Mount Pinatubo is an active stratovolcano

Stratovolcano

A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

 located on the island of Luzon

Luzon

Luzon is the largest island in the Philippines. It is located in the northernmost region of the archipelago, and is also the name for one of the three primary island groups in the country centered on the Island of Luzon...

, near the tripoint

Tripoint

A tripoint, or trijunction , is a geographical point at which the borders of three countries or subnational entities meet....

 of the Philippine provinces of Zambales

Zambales

Zambales is a province of the Philippines located in the Central Luzon region. Its capital is Iba. Zambales borders Pangasinan to the north, Tarlac and Pampanga to the east, and Bataan to the south. The province lies between the South China Sea and the Zambales Mountains. With a land area of...

, Tarlac

Tarlac

Tarlac is a landlocked province of the Philippines located in the Luzon Island. Its capital is Tarlac City. Tarlac borders Pampanga to the south, Nueva Ecija to the east, Pangasinan to the north, and Zambales to the west...

, and Pampanga

Pampanga

Pampanga is a province of the Philippines located in the Central Luzon region. Its capital is the City of San Fernando, Pampanga. Pampanga is bordered by the provinces of Bataan and Zambales to the west, Tarlac and Nueva Ecija to the north, and Bulacan to the southeast...

. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km (26.1 mi) west of the dormant and solitary Mount Arayat

Mount Arayat

Mount Arayat is an extinct stratovolcano on Luzon Island, Philippines, rising to a height of There is no recorded eruption of the volcano, and its last activity probably dates to the Holocene era.The volcano is located in a flat agricultural region at...

. Before 1991, the mountain has no recorded historical eruptions. It was heavily eroded

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

, inconspicuous and obscured from view.