Basic lava, such as basalt, has low silica content and low viscosity. This allows the lava to flow easily over long distances before cooling and solidifying. Consequently, it builds up shield volcanoes characterized by broad bases and gentle slopes, rather than the steep-sided cones formed by more viscous, acidic lava types.
102
What materials typically form the alternating layers of composite volcanoes?
Composite volcanoes, also known as stratovolcanoes, are built up over time by repeated eruptions. They are characterized by their conical shape, formed by alternating layers of viscous lava flows, volcanic ash, and cinder (tephra). The high viscosity of the lava prevents it from flowing far, which contributes to the steep, layered structure of these volcanoes.
103
How are volcanoes that exhibit frequent eruptive activity classified?
Active volcanoes are those that have erupted in recent history or are currently showing signs of volcanic activity, such as gas emissions or seismic tremors. They are distinguished from dormant volcanoes, which are currently inactive but have the potential to erupt, and extinct volcanoes, which are unlikely to erupt again.
104
How does the high viscosity of lava affect the behavior of gases trapped within a volcanic system?
Viscosity refers to a fluid's resistance to flow. Highly viscous (sticky) lava acts as a barrier that traps volcanic gases, preventing them from escaping easily as the magma rises toward the surface. As the magma nears the surface and pressure decreases, these trapped gases expand rapidly. Because the thick lava prevents them from venting gradually, the pressure continues to build until it exceeds the strength of the surrounding rock, leading to explosive volcanic eruptions.
105
Which of the following is a well-known example of a volcano characterized by acid lava?
Acid lava, or felsic lava, is high in silica content, making it viscous and prone to explosive eruptions. Mount Mayon in the Philippines is a classic stratovolcano known for its explosive activity and steep slopes, which are typical of volcanoes formed by the accumulation of viscous, acidic lava flows.
106
What physical change occurs when molten lava comes into direct contact with seawater?
When lava flows into the ocean, the rapid temperature difference between the hot molten rock and the cold seawater causes the lava to cool and solidify almost instantly. This process creates new landforms, such as pillow lavas or volcanic islands, through the rapid hardening of the material.
107
How does the flow behavior of basic lava compare to that of acid lava?
Basic lava has a lower silica content, which results in lower viscosity compared to acid lava. Because it is less viscous, it flows more easily and travels a greater distance across the surface before it loses enough heat to solidify. Acid lava, being highly viscous, tends to pile up near the vent rather than flowing far.
108
What is the term for the underground reservoir of molten rock located beneath a volcanic vent?
A magma chamber is a large pool of liquid rock beneath the surface of the Earth. When pressure builds up, this magma can rise through a conduit or vent to the surface, resulting in a volcanic eruption. It serves as the primary storage area for magma before it is expelled.
109
How many primary categories are volcanoes typically classified into based on their activity levels?
Volcanoes are generally classified into three main categories based on their frequency of eruption: active, dormant, and extinct. Active volcanoes are currently erupting or have erupted in recent history. Dormant volcanoes are currently quiet but are expected to erupt again, while extinct volcanoes are unlikely to erupt in the future.
110
What geological landforms are created when the summits of underwater volcanoes rise above the ocean surface?
Volcanic activity on the ocean floor can lead to the accumulation of lava over time. When these volcanic structures grow high enough to breach the sea surface, they form islands. Examples include the Hawaiian Islands, which were formed by volcanic hotspots.
