Chapter 9, Problem 1RQ

To determine

The description regarding three different kinds of material that can erupt from a volcano.

Answer to Problem 1RQ

Lava is molten rock that flows out of the volcanic flanks in liquid state, cools, and solidifies on the Earth’s surface. Pyroclastic debris forms when molten lava chunks are ejected into the air and cools during flight to form volcanic glass; pyroclastic debris ranges in size from ash to lapilli to larger pieces termed as bombs. Blocks are fragments of pre-existing volcanic rocks from older eruptions that are hurled by the force of eruption. Lastly, the primary gases that erupt from volcanoes are water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide.

Explanation of Solution

A volcanic eruption occurs when hot lava, tephra, and a mixture of gases discharge through a vent.

There are three forms of product of an eruption, namely lava flows, pyroclastic debris, and gas. The term 'lava flow' is used for molten, moving layer of lava as well as for the solid layer of rock that forms when the lava freezes.

Lava flow:

Magma reaching the Earth's surface through fissures and volcanic vent is called lava. Lava flow is the movement of lava outpouring in a quiescent effusive eruption that cools and crystallizes on the surface of the Earth.

Lava behaves in various ways depending on its viscosity (resistant to flow) nature. Lava sometimes flows down the flanks of a volcano like a fast moving, incandescent stream, sometimes it builds into a rubble-covered pile at the summit of volcano, and sometimes it oozes out into blobs like a sticky and blistering paste. In general, lava is in a molten state, which flows out of the volcanic vent in relatively liquid state, cools down due to temperature change, and solidifies on the surface of the Earth.

Pyroclastic debris:

Pyroclastic debris are fragments of lava ejected forcefully into the atmosphere, cools, and solidifies mid-air soon after it reaches the land. Pyroclastic debris occur in various sizes ranging from volcanic ash which are less than 2 mm, pea-size fragments called lapilli and larger pieces called bombs and blocks. Volcanic bombs are made up of molten lava and are cooled once they are ejected, while the volcanic blocks are solid fragments of pre-existing volcanic rocks from old eruptions that are forcefully ejected out. Bombs contain polished surfaces while blocks are chunky angular fragments.

Volcanic gas:

Majority of magma contains mixture of dissolved gases that include water (H₂O), carbon dioxide (CO₂), sulfur dioxide (SO₂), and hydrogen sulfide (H₂S). These dissolved gases are released out of solution when the magma reaches the Earth’s surface.