1. Ch.6, Sec.3 – Causes of Volcanic Eruptions
The Formation of Magma
- changes in the pressure & temperature of the mantle cause magma to form
- soft, igneous rock remains a solid in the Earth because of the intense pressure caused by the weight of the rock above it
- magma is usually created in a liquid form at a plate boundary where the pressure is less and the magma can become less dense and rise to the surface

2. Ch.6, Sec.3 – Causes of Volcanic Eruptions
Where Volcanoes Form
- the majority (95%) of volcanoes form on the boundary between tectonic plates, like along the Ring of Fire

3. Ch.6, Sec.3 – Causes of Volcanic Eruptions
When Tectonic Plates Separate
- deep cracks, or rift zones, often will form at divergent plate boundaries
- most of the volcanic activity on Earth occurs at the mid-ocean ridges along a divergent plate boundary

4. Ch.6, Sec.3 – Causes of Volcanic Eruptions
When Tectonic Plates Collide
- as oceanic crust is subducted under a continental plate, the pressure and temperature increases
- the increase in pressure & temperature allows the water in the oceanic crust to be released
- once released, the water mixes with mantle rock, lowering the rock’s melting point, causing it to melt into magma

5. Ch.6, Sec.3 – Causes of Volcanic Eruptions
Hot Spots
- some volcanoes can occur in spots very far away from tectonic plate boundaries in areas called hot spots
- scientists are not sure if they form from cracks in the crust or from mantle plumes rising to the surface
- the Hawaiian Islands & Yellowstone National Park in Wyoming are underneath hot spots

6. Ch.6, Sec.3 – Causes of Volcanic Eruptions
Predicting Volcanic Eruptions
- the three classes of volcanoes are extinct, dormant, and active (scientists study dormant & active volcanoes)
- small earthquakes are a good sign a volcano will erupt soon
- changes in the gasses found in the magma (sulfur dioxide & carbon dioxide) also can signal a possible eruption
- we can also use GPS to monitor any changes in a volcano’s slope angle

7. Ch.6, Sec.3 – Causes of Volcanic Eruptions

8. Ch.6, Sec.3 – Causes of Volcanic Eruptions