1. Topography of Volcanic Regions Ms. Borges Sept. 29, 2005

2. Topographic Maps Contour Lines – curves that connect all points of the same elevation (height) Contour Interval – difference between adjacent contour lines Topographic Map – shows how steep or gentle a slope is Relief – difference in elevation between the highest and lowest points on the map

4. Important Points Contour Lines NEVER cross Close contour lines = steep slope Craters are marked by “tick marks” pointing downslope Approximately every 5 th line is darker and elevation is marked

5. Magma Composition Shape of volcano affected by: –Erosion (ice, wind, rain) –Landslides –Eruptions –Magma *** Mt. Shasta, California

6. Magma Mixture of liquid, melted rock, & gases Chemical Elements: Silicon & Oxygen –Can combine with: Aluminum, Iron, Calcium, Sodium, Potassium, & Magnesium = Silicate Minerals Igneous Rocks – cooled magma –High in Silica (silicon and oxygen)

7. Properties of Magma as They Relate to Magma Composition Magma Property Magma Composition (percent silica content) Low Silica Medium Silica High Silica Silica Content Viscosity Tendency to form lava Tendency to erupt explosively Melting Temperature Volume of eruption ~ 50 ~ 60 ~ 70 lowest medium highest

8. Types of Volcanic Landforms Shield Volcano –Low silica –Free flowing lava –Broad volcano –Single vent or close vents

9. Composite Cone –Silica Rich –Slow Lava –Tall & Steep Sided –Violent eruptions Mt. St. Helens - BEFOREMt. St. Helens - AFTER

10. Caldera –Magma erupts, overlying rocks collapse –Large depression (larger than erupting vent)