Volcanism and Extrusive Rocks

How do Volcanoes Interact with Earth's Systems?

How do Volcanoes Interact with Earth's Systems? Atmosphere- Earth's first atmosphere was created from gases released by volcanoes. Hydrosphere- Earth's water was likely produced by condensation of volcanic water vapor early in Earth’s history. Biosphere- Volcanoes have both positive and negative effects on the biosphere:

Living With Volcanoes THE GOOD! Tourism– Iceland, Hawaii, Mt. St. Helens Fertile Soils– Hawaii, South America Geothermal Energy- This natural steam provides a clean energy resource (Iceland).

Living With Volcanoes THE UGLY! Direct fatalities: In 1815, 92,000 people were killed by an eruption in Indonesia. Climate: Very large eruptions can result in measurable global cooling (1815 was "the year with no summer") resulting in crop failures and famines.

(AD79)

What is Lava? Lava- Magma that has reached Earth’s surface. It can flow form from a volcano vent, the side of a volcano, or directly from the ground.

How Does Lava Behave? Volcanic eruptions can be: Explosive and unpredictable (Vesuvius). Slow and predictable in nature (Hawaii).

Lava behavior (slow vs explosive) depends on… 1) Amount of dissolved gases (water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide). More gases = more explosive. 2) Viscosity (fluid’s resistance to flow), which depends on two factors: a) Silica (SiO 2 ) content. Higher silica (more felsic) magmas are more viscous than low silica magmas. More viscous = more explosive. b) Lava temperature. Cooler magmas are more viscous (and trap gases better) than hot magmas. Cool magmas = more explosive. Which is usually cooler, mafic or felsic magma?

Comparing Magmas FELSIC- Gases escape violently. MAFIC- Gasses escape easily.

Explosive Volcanoes: Pyroclastic Flows (Felsic Magmas) Pyroclast: Solid/solidified matter blown away from an explosive volcano during an eruption. Pyroclastic flow: Fast moving mixture of pyroclasts and gas (solids float on gas cushion). Temperatures can reach 1000 C (1830F). Speeds can reach 700 km/hr (450 mi/hr). The most deadly aspect of volcanic eruptions!

Volcanic Rock Compositions Can be: Felsic Mafic Intermediate

Volcanic Rock Textures Fine-grained Crystals < 1 mm (extrusive). Obsidian Glassy (no mineral crystals) due to extremely rapid cooling. Porphyritic rocks Large crystals (phenocrysts) surrounded by a fine matrix (groundmass). Indicates two-stage cooling history of magma. Common in andesites. Vesicular rocks Rocks that contain trapped gas bubbles.

Pumice Light to medium colored and porous (rhyolite or andesite) and partly glassy. Formed from very viscous lava in which gases could not escape. May float on water. Scoria Dark colored and porous (basalt). Pyroclasts Volcanic particles blasted apart by explosive eruptions. Dust and ash (<2 mm) Cinders/Lapilli (2-64 mm) Blocks (or bombs, if they are streamlined during flight) (>64 mm)

PYROCLASTIC TEXTURES Volcanic tuff Contains fine-grained pyroclasts (fragments of dust and ash). The hot pyroclastic fragments often weld together (welded tuff). Volcanic breccia Contains coarser-grained pyroclasts.

Parts of a Volcano Volcano-Topographically elevated body of extrusive rock. Forms as lava and/or pyroclastic debris pile up over time. Vent – Any opening through which lava erupts Crater - A basin-like depression over the vent at the summit of the volcano.

A Volcanic Crater

What is a Caldera? Caldera - A volcanic depression (> 1 km) much larger than the original crater (ex. Crater Lake in Oregon).

Caldera Formation A calderas will form if the summit blows off, or… The summit collapses into a vacated magma chamber.

Warmup What factors make a magma flow easily? What factors make a magma explosive?

The 3 Types of Volcanoes Shield Volcanoes- Formed by lava flows (mafic basalt). Cinder Cones- Formed by pyroclasts (felsic rhyolites). Composite Volcanoes (aka stratovolcanoes)- Formed by both flows and pyroclasts (intermediate andesites).

Shield Volcanoes (Hawaii) Large (km-sized) and 5-10 degree slope. Composed of solidified lava flows. Slopes are broad and gentle. Formed by low viscosity lava (usually basalt) that spreads out easily. Lava tube: Tunnel formed by molten lava flowing through lava flows that have already solidified.

Shield Volcanoes (Hawaii) Two types of lava flow basalts: Hot magma  pahoehoe. Cooler magma  aa.

Cinder Cones (Flagstaff) Small volcanoes (usually meters across) with slopes = A pile of loose pyroclasts ejected from a central vent. Slopes are steep. Formed from relatively viscous magma. Erode easily due to their unconsolidated nature.

Composite Volcanoes (Mt.St. Helens) Moderate size and 5-30 degree slope. Alternating layers of pyroclastic debris and solidified lava flows. Moderately to steeply sloping. Composed primarily of intermediate composition volcanic rocks (i.e., andesite). Lava flows result from the low viscosity lavas (hot). Pyroclastic flows result from higher viscosity lavas (cooler). Most common type of volcano at convergent plate boundaries.

What is a “lahar”? It is a mixture of pyroclasts and fluids (H 2 O emitted by volcanic steam). This causes a lahar, or deadly mud flow. Common in explosive volcanoes (cinder cones and composite/stratovolcanoes).

Ground Eruptions: Flood Basalts Very low viscosity mafic magma can seep out of the ground, forming horizontal layers of basalt of extremely large areas. These flood eruptions can create extensive lava plateaus (e.g., Columbia River Plateau and the Rio Grande Rift in New Mexico). In Siberia, extensive flood basalts coincide with Earth's largest mass extinction (the Permian extinction, 251 mya).

The Plates and Plate Boundaries There are 12 main lithosphere plates. Much geology happens at the boundaries between these plates!

Three Types of Plate Boundaries: 1) Transform Boundaries

2) Divergent Boundaries

3) Convergent Boundaries

Tying Volcanism to Plate Tectonics Divergent Boundaries MOR = Mafic = Basalt Convergent Boundaries Felic to Intermediate = Rhyolites + Andesites

Locations of Volcanoes Most volcanism occurs at plate boundaries, including: Divergent (mid ocean ridges)- Basalt Convergent (Subduction zones)- Mostly andesite.

The interiors of plates are mostly devoid of volcanoes. Most (80%) of the Pacific Rim population lives near volcanoes

Eruptions at Mid Ocean Ridges Basaltic (no time to evolve to felsic). Pillow structures indicate the lava flowed and solidified under water.

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