Planet Earth Chapter 17

Plate Tectonics Theory 17.1 Crust of the earth is made of plates floating on magma Move slowly – 1-16 cm each year Divergent boundary – plates move away from each other Convergent boundary – plates move toward each other Transform fault – plates slide past each other

Why do plates move? Scientists don’t know but they hypothesize – The asthenosphere (melted portion of mantle) flows like thick liquid. Plates float on top like ice cubes

Plate boundaries lead to Land Formations Divergent boundaries under the ocean cause oceanic ridges (like mountain ranges under water) Convergent boundaries cause land to rise forming mountains, ocean trenches, and volcanoes

Earthquakes occur along Transform faults Transform faults can lead to earthquakes when tension (pressure) builds up and suddenly the plates slip. The San Andreas Fault in California is an example.

Earthquakes & Volcanoes 17.2 What are earthquakes? Occur at boundaries of tectonic plates – Japan & California have plate boundaries Pressure from plates moving causes pressure that breaks rocks – Seismic waves

Earthquakes generate seismic waves Focus is exact spot where rocks first break Point on the surface immediately above the focus is the epicenter – Damage is greatest here Earthquakes measured as shock waves

3 types of Earthquake Waves Longitudinal waves start at the focus moving all directions – Move faster through rock, first to reach measuring stations – P-waves (primary) Transverse waves start at the focus moving all directions – Travel slower, second to reach measuring station – S-waves Surface waves result from earth surface shaking – Up-&-down and back-&-forth motion causes circular motion like the ocean far from shore

Seismology Science of detecting & measuring earthquakes Seismographs record P waves, S waves, and surface waves. When the ground shakes, the pendulum on the seismograph remains still while a rotating drum of paper at the base records Earth’s movement as the drum vibrates

Richter scale is used to measure magnitude of earthquakes Measure the release of energy at the focus of an earthquake Limited by strength of rocks in Earth’s crust Cannot predict severity of damage which depends on: – Population near focus – Type of construction – Depth of focus

Volcanoes Opening in Earth’s crust where magma can reach surface – Vents Cinder cone volcanoes form after a violent eruption – Small & abundant – Active for short time Shield volcanoes form where eruptions are mild and reoccurring – Forms gently sloping mountain

Minerals & Rocks 17.3 Rocks are composed of minerals Characteristics of minerals – Naturally occurring – nonliving – chemical composition – Internal structure 3500 known minerals 20 are commonly found in rocks 9 most common minerals are – Feldspar – Pyroxene – Mica – Olivine – Dolomite – Quartz – Amphibole – Clay – calcite

Structure and Origin of Rocks Combination of minerals gives rocks a unique set of properties – Porous, smooth, granular – Soft or hard – Density – Color

Igneous Rock Molten rock solidifies when it cools to form igneous rock Crystals form as rock cools – Fewer crystals if cooling is fast Obsidian has few or no crystals Other examples of igneous rock: Granite Basalt

Sedimentary Rock Forms from weathering and erosion of other types of rock Rock is deposited in layers and gets pressed or “glued” together over time May contain fossils of living things that died during the process

Metamorphic Rock Formed when heat & pressure cause change in texture & mineral content of rocks Sedimentary or igneous rock under these conditions change to metamorphic rock. Example: Limestone (sedimentary) turns to marble

How old is that rock? Relative Dating Oldest rocks are in lower layers of soil Can help determine which fossils are older Cannot tell the actual age of the rocks Radioactive Dating Radioactive isotopes of certain minerals help determine actual age As the atoms decay, they give off energy Scientists know the length of time for decay and can use this as a “clock” – Compare the amount of radioactivity that is left to the amount of decayed mineral

Weathering & Erosion 17.4 Physical weathering – Breaks rocks into small pieces – Does not alter chemical composition Frost wedging Plant roots

Weathering & Erosion (cont.) Chemical weathering Chemical composition of rocks changes as they are broken up – Oxygen changes iron by oxidizing it (rust) – Acid rain dissolves rocks by reacting with limestone

Erosion Removal and transportation of soil and rock – Running water – Wind – Waves – Ice – Underground water – Gravity

Underground Caves Limestone is carved away by carbonic acid – Water and dissolved carbon dioxide – Calcium bicarbonate dissolved leaving air pockets