Lesson 4: Intrusive and Extrusive Igneous Rocks

Lesson 4

Intrusive and Extrusive Igneous Rocks

Mount Pinatubo, Philippines

Mount Pinatubo is located just 54 miles northwest of Manila in the Philippines.   Mt. Pinatubo has been dormant for 500 years and belongs to the Cabusilan sub range which consists of Mt. Natib, Mariveles Mountain and Mt. Arayat. They are all Subduction volcanoes resulting from the Eurasian Plate sliding under the Philippine Mobile Belt along the Manila Trench. Before the eruption in 1991 the mountain had an elevation of 5,725 feet. The US had one of it's largest navel bases, Clark Air Base, at the base of the mountain.

On April 2, 1991 Mt. Pinatubo awoke, with phreatic eruptions near the summit along a mile long fissure.

On June 14, at 08:41, after several hours of small earthquakes, an eruption occurred with a column reaching 24 kilometers.  3 hours later the earthquakes began and continued until 13:09 when an eruptive blast generated a 21km (13 mi) high eruption.  The mountain had sent it's last warning, it was time to come alive.

Eruption on June 14 at 13:09

On June 15, 1991 the second largest volcanic eruption of the 20th century occurred. At 13:42 the massive eruption of Mt. Pinatubo began.  The summit collapsed and created a caldera of 1.6 miles in diameter causing several major earthquakes.  The peak of the mountain was reduced 853 feet to 4,872 feet. All the seismographs close to Clark Air Force Base had been rendered completely inoperative by 14:30 by super-massive pyroclastic surges.

Eruption on June 15, 1991

Measurements show that ash was ejected 21 miles high during the most violent phase of the eruption.  The entire eruption lasted three hours and the pyroclastic surges pouring from the summit reaching almost 10 miles away.

Unfortunately, the volcano wasn't the worst part of the day.  On that same day, Typhoon Yunya was passing just 47 miles north of the Volcano.  The typhoon rains mixed with the ash deposits causing more destruction with mud and massive lahars.

20,000 people were evacuated and only approximately 800 were killed.  10,000 people were left homeless. The combination of the Typhoon and ash destroyed most structures. The release of 17 megatons of sulfur dioxide into the atmosphere caused a global cooling of 0.5 degrees C between 1991-1993.

Mt. Pinatubo Hut

Mt. Pinatubo Caldera

Below is a link to a video on YouTube.  It goes into a lot of detail about the eruption.  The main eruption begins about minute 22:00.
https://youtu.be/x90v_cpAZI8