Type of Volcano:

Nevado del Ruiz is a composite volcano, also known as a stratovolcano. Stratovolcanoes, also known as composite cones, have gentle lower slopes, but get very steep near the summit, which give them the cone like shape. Nevado del Ruiz has an overall andesitic and dacitic structure made of layers (strata) of hardened lava, tephra, and volcanic ash. Nevado del Ruiz also has a layered appearance with alternating pyroclastic, mud, lava, and debris flows, and is created by an oceanic to continental convergent boundary.

Geologic Location:

Nevado del Ruiz is located on the Pacific Ring of Fire, is a part of the Andes mountains, and is the northernmost volcano of the Andean Volcanic Belt in Colombia, South America. It is a part of the Los Nevados National Park, west of the main city of Bogota. Nevado del Ruiz sits on a convergent plate boundary, and was formed by the subduction of the oceanic Nazca Plate beneath the continental South American Plate.

Eruption History:

Throughout its existence, Nevado del Ruiz has had 22 eruptions. Its earliest eruption occurred in 6660 BC in the Arenas Crater, its most recent in 1985 as a VEI 3 also in the Arenas Crater. A technique called Tephrochronology has been used to prove this very early eruption, and the first historically documented eruption occurred in 1570. Nevado del Ruiz's largest eruption occurred in 1595 in the Arenas Crater as a VEI 4, but the most well known, and the most recent was the 1985 eruption. As the worst volcanic disaster of the 21 century in South America, beginning at 3:06 pm, this eruption killed over 23,000 people. It consisted of hot pyroclastice flows and surges, as well as deadly floods and lahars that surged through the valleys. The eruption ejected ash for miles and a very small amount of magma, but generated high-volume debris flows that were the the killers of many. Mudflows and lahars traveled over 60 miles, buried whole towns, more than 5,000 homes, killed over 23,000 inhabitants, and injured over 5,000 people. The explosion also left 15,000 animals dead, 8,000 homeless, and the estimated cost of the eruption was 1 billion dollars. The town of Armero, which was closest to the volcanoes base, was hit the hardest. The whole town was buried and destroyed, and three quarters of the 28,700 people who lived there died. Since this lethal eruption, the volcano has has very limited volcanic activity, but the effects of the Nevado del Ruiz eruption can still be seen today.

Volcano Shape:

Nevado del Ruiz is a stratovolcano. Its summit elevation reaches 5,321 meters (17,457 feet), and covers 200 square km in Colombia. Its structure consists of andesitic and dacitic lavas, as well as andestic pyroclastics. Its modern cone includes a broad cluster of lava domes and calderas. At its summit sits its main crater, the Arenas crater, which is 1 km wide and 240 km deep. Nevado del ruiz's prominent cone is a pyroclastic cone calle La Olleta, and is located at its sw point. Although only 500 km from the equator, the uppermost portion of this volcanoe is covered with 25 square km of snow, ice, and glaciers. As a stratovolcano, is has many layers (strata) of hardened lava and tephra. Its modern cone consits of 5 lava domes: Nevado el Cisne, Alto de Laguna, la Olleta, Alto la Pirana, and Alto de Santano. Other hyrdothermic activity that goes on at Nevado del ruiz includes many hot springs and stream vents located on the volcano. The picture shown, shows the upper slopes of Nevado del Ruiz, where you can see scar paths that lahars have made form the 1985 eruption.
http://www.geology.wisc.edu/~g111/Volcanoes/Nevado_del_Ruiz/sw_10_Fx_Web.jpg Photo by Stan Williams.

Neighboring Volcanoes:

There are six major volcanoes in Colombia, the closest major one to Nevado del Ruiz being Tolimo. In between the two major volcanoes, both located southwest of Bogota, are two volcanoes, Santa Isabel and Quindio. Each of these volcanoes are a stratovolcano similar to Nevado del Ruiz, and are located in a region called Parque Nacional Los Nevados.
Below is a map from USGS showing the locations of the six major volcanoes in Colombia.

Eruption Style:

Nevado del Ruiz is an explosive, active, stratovolcano. Stratovolcanos form at a convergent plate margins, and have steep summit slopes and a surprisingly small crater. They are generally known as composite cones for their layered, cone shape appearance. They typically have a layered appearance with alternating lava flows, air fall tephra, pyroclastic flows, volcanic mudflows known as lahars, and debris flows. Nevado del Ruiz produces andesitic and dacitic lava, as well as andesitic pyroclastics. These pyroclastic and lava flows can abolish everything in their path, and travel for miles. Much debris and ash is also ejected from this volcano when it erupts. Lahars are one of the most deadly consequences of Nevado del Ruiz during an eruption. As hot pyroclastic flows sweep down the sides of Nevado del Ruiz, it causes a rapid melting of snow and ice and creates very large floods that sweep down the volcano and through valleys. As these floods flow more quickly, they pick up loose debris and soil, growing in density and volume to form extremely hot mood flows (lahars). These lahars can be as large as 50 meters thick and travel over 50 km per hour. They destroy everything surrounding them, and are the cause of thousands of deaths and destruction of cities. Many hazards come with a Nevado del Ruiz eruption, all harmful, and many deadly. Below is a picture of a lahar from Nevado del Ruiz's 1985 eruption.

http://membres.lycos.fr/lahars/resume2.jpg

Environmental Dangers:

What are the Environmental Dangers of Nevado del Ruiz?

Just as any other volcano, there are many dangers and threats that coincide with Nevado del Ruiz. When this volcano erupts, it can destroy everything in its path for miles. In the 1985 eruption, Nevado del Ruiz projected a huge downpour of ash on Armero, Columbia, and within and hour, a 130-foot wall of ash and mud swept through the city. This concrete flow destroyed homes, cities, cars, people, and many of the natural resources surrounding the area such as trees and crops. When the volcano erupts, pyroclastic flows can also melts snow on the mountain’s icecap and can send floods and mud flows down valleys. These Debris/mud flows create paths for the flows on the sides of the volcano known as Lahars. The eruptions also saturate and loosen the soil, and fill the air with ash and debris. The volcano poses the threat of mass destruction to the population and the environment in the surrounding area. There have been many lives lost, communities destroyed, and cities buried alive with the Nevado del Ruiz eruptions.