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Volcanic Activity
Magma Reaches Earth’s Surface
1. 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.
2. A Volcano erupts because the dissolved gases are
under pressure. When there is a crack, the gases
rush out carrying magma.
Inside a Volcano
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)
Types of Eruptions
• Silica and gas pressure determine how
explosive an eruption will be.
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)
Explosive Eruptions
• If magma is thick, pressure build up until the
volcano explodes.
• The lava flies into air (pyroclastic flow) and
down the volcano.
Lava breaks up into:
1. Ash (size of sand)
2. Cinders (size of pebbles)
3. Bombs (baseball to car)
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.
Hot Spring
Geyser
Old Faithful – Yellowstone
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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