Some Important Terms Related to Volcanoes – 2 (Pillow Lava, Pyroclastic Rocks, Cinder Cone, Repose, Volcanic Explosivity Index)

Volcanoes are awe-inspiring geological formations that have shaped the Earth’s landscape for millions of years. Understanding these terms is crucial for comprehending volcanic processes and their impact on the environment and human civilization.

Pillow Lava

Pillow lava is a type of lava that forms when molten lava is rapidly quenched by water, typically in underwater or subaqueous volcanic eruptions. The lava cools so quickly that it solidifies into distinct pillow-shaped structures. These pillow-like formations are commonly found on the ocean floor, especially along mid-ocean ridges, where volcanic activity occurs beneath the water.

• Example: The Juan de Fuca Ridge in the Pacific Ocean is an excellent example of an underwater volcanic system that produces pillow lava. As the tectonic plates move apart, magma rises to the seafloor, encountering cold seawater. The lava reacts with the water, quickly forming pillow-shaped rocks.

Pyroclastic Rocks

Pyroclastic rocks, also known as pyroclastics, are fragments of volcanic material that are ejected during explosive volcanic eruptions. These fragments can vary in size from tiny particles, like ash and lapilli (small rock fragments), to larger blocks and volcanic bombs. Pyroclastic rocks are often hot and can flow down the sides of a volcano at high speeds, causing devastating pyroclastic flows.

• Example: The eruption of Mount Vesuvius in 79 AD that famously destroyed the Roman cities of Pompeii and Herculaneum is a notable historical event associated with pyroclastic rocks. The volcano expelled a deadly mix of ash, pumice, and volcanic gas, burying the cities and preserving them for centuries.

Cinder Cone

A cinder cone is a small, steep-sided volcanic landform that usually forms from a single, relatively short-lived eruption. These cones are primarily composed of pyroclastic material such as volcanic ash, cinders, and volcanic bombs. Cinder cones typically have a bowl-shaped crater at the summit, and their slopes are steep due to the loose, fragmentary nature of the material from which they are formed.

• Example: Parícutin in Mexico is a famous cinder cone that appeared in a farmer’s field in 1943. The eruption began with the sudden appearance of a fissure that spewed ash, lava, and gases, eventually forming a cinder cone that grew to a height of over 1,000 feet in just one year.

Repose

In the context of volcanoes, repose refers to the period of inactivity or dormancy between volcanic eruptions. It is the interval during which a volcano remains quiet and shows no signs of eruptive activity. Repose periods can vary significantly between different volcanoes, ranging from several years to thousands of years.

• Example: Mount St. Helens in the state of Washington, USA, had a long repose period before its catastrophic eruption in 1980. The volcano had been dormant for over 120 years before it suddenly woke up and unleashed one of the most powerful volcanic eruptions in the history of the United States.

Volcanic Explosivity Index (VEI)

The Volcanic Explosivity Index (VEI) is a scale used by volcanologists to measure the explosiveness and size of volcanic eruptions. The index ranges from 0 to 8, with 0 representing non-explosive eruptions and 8 signifying colossal, super-eruptions. VEI takes into account various factors, such as eruption cloud height, volume of erupted material, and duration of the eruption.

• Example: The eruption of Mount Pinatubo in the Philippines in 1991 was a VEI 6 event. It released approximately 10 billion tons of magma, producing an ash cloud that reached heights of 34 kilometers (21 miles) into the atmosphere. This eruption had significant global climatic effects, temporarily lowering the Earth’s temperature.

The following table represents Volcanic Explosivity Index (VEI) Categories and Associated Eruption Characteristics

Understanding these important terms related to volcanoes is essential for comprehending the diverse and fascinating world of volcanic phenomena. Pillow lava, pyroclastic rocks, cinder cones, repose, and the Volcanic Explosivity Index all play crucial roles in shaping our planet’s geology and impacting the environment and human civilization.