Physical Geology Chapter 4 Volcanism & Extrusive Rocks

 

Volcanism & extrusive rocks

 

Extrusive – lava or ash onto the Earth surface

 

pyroclastic

            pyro – fire

            clastic – small pieces of rock

              (same root word will be used to describe sediments)

 

Words to describe volcanic rocks

 

Table 4.1  Summary of textures in volcanic rocks    *** Know these ***

 

Fine-grained

Porphyritic

Obsidian

Vesicular

Pumice

Tuff

Volcanic breccia

 

Volcanic glass – obsidian

    Figure 4.9

 

Andesite porphyry

    Figure 4.10

 

Porphyry thin section

    Figure 4.10

Vesicular basalt    Figure 4.11

 

 

            Bubbles produced by expanding gas in the magma

Pumice    Figure 4.12

           

A volcanic glass full of bubbles

           

Usually rhyolitic (lots of silica and very viscous)

Volcanic bombs    Figure 4.13

Volcanic tuff    Figure 4.14

Volcanic breccia   

            Big chunks of broken rock in a matrix of ash

Types of volcanoes

 

 

 

 

 

 

 

 

 

 

    Table 4.2

            Another word for composite volcano:    stratovolcano

Shield volcano – created by basalt flows

           

 

Cinder cone – steep pile of coarse ash

 

 

Composite volcano – layered lava and ash

    andesite or rhyolite

    typically explosive eruptions

 

 

 

Differences between magmas

 

Basaltic lava flows easily    Figure 4.1

 

Rhyolitic and andesitic lavas tend to explode

    water & volatiles under pressure, viscous magma

 

 

Cascades
  volcanoes of the Pacific Northwest    Figure 4.5

 

 

  produced by subduction

 of the Juan de Fuca plate

  Juan de Fuca plate is young, hot, low density, buoyant

 

Composite or strato-volcanoes

 

Mt. Shasta, northern California

Mt. Fuji, Japan

Internal structure    Figure 4.21

 

 

 

 

 

 

Mount St. Helens before erupting in 1980

After eruption (7 years later)

  northeastern corner of the cone was blown away

  extensive ash deposition and pyroclastic flows

 

Mt. St. Helens eruption sequence    Box 4.1

 

Forming a dome – because of viscous lava    Figure 4.26

 

Pyroclastic flow – a cloud of hot gas and ash collapses and flows down the side

    Responsible for many of the volcano-related disasters in history   

Pompeii

Figure 4.7

 

 

Mt. Pelée  1902   

Martinique, West Indies  Figure 4.8

 

 

Crater Lake, Oregon

 – a caldera formed by collapse

Figure 4.4    forming a caldera

 

 

Crater Lake perspective view  from USGS bathymetry

 

Cinder cones

 

Shield volcanoes

produced by repeated basalt flows

Figure 4.16

Galapagos Islands

Low-relief slopes (because basaltic lava flows easily)

Hawaii – Mauna Loa and Mauna Kea

            Kilauea caldera

 

 

 

 

 

 

           

Southwest rift lava flows

 

Basaltic lava – Pahoehoe “pah hoy hoy”   

 (ropy texture)

 

 

 

 

 

 

 

 

 

 

 

 

 

            aa    “ah-ah”  partially cooled, crumbled surface texture

 

Crater and lava pool

            spattercone

            firefountain

 

Lava pool tectonics – a “rifting zone”

            a “triple junction”

 

Lava tube – flowing lava drains out of cooled outer tube

 

Basalt – columnar jointing    Figure 4.28

           

produces hexagons

            Note: based on triple junctions (120° angles)