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 crater Crater 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