1. Volcanic Activity

2. Magma Reaches Earth’s Surface
Magma rises because it is liquid and because it is hot and is less dense. It rises until it reaches the surface or is trapped.
A Volcano erupts because the dissolved gases are under pressure. When there is a crack, the gases rush out carrying magma.

3. Inside a Volcano

4. Characteristics of Magma
Thick magma (High Viscosity) is relatively low in temperature or high in silica content. (Granite)
Thin magma (Low Viscosity) is relatively high in temperature or low in silica content. (Basalt)

5. Types of Eruptions
Silica and gas pressure determine how explosive an eruption will be.

6. Quiet Eruptions
Quiet if magma flows easily. Example Mount Kilauea on Hawaii.
Two different types of lava.
1. Pahoehoe-fast moving hot lava. (smooth)
2. aa-cooler and slow moving lava. (rough)

7. Explosive Eruptions
If magma is thick, pressure build up until the volcano explodes.
The lava flies into air (pyroclastic flow) and down the volcano.

8. Lava breaks up into: Ash (size of sand) Cinders (size of pebbles)
Bombs (baseball to car)

9. Stage of a Volcano Active-Erupting or signs of near future eruption.
Dormant- Likely to erupt again. May be thousands of years between eruptions.
Extinct-Unlikely to erupt again.

10. Hot Spring

11. Geyser

12. Old Faithful – Yellowstone
Judge, then, what must have been our astonishment, as we entered the basin at mid-afternoon of our second day's travel, to see in the clear sunlight, at no great distance, an immense volume of clear, sparkling water projected into the air to the height of one hundred and twenty-five feet. "Geysers! geysers!" exclaimed one of our company, and, spurring our jaded horses, we soon gathered around this wonderful phenomenon. It was indeed a perfect geyser. The aperture through which the jet was projected was an irregular oval, three feet by seven in diameter. The margin of sinter was curiously piled up, and the exterior crust was filled with little hollows full of water, in which were small globules of sediment, some having gathered around bits of wood and other nuclei. This geyser is elevated thirty feet above the level of the surrounding plain, and the crater rises five or six feet above the mound. It spouted at regular intervals nine times during our stay, the columns of boiling water being thrown from ninety to one hundred and twenty-five feet at each discharge, which lasted from fifteen to twenty minutes. We gave it the name of "Old Faithful."
– Nathaniel P. Langford, 1871