Earth System Hazards: Earthquakes, Volcanoes, and Ground Subsidence

This lesson explains three major geologic hazards: earthquakes, volcanic activity, and ground subsidence. Using information from Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes, we can distinguish how seismic waves move, describe important volcanic processes and impacts, and recognize how sinkholes and subsidence develop and threaten people, buildings, and infrastructure.

1. Earthquakes and Seismic Wave Behavior

Earthquakes release energy that travels through Earth as seismic waves. A key part of understanding earthquake hazards is distinguishing the different kinds of waves and how they move materials.

P and S waves

• P waves travel through Earth as compressional waves. They move material in the same general direction as the wave travels.
• S waves, also called shear or secondary waves, move material at right angles to the direction of travel. According to Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes, S waves produce an up-and-down or sideways shear motion, similar to a jump rope whipping back and forth.

S waves are especially important because they can travel only through solid materials. The source states that they move more slowly than P waves and have an average velocity through Earth’s crust of about 3 km per second. This difference in speed and motion helps scientists interpret earthquake behavior and Earth’s internal structure.

Surface waves

The source also describes surface waves as producing a rolling motion along the land surface. In this motion, atoms in the solid Earth vibrate at right angles to the direction the wave is moving. Because these waves affect the ground surface directly, they are often strongly associated with damaging shaking experienced by communities.

Why wave behavior matters

• Different wave types arrive at different times.
• They shake the ground in different ways.
• Surface motion can strongly influence damage to structures and infrastructure.

In short, earthquake hazard analysis depends on understanding that seismic waves differ in speed, direction of particle motion, and the materials through which they can travel, as described in Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes.

2. Volcanic Activity and Its Impacts

Volcanoes are hazardous not only because of lava and explosions, but also because of ash, heat, and interactions with surrounding environments. The source highlights the 2010 eruption of Eyjafjallajökull in Iceland as an example of how volcanic activity can produce wide-reaching impacts.

Warning signs of activity

According to Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes, increased seismic activity around Eyjafjallajökull in December 2009 signaled that the volcano was awakening. This shows that earthquakes and volcanic hazards are often connected: movement of magma may be accompanied by seismic unrest.

Explosive ash eruptions

The source explains that the 2010 eruption was notable because the height of the ash plume was surprising. Earlier eruptions over the previous 1500 years had been comparatively small, but this event sent ash high into the atmosphere. Large ash plumes are especially hazardous because they can spread far beyond the volcano itself.

Human and economic impacts

The eruption disrupted modern transportation on a massive scale. The source reports that it caused the greatest disruption to air traffic in Europe since World War II, with about 95,000 flights canceled. Hundreds of thousands of travelers were stranded, and the airline industry lost more than a billion dollars in one week. This demonstrates that volcanic hazards can create severe social and economic consequences even far from the eruption site.

Volcano-ice interactions

Many of Iceland’s large volcanoes lie beneath thick glacial ice caps. The source notes that Eyjafjallajökull itself is associated with a glacier, and that during eruptions, heat escaping from Earth’s interior can interact with this ice. This is an important reminder that volcanic impacts depend partly on local environmental conditions. Ice, water, and rock can combine to make an eruption more complex and hazardous.

Main volcanic hazards recognized in the source

• Seismic unrest before eruption
• Explosive ash production
• Disruption of transportation systems
• Major economic losses
• Additional hazards where volcanoes interact with glacial ice

3. Ground Subsidence and Sinkholes

Ground subsidence is the sinking or settling of the ground surface. One dramatic form of subsidence is the sinkhole, which can form gradually or collapse suddenly. The examples in Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes show that subsidence hazards are often tied to both natural geologic conditions and human activities.

How sinkholes develop

The source describes cases in which underlying limestone is dissolved, creating underground voids. If the ground above those voids weakens enough, collapse can occur. Water is a key factor because it can continue dissolving limestone and enlarging openings below the surface.

The role of human activity

Human actions can hide or worsen subsidence hazards. In the Lehigh Valley of eastern Pennsylvania, a sinkhole identified in the 1940s was filled with stumps, asphalt blocks, and trash, then covered with a cornfield. Although the depression was no longer recognizable as a sinkhole, it still remained a hazard.

The source explains that runoff from nearby pavement and buildings collected in the filled area. This runoff loosened the plug of soil and trash and continued dissolving the underlying limestone. At the same time, increased demand for groundwater lowered the groundwater table. Together, these conditions contributed to a catastrophic collapse.

Example of rapid growth

The source also describes the 1981 Winter Park, Florida, sinkhole, which grew rapidly over three days. It swallowed part of a swimming pool as well as businesses, houses, and vehicles. This example shows how fast subsidence hazards can become disasters in developed areas.

Why subsidence is dangerous to communities

• It can damage homes, roads, and utilities.
• It may appear suddenly after years of hidden underground change.
• Urban runoff and groundwater withdrawal can increase the risk.
• Previously filled or buried sinkholes may still remain unstable.

4. Connecting the Hazards

These geologic hazards differ in process, but they share an important pattern: natural Earth systems can become especially dangerous where people build, travel, and extract resources.

• Earthquakes threaten communities through ground shaking produced by different seismic waves.
• Volcanoes threaten communities through ash, eruption-related seismicity, and wide-ranging disruption to transportation and economies.
• Subsidence and sinkholes threaten communities when dissolution, groundwater changes, and surface runoff weaken the ground beneath developed land.

As shown in Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes, understanding the physical process behind each hazard helps explain why impacts vary from place to place and why risk often increases when human systems interact with unstable natural conditions.

5. Key Takeaways

• S waves are shear waves that move material at right angles to wave travel and can move only through solids.
• Surface waves create rolling motion along the land surface and are closely tied to damaging ground shaking.
• Volcanic eruptions may be preceded by seismic activity and can produce far-reaching ash hazards.
• The 2010 Eyjafjallajökull eruption shows that volcanic hazards can disrupt transportation and cause major economic losses.
• Subsidence and sinkholes commonly develop where water dissolves underlying rock such as limestone.
• Human activities, including burying old sinkholes, increasing runoff, and lowering groundwater tables, can worsen collapse hazards.

Overall, Natural Hazards: Earth’s Processes as Hazards, Disasters, and Catastrophes shows that effective hazard understanding begins with recognizing the underlying Earth processes and how they interact with human environments.