Key Concept: Tectonic plate motions can result in volcanic activity at plate boundaries.

• Heat and pressure cause rocks to melt and form magma.
• Magma needs to get out, too much pressure!!!!!!
• Rise in temperature or drop in pressure causes magma to form faster.

• Magma is forced onto
Earth’s surface.
• It dries and hardens, this happens many times over thousands of years.
• Eventually a mountain called a volcano is formed.

Parts of a Volcano
• Most volcanoes share a specific set of features.
• The magma that feeds the eruptions pools deep underground in a structure called a magma chamber .
• At Earth’s surface, lava is released through openings called vents .
• Flowing lava in the interior travels through long, pipelike structures known as lava tubes .

Where do most volcanoes occur?
• Volcanoes occur at both divergent and convergent boundaries and also at hot spots.

Why do we have different types of
Volcanoes?
• The process of magma formation is different at each type of plate boundary.
• Therefore, the composition of magma differs in each tectonic setting.
• Tectonic settings determine the types of volcanoes that form and the types of eruptions that take place.

Volcanoes at Divergent Boundaries
• At a divergent boundary, the lithosphere becomes thinner as two plates pull away from each other.
• A set of deep cracks form in an area called a rift zone .
• Hot mantle rock rises to fill these cracks.
• As the rock rises, a decrease in pressure causes hot mantle rock to melt and form magma.
• The magma that reaches Earth’s surface is called lava .
Basaltic magma rises to Earth’s surface through these fissures and erupts nonexplosively

• Shield volcano: usually form at hot spots, from non explosive eruptions
• Cinder cone volcano: form from explosive eruptions, very steep.
• Composite volcano: form from both explosive and non explosive eruptions.

• Form from many layers of “runny” lava.
• Very wide, not to steep.
• Biggest type of volcanoes
• Tallest mountain in the world is Mauna Kea
(measures from sea floor to top)
• non explosive eruptions

• Smallest type of volcano
• Most common
• Made from pyroclastic material (material shot out of a volcano)
• Form a large crater
• Explosive!

• Eruptions alternate between explosive and non-explosive.
• Sometimes they have runny lava layers, other times the have pyroclastic materials form layers.
• Have a wide base and steep sides.
• Have a crater
• Mount Fuji

• Explosive: volcanoes that build enough pressure to blow its top, sending pyroclastic material into the air.
• Non explosive: Build only enough pressure to allow lava to run down its sides.

• Mafic: refers to rocks and magma rich in iron and magnesium.
• This type of lava that is very runny.
• As magma nears the surface there is little pressure, causing gasses escape easily.
• Magma low in Silica have quiet eruptions

• Felsic: means magma with high silica and feldspar content.
• Felsic magma traps water and gas bubbles, which leads to lots of pressure.
• Silica acts like a cork
• Explosive eruptions are caused by a build up of high pressure.
• Convergent zones contain lots of water, therefore have explosive eruptions.

• Material that is thrown into the air during an explosion.
• Volcanic bombs: large blobs of magma that harden in the air.
• Lapilli: pebble size rocks
• Volcanic ash: tiny powder like material

• Aa: lava that is thick and sharp
• Pahoehoe: lava that forms thin crust and wrinkles
• Pillow lava: lava that erupts under water, has a round shape
• Blocky lava: cooler, lava that does not travel far from eruption, jagged when it dries.

• lava that is thick and sharp

• lava that forms thin crust and wrinkles

• lava that erupts under water, has a round shape

• cooler, lava that does not travel far from eruption, jagged when it dries.