VOLCANOES CHAPTER 10

• Viscosity – the resistance to the flow. As temperature decreases, viscosity increases.

• As silica content increases, viscosity increases. Therefore, continental (rhyolitic) volcanoes with high silica content are more explosive than basaltic volcanoes.

Dissolved Gases• Dissolved gas (mostly water vapor and

carbon dioxide) comes out of solution as the pressure decreases near the Earth’s surface.

• Continental eruptions are explosive because of the trapped gas. Basaltic eruptions like Hawaii, the gases escape and the lava flows quietly.

Volcanic Material• Lava Flows• Continental volcano lava, with high

silica content flows slowly.

• Basaltic volcano lava, like Hawaii, flows quickly. Types of lava flows are

pahoehoe and aa.

• Gases emitted by volcanoes are water vapor, carbon dioxide, nitrogen, sulfur, chlorine, hydrogen, and argon.

• Pyroclastic material are fragments ejected during volcanic eruptions. Types of pyroclastic material include ash, cinders(lapelli), blocks, and bombs.

Anatomy of a Volcano• Crater, Vent, Volcano

• Shield volcanoes – produced by the accumulation of basaltic lava, examples are Hawaiian Islands and Iceland

• Cinder cones – steep sloped, small, eruptions last only a short time

Composite cones – located along the Pacific Ring of Fire, examples are volcanoes in the Cascade region of the Pacific Northwest and the Andes

Large, silica-rich, most explosive eruption

Dangers from Composite ones• Pyroclastic flows• Lahars – mudflows of volcanic

material

Volcanic Landforms• Calderas – large depression in a

volcano, formed by a collapse of a craterCrater Lake is in a caldera

• Necks and pipes - formed from hardened magma in vents

• Lava Plateaus – formed from successive lava flows extruded by fissures over a large area

Intrusive Igneous Bodies – Plutons• Sills – forms when magma is injected

along sedimentary bedding planes• Laccoliths – more viscous lava

injected along sedimentary bedding planes• Dikes – form when magma is injected

into fractures• Batholiths – largest intrusive igneous

body,

• Origin of Magma – Geologists conclude that magma originates when essentially solid rock, located in the crust and upper mantle, partially melts.

• Source of heat to melt rocks– temperatures get higher with depth, 20 to 30 degrees Celsius per kilometer. Additional heat is generated by friction and rising magma

• Reducing pressure lowers a rock’s melting temperature

• Water causes a rock to melt at a lower temperature.

• Most volcanoes are located along the margins of the ocean basins at oceanic-continental convergent plate boundaries. As the oceanic plate subducts the continental crust, some rock melts. The magma rises, forming silica-rich volcanoes.

At oceanic-oceanic convergent plate boundaries a chain of volcanoes, volcanic arcs form.

At divergent boundaries, magma is produced along ridges during sea floor spreading.

Interplate igneous activity is associated with mantle plumes, or hot spots. Examples are Hawaiian Islands