Unit 4: Volcanoes
Why magma forms at all, what decides whether an eruption oozes or explodes, and how a volcano's shape reveals the tectonic setting underneath it.
6 topics · 18 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Why Magma Forms and Rises
- How it worksHow do decompression, flux melting, and heating each push solid mantle rock past its melting point?
- How it worksWalk me through why molten magma rises through solid rock instead of staying put underground.
- Common mistakeWhy isn't magma forming because the whole mantle is one giant sea of liquid rock?
Your turn1 of 2
Rock at a given depth beneath a mid-ocean ridge melts and rises as magma, but rock at that identical depth beneath a stable continental interior stays solid. Why does rock deep underground melt at a subduction zone or ridge but not at that same depth everywhere else?Magma Composition and Eruption Style
- How it worksHow does a magma's silica content control how thick, gassy, and explosive an eruption becomes?
- Going furtherWhat happens to an eruption's explosiveness as trapped gas gets less and less time to escape thick magma?
- Ties togetherHow does magma composition connect the type of plate boundary above it to the volcano's eruption style?
Your turn1 of 2
A basaltic lava flow in Hawaii oozes gently downhill for miles, while a rhyolitic eruption explodes violently. Why does one volcano erupt as a slow lava flow while another blows its top apart?Shield Volcanoes
- How it worksHow does thin, runny basaltic lava spread far before it cools, building a wide, low profile?
- Worked exampleFind the average slope of a shield volcano that rises 4 km over a base 120 km wide.
- Common mistakeWhy isn't a shield volcano's gentle slope a sign that it's a weak or harmless volcano?
Your turn1 of 2
A shield volcano rises 5 km over a base 125 km wide. What is its average slope, expressed as a percentage, rise divided by run times 100?Composite Volcanoes and Cinder Cones
- How it worksHow do alternating layers of thick lava and ash build up a composite volcano's steep sides?
- How it worksWalk me through how a single explosive eruption of cinders can pile up an entire cinder cone in months.
- Going furtherWhat happens to a composite volcano's shape after a violent eruption blows out one side of its cone?
Your turn1 of 2
A cinder cone builds up rapidly, growing roughly 4 meters in height per day of continuous eruption. Over 65 days of continuous eruption, about how many meters tall does the cinder cone grow?Volcanic Hazards
- How it worksHow does a pyroclastic flow of hot gas and ash outrun anyone trying to escape it?
- How it worksWalk me through how volcanic ash mixing with rain or melted snow becomes a destructive lahar.
- Common mistakeWhy isn't lava itself the deadliest part of most volcanic eruptions?
Your turn1 of 2
A pyroclastic flow of hot gas and ash can travel at speeds up to 164 kilometers per hour. If someone tries to flee on foot at 13 kilometers per hour, how many kilometers per hour slower is the fleeing person than the pyroclastic flow?The Link Between Plate Boundaries and Volcanoes
- How it worksHow does each of the three plate boundary types produce magma through a different melting process?
- Going furtherWhat kind of volcano would you expect to find above a mid-ocean ridge instead of a subduction zone?
- Ties togetherHow does the Ring of Fire connect a whole chain of subduction zones circling the Pacific Plate?
Your turn1 of 2
About 75 percent of Earth's active volcanoes above sea level sit along the Ring of Fire circling the Pacific Plate. Out of 618 active volcanoes surveyed worldwide, about how many would be expected to lie along the Ring of Fire?