FOUNDATIONS OF GEOLOGY CHAPTER 2

FOUNDATIONS OF GEOLOGY CHAPTER 2

2.3A- VOLCANOES Volcanoes all share common characteristics. Vent- a central channel through which gases, ash and rock are ejected. Magma- molten rock which lies several kilometers beneath earth s surface. Magma Chamber- reservoir of magma. The source of the eruption. Cone- formed when materials ejected from the volcano accumulate to form a conical mountain. A bowl shaped crater often caps the cone.

TYPES OF VOLCANOES Volcanoes are classified according to the materials from which their cones are made of. Cinder-Cone Volcanoes- consist primarily of erupted volcanic ash and rock fragments called cinders. This cone is composed of loose materials and does not grow as large as other volcanoes. Erosion occurs rapidly. Most are formed by a single eruptor which may last several years.

TYPES OF VOLCANOES Shield Volcanoes- pours out large amounts of highly fluid lava in mild eruptions. Produces a broad, gently sloping cone. The slopes of a shield volcano are usually built up over time and flow after flow of lava. Eruptions occur at central vents and from fissues along the volcano s sides (called flanks).

TYPES OF VOLCANOES Composite Volcanoes- produce both cinder and lava. Also known as stratovolcanoes. Produce symmetrical, widebased mountains that consist of alternate layers of lava and cinders. Cone is formed from eruptions from the crater and mild lava flows out of cracks in the volcanoes flanks. Produces a steeper cone than a shield volcano.

VOLCANIC ACTIVITY Scientists classify volcanoes as active, dormant, or extinct, according to their level of activity. Active- when a volcano has erupted within recorded history or is currently producing seismic activity. Dormant- an inactive volcano that is believed to have the potential to erupt again. Extinct- a volcano that will probably not erupt again.

VOLCANIC ACTIVITY Almost all of the earth s volcanoes are concentrated into three main belts that coincide with the earths earthquake belts. More than half of the world s active volcanoes are in the Ring of Fire. Another belt corresponds with the Alpide earthquake belt, The third belt is associated with the mid-atlantic ridge.

2.3 B- VOLCANIC ERUPTIONS Volcanoes rarely erupt without warning. The first sign of an impending eruption is when part of a volcano swells and bulges. Minor earthquakes may also precede the eruption by a few days or hours.

VOLCANIC ERUPTIONS The first explosions are usually of volcanic gases, steam and dust. Eruptions may occur at intervals, or a continuous eruption may follow. Volcanic eruptions may continue for weeks, months, or years until the volcano is quiet once again. Like earthquakes, violet eruptions by undersea or island volcanoes can generate huge tsunamis.

VOLCANIC ERUPTIONS Not all volcanoes erupt explosively. The violence of a volcano s eruption depends on two factors: The viscosity ( syrupiness ) of the magma. Less viscous magma is thin and flows easily out of the cone. Thicker magma does not flow easily and results in more violet eruptions. The amount of dissolved gas contained in the magma.

EJECTA Ejecta- any substance emitted by a volcano. Gases- can be poisonous. Lava- molten rock. Glows bright yellow initially, then turns red-orange, to dull red, then to a black/gray as it solidifies, forming volcanic rock. Pahoehoe- lava that hardens to form a smooth or ropy surface. AA- lava that hardens into rough, jagged rocks with a crumbly texture. Pillow Lava- lava that is emitted underwater or flows into water before solidifying.

EJECTA Pyroclasts- particles or blocks of solid volcanic ejecta. Volcanic Ash- fine pyroclasts that are light enough to be carried by wind. (less than 2 mm) Lapilli- larger volcanic ash particles (less than 64 mm) Volcanic Blocks- form when lava is hardened within the volcano and is ejected in a solid state. Solid and irregularly shaped. Volcanic Bombs- form when lava is through high into the air in a liquid or semiliquid state and hardens into rock before it hits the ground. Almond or tear shaped. Pyroclastic Flow- a superheated cloud of gas and volcanic ash that travels down the volcanic slope as an avalanche.

EJECTA Volcanic Explosivity Index- scale from 0 to 8 used to measure the explosiveness of a volcano s eruptions. Each number is 10x more explosive than the previous.

VOLCANIC STRUCTURE Calderas- huge bowl-shaped craters. Formed when magma chamber is emptied and collapses. Lava Tunnels- form when the surface of a large lava flow hardens, but the lava beneath continues to flow.

VOLCANIC STRUCTURE Igneous Intrusions- masses of volcanic rock beneath earth s surface. Dike- when magma hardens in a vertical crack or fissure. Sill- when magma squeezes between two horizontal layers of rock, forming a sheet-like mass. Laccolith- a dome like intrusion formed by thick, stiff magma that does not flow easily. The uplifted rocks above laccoliths form round hills or domed mountains. Batholiths- larger versions of laccoliths.