02/20/2026

 

Samuel Clifford

 

Chapter 5: Volcanoes and Volcanic Hazards

 

5.1

 

On May 18, 1980 one of the biggest geological disasters occurred in North America. Mount St. Helens in Washington State erupted in a violent fashion that changed the entire landscape of the volcano and lowered its summit 1,350 feet. It was catastrophic, stripping trees of branches and bark with large mudflows and claimed 59 lives. Ash and debris was ejected 11 miles upward into the stratosphere. Yet, this catastrophic example of volcanic eruptions is not the rule. Hawaii’s Kilauea Volcano has generally calm outpourings of lava. There are few to no explosions and is minimal in damage especially compared to Mount St. Helens’ eruption. 

 

5.2

 

Magma, molten rock, can contain varying amounts of dissolved gas. Erupted magma is called Lava. Mafic, intermediate, and felsic magmas differ mainly in their silica content, mineral makeup, gas levels, and eruption temperatures. Mafic magmas are low in silica, rich in iron‑ and magnesium‑bearing minerals, and produce dark rocks that erupt at the highest temperatures with relatively little dissolved gas. Felsic magmas contain much more silica, are dominated by light-colored minerals like quartz and potassium feldspar, trap the most gas, and erupt at lower temperatures. Intermediate magmas fall between these two extremes in composition, gas content, and eruptive behavior.

 

Quiet (non-violent) eruptions are called effusive eruptions (effus = pour forth). Explosive eruptions are at the other end of the spectrum. The two factors that determine the way magma erupts are viscosity and gas content. 

 

Viscosity is a measure of how mobile a fluid is. The greater viscosity a fluid has the more resistance it has to flowing. Maple Syrup is more viscous than water as it is more resistant to flow. The viscosity of magma is dependent on its temperature and silica content. The more silica in magma the more viscous it is. Normatively, the higher the temperature of a liquid the less viscous it will be and the cooler the temperature the more viscous it will be. 

 

Volcanic eruptions are also strongly influenced by how much dissolved gas a magma contains and how easily that gas can escape. High pressure deep underground keeps gases, mainly water vapor and carbon dioxide, dissolved in the melt, but as magma rises and pressure drops, those gases come out of solution just like carbonation escaping from an opened soda. Magma composition controls this behavior: fluid mafic magmas hold relatively little gas, while stickier, silica‑rich magmas trap much more, making them far more explosive when the pressure is finally released.

 

5.3

 

Aa and pāhoehoe are two basaltic lava‑flow styles that reflect how fluid the molten rock is as it moves across the ground. Pāhoehoe is hot, very fluid, and travels smoothly, forming thin, ropy, billowy surfaces that look almost like stretched taffy. Aa is cooler and more viscous, so it advances slowly as a jumble of sharp, broken clinker; the surface is rough, jagged, and noisy as the flow front grinds forward. Both come from the same magma, but differences in temperature, gas content, and flow rate determine whether a lava stream stays silky and ropy like pāhoehoe or breaks apart into the rubble-like texture of aa.

 

Block lava and pillow lava form in very different environments, and their textures reflect the conditions under which they solidify. Block lava is the silicic, high‑viscosity counterpart to aa: it forms in cooler, thicker andesite or rhyolite flows that can’t stretch or fold, so the surface fractures into large, angular slabs that ride atop a slow, creeping interior. Pillow lava forms when basalt erupts underwater; the outer skin quenches instantly into glass, trapping the still‑molten interior, which then bulges outward into rounded, interconnected “pillows.” Each pillow breaks through its chilled rind to form the next, creating a stack of bulbous, lobate forms that record rapid cooling and submarine eruption.

 

Volatiles are the dissolved gases, primarily water vapor, carbon dioxide, and sulfur‑bearing compounds, that are trapped within magma and profoundly shape how it behaves as it rises and erupts. Under high pressure at depth, these gases stay dissolved, but as magma ascends and pressure drops, they exsolve into bubbles, expanding dramatically and driving everything from gentle lava fountaining to catastrophic explosive eruptions. Magmas rich in volatiles tend to fragment violently because the expanding gas shreds the melt into ash and pumice, while low‑volatile magmas can ooze quietly as lava flows. In essence, volatiles are the hidden energy source of volcanism, controlling buoyancy, viscosity, eruptive style, and the entire spectrum of volcanic hazards.

 

5.4

 

Volcanic activity normally begins when a crack develops in the earth's crust. The crack is called a fissure and magma from below usually causes it as the magma moves forcefully toward the surface of the earth. The magma is usually localized in its movement into a pipe-shaped conduit that ends at a vent. A volcanic cone is sometimes created by the successive eruptions of lava. At the summit of most volcanic cones is a funnel-shaped depression called a crater. A Caldera is “a large depression typically caused by collapse or ejection of the summit area of a volcano” (Tarbuck, Lutgens, and Linneman 144).

 

5.5

 

Shield volcanoes are broad, gently sloping volcanic structures built almost entirely from low‑viscosity basaltic lava that can flow long distances before cooling. Their eruptions tend to be effusive rather than explosive because the magma contains relatively little dissolved gas and moves easily through the crust. Over thousands to millions of years, repeated outpourings of these fluid lava flows stack into wide, dome‑like profiles that resemble a warrior’s shield laid on the ground. Many of the largest volcanoes on Earth, and even in the solar system, fit this category, including the volcanoes of Hawai‘i such as Mauna Loa, whose immense size comes from countless overlapping basalt flows spreading outward in all directions.

 

5.6

 

Cinder cones are small, steep‑sided volcanic structures built almost entirely from loose fragments of basaltic lava, called cinders or scoria, that are explosively ejected from a single vent. As these gas‑rich lava blobs cool in the air and fall back around the opening, they accumulate into a cone with a classic bowl‑shaped crater at the top. They tend to erupt only once or a few times, producing short‑lived but visually striking volcanoes that rarely exceed a few hundred feet in height. Because they form quickly and from relatively simple eruptive processes, cinder cones are among the most common and easily recognized volcanic landforms on Earth.

 

5.7

 

Composite volcanoes, also called stratovolcanoes, are large, steep‑profiled volcanoes built from alternating layers of viscous lava flows, ash, pumice, and volcanic debris. Their magma is typically andesitic to rhyolitic, meaning it’s thicker and more gas‑rich than the basalt feeding shield or cinder cone eruptions. That viscosity traps pressure, so these volcanoes often produce powerful, explosive eruptions capable of generating pyroclastic flows, ash columns, and lahars. Because they grow through many eruptive cycles over long periods, they develop the classic cone shape seen in volcanoes like Mount St. Helens or Mount Fuji. Their layered internal structure reflects this history of alternating quiet lava effusion and violent fragmentation, making them some of the most complex and hazardous volcanic systems on Earth.

 

Chapter 11: Earthquakes and Earthquake Hazards

11.1

 

An earthquake occurs when the ground shakes because one block of rock suddenly and rapidly slips past another along fractures in the Earth’s crust known as faults. Most of the time, a fault doesn’t move because the pressure from the rocks above keeps it tightly squeezed shut. But as stress builds up inside the crust, it eventually becomes stronger than the friction holding the rocks in place, so the fault suddenly slips, and that slip is an earthquake. Where the rock begins to slip is called the hypocenter or focus. The location on the Earth's surface that is directly above the hypocenter is called the epicenter. Earthquakes that are large then release large amounts of stored-up energy as seismic waves which travel through the lithosphere and interior of the Earth.

 

These waves spread out in all directions from the hypocenter and cause the material that transmits these waves to vibrate. Even though thousands of earthquakes occur everyday, only around 15 are strong earthquakes without a magnitude of 7 or greater. The cause of earthquakes was not determined by scientists until H. F. Reid studied the earthquake that occurred in San Francisco in 1906. He noticed that the Pacific plate lurched 32 feet northward past the adjacent North American plate. Over long periods, stress builds up on both sides of  a fault which slowly bends the crust. Friction along the fault keeps it from slipping even as the stress increases. Eventually, the accumulated stress exceeds the frictional resistance and the fault suddenly slips causing the bent rock to resume its original shape. The rapid springing back releases energy and is what was called “elastic rebound” by H. F. Reid. 

 

Foreshocks sometimes occur an uncertain amount of time prior to an earthquake. It can be days to several years and they are like small earthquakes. After a strong earthquake there are Aftershocks which are earthquakes of lesser magnitude. Aftershocks are caused by the stress that was added to the rocks on both sides of the fault by the initial earthquake.

 

11.2

 

Seismology is the study of earthquake waves and dates back almost 2000 years to China where they attempted to determine the direction the waves were coming from. In modern times, seismographs or seisometers are instruments used to record earthquakes. Seismographs work by anchoring a heavy mass in place while the ground, and the rest of the device, moves during seismic waves. Because the mass stays still due to inertia, the relative motion between the mass and the frame is traced onto a rotating drum or digital sensor, creating a seismogram. This record shows how strong the shaking was and how it changed over time, allowing scientists to determine an earthquake’s size, location, and characteristics. Seismograms reveal two major types of seismic waves:

 

1. Body Waves which travel through the Earth’s interior

 

2. Surface Waves which travel in the rock layers just below the surface of the Earth

 

Body waves have two main types. P‑waves (primary waves) are the fastest and move by compressing and expanding the material they travel through, like a push‑pull motion; they can move through solids, liquids, and gases. S‑waves (secondary waves) are slower and move the ground side‑to‑side or up‑and‑down, but they can only travel through solids because liquids can’t support their shearing motion. 

 

11.3

 

Scientists determine an earthquake’s epicenter by comparing seismic records from several different seismograph stations. Each station measures how long it takes for the faster P‑waves and the slower S‑waves to arrive, and the time gap between them reveals how far that station is from the earthquake. Using that distance, scientists draw a circle around each station with a radius equal to how far away the quake occurred. When at least three circles are drawn, they intersect at a single point on the map, and that point is the earthquake’s epicenter.

 

11.4

 

Intensity and magnitude describe two different ways of measuring an earthquake. Magnitude is the amount of energy an earthquake releases at its source, calculated from seismic data; it is a single number that does not change no matter where you are. Intensity, on the other hand, describes how strongly the earthquake is felt at different locations on the surface, so it varies from place to place depending on distance from the epicenter, local geology, and building structures. In short, magnitude measures the earthquake’s actual power, while intensity measures its effects on people and the environment.

 

11.5

 

Within the 12 to 30 miles of the epicenter of an earthquake the degree of shaking experienced within that region is roughly the same. However, the vibrations beyond that limit begin to diminish rapidly. The amount of damages caused to man-made structures by earthquakes varies due to three major factors:

 

1. Intensity and Duration of Vibrations

 

Earthquakes that are strong have longer durations. This causes more potential for structural damage.

 

2. The Construction Practices of the Region

 

Steel rods are necessary to the safety of a building during an earthquake. The buildings are reinforced and normally don’t have any damage to the structure after an earthquake. Countries without these practices are more prone to damage.

 

3. The Nature of the Material on Which Structures Rest

 

The type of ground beneath a building greatly affects how much shaking it experiences during an earthquake. Solid bedrock tends to shake less and provides a more stable foundation, while loose materials like sand, clay, or water‑saturated soil can amplify seismic waves and make the shaking much stronger. In some cases, soft, wet ground can even behave like a liquid, a process called liquefaction, which causes buildings to tilt or collapse. Because of this, the stability of the underlying material is a major factor in how much damage an earthquake can cause.

 

The greatest earthquake-related damage is usually caused by landslides. Destructive landslides happen when rock, soil, or debris suddenly moves downhill, often triggered by heavy rain, earthquakes, or unstable slopes. They can wipe out roads, homes, and vegetation in seconds because the fast‑moving material carries enormous force and sweeps away anything in its path.

 

A tsunami is a series of large ocean waves usually caused by underwater earthquakes, landslides, or volcanic eruptions that suddenly displace huge amounts of water. These waves travel across the ocean at high speeds and grow dramatically in height as they reach shallow coastal areas, where they can flood shorelines, destroy buildings, and sweep away anything in their path.

 

11.6

 

“The zone of greatest seismic activity, called the circum-Pacific belt, encompasses the coastal regions of Chile, Central America, Indonesia, Japan, and Alaska…” (Tarbuck, Lutgens, and Linneman 329).