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.
Understanding Volcanic Hot Spots & Mantle Plumes
Mantle rock is solid even though it is extremely hot. Deep underground, the immense weight of overlying rock keeps its minerals packed tightly enough to prevent much melting. When hot mantle rises, the pressure on it falls.
A small fraction can then melt and collect into magma. This magma is less dense than surrounding rock, so it continues upward through cracks and weak zones in the crust. Much of the magma cools below ground, where it forms igneous rock.
The portion that reaches the surface can erupt repeatedly from the same broad area over a very long time. Scientists use the term mantle plume for one possible source of this unusually hot rising material. Plumes are supported by several observations, but geologists still discuss whether every hot spot has the same deep origin.
The type of magma affects the shape and behavior of a volcano. Hot spot magma is commonly rich in basalt, a dark rock with relatively low silica content. Basaltic lava can flow easily across long distances before it cools.
Repeated thin flows build wide volcanoes with gentle slopes, called shield volcanoes. Hawaiian volcanoes are a well known example. As a volcano moves away from its magma supply, eruptions slow down and eventually stop.
Rain, waves, landslides, and weathering then wear the volcanic island down. The heavy volcano can make the oceanic crust sink gradually as well. An old underwater volcano is called a seamount.
If wave erosion once flattened its top near sea level, it is called a guyot. These landforms preserve a record even after an island has disappeared beneath the ocean.
Age patterns in a chain are useful, but they are not perfectly simple. Scientists determine volcanic rock ages mainly by radiometric dating. This method measures changes in radioactive atoms trapped in minerals after the rock cooled.
Dates from several locations can show whether age increases in one general direction. A sharp bend may record a major shift in the movement of a plate. The Hawaiian Emperor chain contains such a bend.
However, the pattern does not prove that the mantle source stayed completely still. A plume may drift slowly, volcanoes may form some distance from the main melting area, and parts of the chain may be buried or altered. Good interpretations compare rock ages, seafloor maps, earthquake data, and evidence from other plate boundaries.
When reading a hot spot diagram, first identify the active volcanic center and the direction of the chain. Then check whether the labeled ages become older along that direction. Keep the units consistent when calculating a rate.
A distance in kilometers divided by an age in millions of years gives kilometers per million years. This can be converted to centimeters per year if needed. One kilometer per million years is about one tenth of a centimeter per year.
Do not confuse a hot spot chain with a mid ocean ridge. At a ridge, new crust forms along a long boundary and spreads away on both sides.
At a hot spot, a moving plate carries volcanoes away from a more localized source. Both settings produce basalt, but their map patterns tell different stories about Earth’s moving surface.