What happens when volcanoes move away from a hot spot?
They get older and, because of erosion, the plate cools down and sinks.
Coral reef on volcano
Pahoehoe lava (rubbly lava) - relatively hot fluid
A'a lava (blocky lava) - relatively cold & more solid
Tunnels - outer part of magma, inner part keeps flowing
What is a volcano?
A vent at which molten rock, gas, and ash are emitted (erupted) from inside the Earth. A mountain built from erupted material is also called a volcano.
Magma & lava
Magma: molten rock inside the earth
Lava: magma that has erupted
Which type of igneous rock is volcanic?
Extrusive. Lava cools to form extrusive igneous rocks (basalt, andesite, rhyolite)
Gas includes mostly...
CO2, H2O, O2, SO2
What does ash consist of?
Glass shards formed from spray of lava that cools instantly.
Pyroclastic debris or tephra
Fragments of various shapes - pumice, ash, cinders, bombs, and blocks
Why do rocks melt?
Heat (increase in temperature), Decrease in pressure, Water (lowers the melting temp)
Locations for volcanoes
In general, they form above regions of melting.
3 Main Reasons for Volcanoes
1. Subduction zones (convergent plate boundaries
2. Mid-ocean ridges (most are submarine, along divergent plate boundaries)
3. Hot Spots
Away from plate boundaries, or superimposed on plate boundaries. A plume of hot mantle rises from near the core-mantle boundary. Decompression causes melting. Magma rise and eventually erupts at the surface. Plate drifts over hot spot so you get a chain of extinct volcanoes in the wake of an active one ("hotspot track")
Hot Spot Track
A chain of extinct volcanoes in the wake of an active one
The most violent eruptions
Those of high-viscosity rocks. In lavas, viscosity increases with silica (SiO2) content - felsic melts have higher viscosity than mafic melts. Felsic rocks have a lower melting temp as well.
Name 2 main ways eruptions can occur
1. Effusive (non-violent) eruptions
2. Explosive (violent) eruptions
Relatively quiet eruptions of lava, though lava can fountain at the vent. Rivers and pools of lava may form - rivers can flow fast. Most effusive eruptions produce basalt (mafic lava is less viscous).
Lots of gas pressure builds up (either gas separating from the magma or steam from water that gets into magma chamber). Volcano explodes and emits huge cloud of ash. 2 kinds of ash clouds exist. Some may rise high in the atmosphere. Alternatively, a nuee ardent (glowing avalanche) may form which is an avalanche of hot ash that runs down the flank of the volcano.
3 volcanic rock names
Name 5 types of volcanoes
1. Shield volcano
2. Cinder cone
3. Composite volcano
4. Volcanic dome
broad gentle cone, composed of layers of basalt lava.
small steep cone of tephra and ash. Forms from basaltic lava with a high gas content. Typically has a crater at the top.A crater is a small depression formed because eruptive material builds up around it.
(also called “stratovolcano”): large cone of alternating ash and andesitic lava.The lava armors the volcano so it doesn't erode away. Typically has a crater at the top.