists use the term to mean a mudflow on an active volcano. Sometimes the word is reserved only for mudflows that are directly associated with an ongoing eruption (which are therefore usually hot), but that starts to make things confusing. It is probably simplest to just call any mudflow on a volcano a lahar. Lahars are so dangerous because they move quickly, and often times a small eruption or relatively small rainstorm can generate a huge lahar.
The most recent huge volcanic disaster occurred at a Colombian volcano called Nevado del Ruiz in 1985. This disaster has been well-documented by numerous post-eruption studies. Somewhere around 23,000 people were almost instantly killed.
This photo shows cleary lahar mudflow at Mount Ruapehu (New Zealand) in 2007 eruption.
An estimated 1.4 million cubic metres of mud, rock, and water thundered down the Whangaehu river.
Mount Meru
Mount Meru was once higher than its nearby famous neighbour Mt. Kilimanjaro but a collapse of the main caldera about 7800 years ago left Mt Meru Summit at 4566 m.a.s.l.
The earliest volcanic activity occurred at west Meru plateau 1.5 million years ago. The total extent of the eruption is obscured by later ash and lahar deposits. About 100 m is exposed, and may be up to 700 m deep.
The main cone of Meru was built between 160,000 and 60,000 years ago. Summit volcanic activity at Meru began 74,000 years ago and continued for 20,000 years. Parasitic cones and lava domes are located on all sides
About 7800 years ago the east side of Meru and its crater collapsed creating lahars which extended eastwards to Kilimanjaro. These catastrophic events were connected with the plinian eruption of thick ashes that covers much of the volcano, especially to the west.
A minor eruption occurred at Mt Meru in 1910 (at Ash Cone) and the previous ones in 1886 and 1878 (North West of Ash Cone).
On 6th February 2008 snow fell on the summit of Mt Meru for the first time in decades. The mountain lost its glacier in the late 1960's.
Mount Meru is considered as an active stratovolcano.
The local Waarusha people maintain that the volcano is sacred.
Mount Meru is the topographic centerpiece of Arusha National Park. Its fertile slopes rise above the surrounding savanna and support a forest that hosts diverse wildlife, including nearly 400 species of birds, and also black and white colobus monkeys, giraffes, zebras and water buffalos.
The Summit of Mount Meru