Landform Geography

Earthquakes and VolcanoesJuly 9, 2013

Earthquakes

• Sudden release of tectonic stress creates movement in Earth’s crust & shockwaves through lithosphere

Earthquake Processes

Earthquake Energy• Waves released by an earthquake:

– P-waves – primary, compressional waves that travel 1.5-8 km/sec

– S-waves – secondary, vertical waves that travel 60-70% slower than P waves

Triangulation to Locate ‘QuakeKnown distance to stations A, B and C shows location of epicenter

Measuring Earthquakes• Richter Scale – logarithmic measure where

each whole number represents 10X the shaking of the next smaller number

Faulting• Earthquakes occur along faults – cracks in

Earth’s crust where rocks or plates are displaced

• Fault Types:– Normal –

– Reverse –

– Strike-Slip –

– Overthrust –

Normal Fault

Reverse Fault

Strike-slip Fault

San Andreas Fault(Transform Fault)

Movement

Movement

Overthrust fault

1886 Charleston Earthquake

Volcanoes

• Mts or hills w/ a conduit down into upper mantle through which magma, ash & gases are ejected

• 3 basic types:– Cinder-cone Volcanoes– Composite (strato) Volcanoes– Shield Volcanoes

• Where to find volcanoes– Subduction zones, Sea-floor spreading, and Hot spots

Cinder-Cone Volcanoes• Small, steep-sided

volcano made of magma fragments & rock debris from central vent

Composite Volcanoes• Large, steep-sided volcano built up by layers of lava &

rock debris – over subduction zones – viscous, silicate magma - explosive eruptions

Cross Section Mt. Fuji

Pacific “Ring of Fire”• Concentration of composite volcanoes around the

Pacific Basin over subduction zones

Shield and Composite Volcanoes

Mauna Loa, Hawaii

Shield Volcanoes• Broad, gentle-sided volcanoes formed from low-

silica, low-viscosity magma – lava flows cool & harden to become basalt

Volcanic Output

• Volcanic ash consists of powder-size to sand-size particles of igneous rock material that have been blown into the air by an erupting volcano.

• An ash field is a geographic area where the ground has been blanketed by the fallout of an ash plume.

• Silicosis

• Dry ash weighs about ten times the density of fresh snow

Volcanic Ash

Volcanic Lightning

• Volcanoes produce 2 kinds of lightning:– First occurs in the smoke– The second is produced by the mouth of the

volcano

• Not all volcanoes produce lightning

Hot Spots• Stationary points in asthenosphere from which a

magma plume intermittently pushes through the crust above

• Plates move over hot spots, carrying deposits of basalt with them

• Hawaii (& the Emperor Seamount Chain) & Yellowstone have been shaped by hotspots

Formation of HawaiiEmperor Seamount Chain70 M yrs old – Pac. Plate

1st moved North, then NW

HawaiiKauai Oldest – Big Island (Hawaii) still over hot spot

70 mya

Present

Hot Spot Tracks

Yellowstone Hot SpotNorth American Plate has moved west, then northwest over past

16.5 M yrs

Calderas from eruptionsin past 2 M yrs

Supervolcano• A supervolcano is an eruption that rates a magnitude

of 8 on the Volcanic Explosivity Index. • The VEI is a scale that rates eruptions on their ejected

volume, plume height and duration. The scale ranges from 0 through 8.

• Only a few dozen eruptions in all of Earth's history are known to have a VEI of 8.

• Two of those eruptions, the Lava Creek eruption (640,000 years ago) and the Huckleberry Ridge eruption (2.2 million years ago), occurred at Yellowstone.

• These eruptions were given the VEI rating because their ejected volume exceeded 1000 cubic kilometers

Geyser Cross Section

Calderas• Large depression formed when a volcanic

mountain collapses after erupting

Crater Lake, Oregon