Eruptions!

1. Eruptions!

2. Rift Eruptions • This type of eruption occurs along narrow fractures in the Earth’s crust. • Examples of eruption sites: • Mid-ocean ridges such as the mid-Atlantic ridge. • Rift valleys such as the East African Rift System. • This will eventually form a mid-ocean ridge as the rift valley becomes submerged.

3. Rift Eruptions

4. Rift Eruptions • Lava in a rift eruption is basaltic • It has a smooth and quiet flow. • It contains few gasses. • When the eruption occurs on land, it can cover thousands of square kilometers. • When the eruption occurs in water, it cools quickly into pillow-shaped rocks. • The Hawaiians have two names for this type of lava: • Aa: “stony rough lava” • Pahoehoe: “smooth unbroken lava”

5. Aa Lava

6. Pahoehoe Lava

7. Aa and Pahoehoe can appear at the same time and from the same eruption.

8. Pillow Rocks

9. Rift Eruptions • Basalt Plateau • A large amount of basaltic lava erupts, resulting in a basalt plateau. • Basically, this lava covers an area and hardens. • Since this lava flows evenly, the area that is covered now has a plateau there. • The Columbian Plateau: lava covers 200,000km2 with 1500m of basaltic lava.

10. Rift Eruptions • Basaltic lava flows often leave closely-packed, six-sided columns called columnar joining. • This is thought to be the result when shrinking lava cools, and cracks. • The columns are as tall as the lava flow is thick.

11. Basalt Plateau

12. Basalt Plateau The different layers of the plateau show different eruptions. The exposed rock shows the six-sided columns that are the result of the lava cooling.

13. Subduction Boundary Eruptions • At a subduction boundary, one plate is driven deep into the Earth. • As the plate is pushed deeper, it begins to heat up. • Eventually, the heat and pressure are so great that the plate melts and turns to magma. • Water that was trapped in the plate as it subducted (remember, subduction zones occur where there is an oceanic plate) heats up as well.

14. Subduction Boundary Eruptions • Magma from a subducted plate is called felsic magma due to its high silica content. • This type of magma is very viscous • It contains a lot of dissolved gasses, including water. • When it erupts, this type of magma produces a very explosive eruption.

15. Subduction Boundary Eruptions • There is less lava flow in an explosive eruption (as compared to a rift eruption) • Most of the erupted material is tephra (lava fragments) • It can also have a pyroclastic flow: A very hot and fast moving mixture of volcanic gasses and tephra. • This type of volcano has very steep sides.

16. Subduction Boundary Eruptions • As the name of the eruption implies, these volcanoes occur near subduction boundaries. • Examples include Indonesia, the Philippines, and Japan. • The Cascade Mountain Range in Washington and Oregon is also the result of a subduction boundary and the resulting volcanoes.

19. Hot Spots • Sometimes, volcanoes are created that are nowhere near plate boundaries. • These volcanoes are due to a process in the crust and mantle called a hot spot. • A hot spot is quite literally a hot spot in the earth’s crust and mantle.

20. Hot Spots • When lava erupts from a hot spot, it is similar to lava from a rift eruption. • The lava is smoothly flowing and has little dissolved gas in it. • This lava builds up a rounded volcano as it erupts and can eventually build an island.

21. Hot Spots • A hot spot works through this process: • Magma is less dense than the surrounding rock, so it pushes its way up to the surface. • If it is near the center of a lithospheric plate, it remains in the mantle. • One particular spot, however, seems to generate enough heat and pressure to push its way through the lithosphere.

22. Hot Spots • If the lithosphere moves, then the magma in the astenosphere remains stationary. • This can result in extinct volcanoes along the lithosphere as an active volcano is forming nearby. • Hawaii is an example of land forming through a hot spot (under Kilauea)

23. Hot Spots