Volcanic hot spots are places where magma rises through a tectonic plate and creates volcanoes far from many plate boundaries. Mantle plumes are one explanation for these hot spots because they bring unusually hot rock upward from deep within Earth. This cheat sheet helps students connect hot spot volcanism to plate motion, island chains, seamounts, and Earth’s internal heat.
It is useful for interpreting maps, diagrams, and age data from volcanic island chains such as Hawaii.
The most important idea is that a hot spot can stay nearly fixed while a tectonic plate moves over it. The youngest volcano is usually closest to the active hot spot, while older volcanoes are farther away in the direction the plate has moved. Plate speed can be estimated using speed = distance / time when the ages and distances of volcanoes are known.
Hot spots provide evidence for both mantle processes and the direction and rate of plate motion.
Key Facts
- A volcanic hot spot is an area of long-lasting volcanism that may occur in the middle of a tectonic plate or near a plate boundary.
- A mantle plume is a rising column of hot, less dense mantle material that may partially melt as pressure decreases near the surface.
- Decompression melting occurs when hot mantle rock rises, pressure decreases, and some rock melts without needing a major temperature increase.
- In a hot spot chain, the youngest volcano is usually above or closest to the active plume, and older volcanoes are farther away.
- Plate speed can be estimated with the formula speed = distance / time, using distance from the active hot spot and the volcano’s age.
- The direction from the youngest volcano toward older volcanoes usually shows the direction of past plate motion.
- A bend in a hot spot island or seamount chain may show a change in plate motion direction over geologic time.
- Hot spot volcanoes often produce basaltic lava because the magma commonly comes from partial melting of mantle rock.
Vocabulary
- Hot spot
- A volcanic region where magma rises through the crust for a long time, often away from a plate boundary.
- Mantle plume
- A narrow, rising zone of unusually hot mantle material that may cause melting and volcanism near Earth’s surface.
- Decompression melting
- Melting caused when hot mantle rock rises and pressure drops, allowing some of the rock to melt.
- Seamount
- An underwater volcanic mountain that may form as a plate moves over a hot spot.
- Island chain
- A line of volcanic islands formed as a moving tectonic plate passes over a hot spot.
- Plate motion
- The movement of a tectonic plate across Earth’s surface, measured by direction and speed.
Common Mistakes to Avoid
- Assuming every volcano forms at a plate boundary is wrong because hot spot volcanoes can form within plates, such as the Hawaiian Islands.
- Reversing the age pattern in a hot spot chain is wrong because the youngest volcano is usually near the active hot spot and older volcanoes are farther away.
- Using speed = time / distance is wrong because plate speed is calculated as speed = distance / time.
- Forgetting unit consistency is wrong because distances and times must be converted correctly, such as kilometers and millions of years, before comparing plate speeds.
- Thinking the plume moves with the plate is wrong because the standard hot spot model treats the plume as relatively fixed while the plate moves over it.
Practice Questions
- 1 A volcano is 600 km from the active hot spot and is 12 million years old. What is the plate speed in km per million years?
- 2 Two extinct volcanoes in a hot spot chain are 450 km apart. Their ages differ by 9 million years. What average plate speed does this suggest?
- 3 In an island chain, the active volcano is at the southeast end and the islands get older toward the northwest. What is the plate motion direction?
- 4 Explain why a line of progressively older volcanoes can be used as evidence that a tectonic plate moved over a relatively fixed hot spot.