Geology Field Investigation Parent Training Wellesley Elementary Schools Fall 2016 1
Training Objectives • To provide background information for the fourth grade geology field investigation • To familiarize parent volunteers with important geological sites in Wellesley 2
Presentation Outline • Geology 101: Structure of the Earth • Changes to the Earth’s Surface • Wellesley’s geological sites • Geology of Wellesley video 3
The Structure of the Earth Crust: relatively cold, thin, brittle solid fractures with earthquakes made of calcium, sodium, and aluminum silicate minerals 5-20 miles (8-32km) thick thickest at mountains thinnest at ocean floor Mantle : an elastic solid most of the earth’s mass. made of iron, magnesium aluminum, silicon, and oxygen convection currents cause crustal plates to move 1832° F (1000° C) 1800 miles (2896km) thick Liquid outer core: made of iron and sulfur 6692 ° F (3700 ° C) 1400 miles (2252km) thick Solid inner core: made of iron remains solid due to immense pressure and high heat 9000 ° F (5000 ° C) 800 miles (1287km) to center 4
Plate Tectonics • Plate tectonics is the movement of the earth’s crustal plates due to convection currents in the mantle • As plates move they may collide, pull apart or rub past each other • The earth ’ s land masses sit atop these crustal plates and are deformed by crustal motion 5
Stories in Stone Every rock tells a story: how it formed and what has happened to it since 6
Changing Earth • How do mountains shrink? • How do boulders become small pebbles? • How do sharp rocks become smooth and rounded? • What makes sand? Answer? 7
Weathering • Weathering : “breaking down” forces – things that break rocks & minerals into smaller particles • Physical • Chemical • Biological 8
Physical Weathering rain, rivers, waves and storms - moving water greatest cause of physical weathering wind carrying pieces wind of debris can sand- blast surfaces freezing/melting temperature cycles- (ice) water expands when it freezes 9
Chemical Weathering when oxygen reacts with oxidation iron-rich minerals - rust gases in the air (from pollution or natural causes 1908 1969 acid rain such as volcanoes) mix with rain, snow or other precipitation 10
Biological Weathering organisms growing lichens, on rocks produce bacteria, acids and enzymes that break down and fungi dissolve the rock trees tree roots can break and apart huge boulders plants 11
Photos courtesy of Irene Gruenfeld 12
Erosion • Erosion : “carrying away” forces – things that take the broken off pieces and move them elsewhere 13
Deposition • Deposition : “leaving behind”–rock material is laid down (volcanoes also leave new rock) 14
Hemlock Gorge • cave – rock material has been worn away 15
Sedimentary rock • Made from sediments: bits and pieces of rock and organic matter cemented and compacted together • Rocks break apart and form sediments - weathering • Sediments transported by wind, water, and glaciers - erosion • Heavier material drops out first - deposition • Layer upon layers, flat and parallel • Weight of water compresses the layers 16
Hemlock Gorge • Around 580,000,000 years old • Sedimentary – Roxbury Puddingstone • Large rounded rocks embedded in sandstone • Embedded rocks resembled raisins in “pudding” Similar rock is found in Africa! 17
Hemlock Gorge Roxbury Puddingstone Is this an example of weathering, erosion or deposition? 18
Shaping New England Glaciers 19
Glacial Formations • 22,000 years ago – peak of the Ice Age • Glaciers covered New England • Glaciers move – push rocks and earth aside • Rocks get frozen in the ice and carried long distances • Receding glaciers and melting ice create new landforms 20
Abrasion and Plucking • Glacier came from NW passing slowly over bedrock – Polishing the surface – Scraping in the rock – glacial striations – Plucking the backside 21
Devil’s Slide St Mary’s Cemetery 22
Different Types of Rock 23
Intrusions • Magma under the surface is pushed up into any available cracks and spaces in existing rock • Can vary from mountain- range-size to vein-like fracture fillings 24
Devil’s Slide • 600,000,000 years old • Oldest rocks in Wellesley • Formed deep below the surface • Diorite intrusion into surrounding granite • Millions of years of weathering for our granite and diorite to become exposed at the surface 25
A Volcano? in Wellesley? 26
Volcanic Breccia • As lava moves up through cracks in bedrock, pieces of bedrock break off • Bedrock pieces show signs of stretching from the heat • Some lava never reaches the surface - cools slowly, hardening underground with pieces of bedrock embedded in it • Over time, the volcano has eroded away – only plug remains 27
St. Mary’s Cemetery • 580,000,000 years old • Igneous – volcanic breccia • Hardened lava that never made it to the surface • More breccia is found near the school in nearby Needham – it was a large volcano 28
Devil’s Slide St Mary’s Cemetery Are these examples of weathering, erosion or deposition? 29
Eskers • Ridges of gravel and sand • Rivers and streams flowed on, in and beneath glaciers • Can be over 100 ft. high, hundreds of feet wide, and several miles long 30
Longfellow Pond – Esker Trail 31
Glacial Erratic Kelly Memorial Park • Erratic: a rock that differs from native rock in size and type • Carried by glacial ice, often hundreds of miles Note: Fairy Rock has very large pink feldspar crystals (this tells us it cooled very slowly below the surface) 32
Longfellow Pond Kelly Memorial Park Esker Trail Glacial Erratic Are these examples of weathering, erosion or deposition? 33
Kelly Field: Kettle Hole • Formed by a retreating (melting) glacier • Blocks of ice break off, getting surrounded by sand, gravel and rocks from the melting glacier • Blocks eventually melt leaving a depression 34
Weathering & Erosion • Devil’s Slide – trees growing into the rock - lichen growing on the rock - rocks broken off the backside • St. Mary’s Cemetery - channel cutting across the rock - signs of rocks eroding at different rates • Hemlock Gorge - the cave formed by water - rocks falling out 35
Glaciers • Devil’s Slide • St. Mary’s Cemetery - - smoothing on front smoothing on front - - glacial striations glacial striations - - plucking on back plucking on back • Longfellow Pond • Kelly Field - esker - glacial erratic - kettle hole 36
The Geologic Time Scale a trip through Wellesley’s geologic history 37
Final thoughts: • Stories in Stone: every rock tells a story of how it formed and what has happened to it since • Understand what a rock, or rock formations, tell you about the geologic history of the area (i.e. glacial activity, volcanic activity, crustal motion, water action) • Don’t worry about specific rock names but perhaps look for identifying traits for each type of rock: igneous – visible crystals sedimentary – layers, fossils, rounded particles • Generate excitement and curiosity as you learn about Wellesley ’ s history through its rocks, exploring it’s volcanic past, glacial past and present day geology 38
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