Earth’s Changing Surface Fourth grade scientists are introduced to the scope of geologic time and learn about forces that change our Earth’s surface. During their study, students use scientific modeling and research to study the forces that change landscapes over long periods of time through weathering, erosion, and deposition, including the role of glaciers in shaping New England. Students apply their understanding of how landscapes can slowly change through hands-on experiences which culminate in finding evidence these changes as they explore geologic features in Wellesley during the Geology Field Investigation. Guiding Question: How did Earth’s surface get the shape it has now? Skills Students will be able to… • Design an investigation to determine factors that affect how moving water shapes the Earth’s surface • Model changes made by water and ice (glaciers) • Test and compare multiple solutions to prevent earthquake damage • Construct an explanation based on firsthand observations and support their thinking with evidence • Actively engage in scientific discussions Knowledge Students will know… • Earth has changed over time. Landforms develop and change. • Water, ice, wind and living things can break down rocks, soils and sediments (weathering) and transport materials elsewhere (erosion). • Materials can be deposited somewhere else. • Glaciers shaped many of the landforms found in Wellesley. • Volcanoes and earthquakes are rapid events that cause significant damage Geology Background Information Every rock tells a story – how it formed and what has happened to it since. Our field investigation is focused on the latter – the processes of weathering, erosion and deposition, along with other landforms created by glaciers. The information on this page will provide you with some background on the former – how rocks are formed in the first place. All the rocks we see on the Geology Field Investigation were formed a long time ago, but not all rocks were formed at the same time. Similar processes are at work now. New rocks are continually being formed below the surface. On the surface rocks are being weathered, eroded, and otherwise changed. Bedrock The crust of the earth is solid rock called bedrock. Bedrock is usually covered by forests, meadows, houses, oceans, ponds, lakes, etc., but it is always there. Sometimes bedrock sticks up where we can see it. When it does we call it mountains or a ledge. In Wellesley the bedrock is very close to the surface and we often see it poking up through the soil. Plate Movement and the Pangaea Supercontinent (for Hemlock Gorge) The crust of the earth is made of large continental plates. These hard plates move because of changes beneath them. In some places the plates are moving apart. In other places they are moving toward each other. Earthquakes and volcanoes tend to occur where the plates meet. Over 200 million years ago the seven continents we have on Earth today were connected in one giant continent called Pangaea. The east coast of North America, where we are, was connected to the northwest coast of Africa. Scientists believe at one point these two plates moved apart and then collided again. When they split apart for the final time, they split in a different place, leaving part of what was once Africa connected to northeastern North America. How rocks are made: There are three types of rock forming processes: igneous, sedimentary, metamorphic. Igneous Rocks are formed from molten materials hardening into rocks as they cool: both deep underground and as volcanic eruptions at the surface. The longer the molten material takes to cool, the bigger the crystals in the rock. Rocks cooled under the earth’s surface have crystals we can see. Different minerals cool at different temperatures. The last mineral to cool fills the remaining spaces. The type of rock depends on what minerals are in it. When minerals from the magma move rapidly to the surface and cool above ground, they cool so quickly that there are no visible crystals – these are called lava rocks. Igneous rocks in Wellesley: granite – pinkish color: made of feldspar, mica, quartz diorite – dark blackish gray: made of feldspar, hornblende volcanic breccia – hardened molten material from a volcano, but cooled below the surface (St. Mary’s Cemetery) Sedimentary Rocks are formed when surface rocks are weathered and eroded into smaller particles and are moved (primarily by water). The rock particles are rounded by wind and water and are laid down in layers of sediment in lakes or oceans. Sedimentary layers are always flat and parallel. The pressure of the top layers compresses the layers below and it hardens into solid rock. The type of rock depends on what minerals were present and the size of the particles. Sedimentary rocks in Wellesley: Roxbury Puddingstone – conglomerate rock: made of sandstone with larger rocks cemented into it Metamorphic Rocks are formed when existing rocks are changed by extreme heat and pressure. Marble and slate are examples of metamorphic rock. We do not have any natural metamorphic rock formations in Wellesley (only marble statues or tombstones). Devil’s!Slide! ! 600,000,000!years!old!–!oldest!rocks!in! Wellesley.! Devil’s!Slide!is!a!diorite!intrusion!into!granite! rock.!! This!site!shows!glacial!smoothing!and!plucking.! ! As!glaciers!move,!they!push!debris!(boulders,!rocks,!pebbles,!soil)!ahead!of!it!like!a!bulldozer.!!A! glacier!cannot!move!bedrock!so!it!passes!over!it.!!As!the!glacier!passed!over!the!bedrock!here,!it! smoothed,!polished,!and!scraped!it.!!After!passing!over!it,!the!glacier!then!broke!off!large!pieces! of!rock–!this!is!called!plucking.! ! Walking+in:+ On!the!rock!wall!to!your!left!–!what!kind!of!rock!do!we!find?!(granite)! How!do!you!think!this!rock!got!here?!!Naturally!or!moved!by!people?!(both)! Granite!is!formed!underground.!!How!come!we!see!the!granite!exposed!today?! ! Diorite!is!what!makes!up!Devil’s!Slide! Diorite!=!feldspar!+!hornblende;!looks!rusty! ! Granite!has!more!color! Granite!=!quartz!+!feldspar!+!mica!(all!minerals)! Much!of!the!granite!was!scraped!by!the!glacier!and!has!been!weathered!away.!!! ! Along+the+path+with+the+ledge:+ What!do!you!see!here?!!(Granite)! Any!idea!where!the!rocks!on!the!side!came!from?!(plucking!from!the!glacier)! What!is!happening!to!the!ledge?!! Are!any!pieces!of!rock!falling!off?!(yes!–!from!weathering,!erosion,!tree!roots)! Does!anyone!know!what!bedrock!is?!!(bedrock!forms!the!crust!of!the!earth)! ! At+the+base+of+Devil’s+Slide:++ Is!this!the!same!rock!as!the!ledge?!!What!is!this!rock!called?!!(diorite)! Which!rock!is!older?!!(granite)! ! Going+up+to+Devil’s+Slide:+ Notice!the!granite!bedrock!under!foot!abutting!the!diorite.!!Here!you!can!see!that!molten!diorite! really!did!flow!into!a!crack!in!the!granite!when!these!rocks!were!still!deep!under!the!crust!of!the! earth.! ! On+Top+of+Devil’s+Slide:+ Discover!the!date!+!initials!carved!into!the!rock.! Look!at!the!roots!of!the!tree.!!What!surprises!you!about!the!tree?! What!is!happening!to!the!rock?!!What!has!happened!to!the!diorite!since!it!was!formed?!!What!is! happening!to!it!now?!! Do!you!see!any!signs!of!weathering?!!Will!any!more!pieces!break!off?! ! ! Fault!Line!by!Train!Station! ! Point!out!the!many!rock!outcroppings!in!the!yards!of!the!houses!north!of!the!railroad!track.!!Look! for!the!rock!cliff!along!the!railroad!tracks!which!mark!the!fault!line.!!Notice!the!height!of!the!land! north!of!the!tracks!and!large!outcroppings!of!bedrock.!!Rocks!blasted!along!the!fault!line!to!make! the!railroad!bed!were!taken!to!Boston!and!used!as!fill!for!the!Public!Garden.! ! Driving!by!Mass!Bay!Community!College!–! Look!across!to!Boston!and!its!tall!buildings!–!imagine!a!glacier!covering!the!whole!area!taller! than!those!buildings.! ! ! ! ! ! ! Hemlock!Gorge! ! ! Be#careful#of#the#poison#ivy#between#the#path#and#the#water#and#to#the#left#of#the#cave.# ! 580,000,000!years!old—!!2nd!oldest!rock!in!Wellesley.! Roxbury!Puddingstone:!!sedimentary!rock!laid!down!by!fast!moving!streams!flowing!from!the! mountains!down!into!the!ocean.!!!Sandstone!that!contains!large!rounded!stones.!! First!identified!in!Roxbury,!it!is!so!named!because!it!looked!like!raisins!in!a!pudding.!!! Puddingstone!is!found!all!over!the!Boston!area.! During!earth!movements,!the!puddingstone!was!not!only!exposed!at!the!surface!but!also!tilted!so! the!layers!were!no!longer!flat.! Roxbury!Puddingstone!is!the!state!rock!of!Massachusetts.! ! On+the+path+to+the+caves:+ Look!at!the!rocks!under!foot!–!what!do!you!see?! What!kind!of!rock!is!this?!!How!can!you!tell?!!How!is!it!different!from!granite!and!diorite?! ! At+the+caves:+ What!do!you!see!here?!!What!can!you!tell!about!the!history!of!these!rocks?!!! How!does!a!rock!come!to!be!rounded?!(water)! Are!the!rounded!rocks!all!the!same!size?!(no)!! Why!have!some!rocks!fallen!out?!(erosion,!people,!plants)! How!can!you!describe!this!rock?!! Is!it!smooth!or!gritty?!! What!kind!of!rock!is!it?!!(sedimentary!sandstone)! Why!are!there!layers?!! Can!you!tell!how!it!was!formed?! Why!do!you!think!the!layers!are!tilted?!(plate!movement)! Back+up+the+path+–+look+across+the+water+to+the+other+side:+ How!come!the!level!of!ground!on!this!side!is!like!the!level!on!the!other!side!of!the!water!–!what! do!you!think!happened!over!time?!!How?!!(The!water!eroded!the!rock!to!form!a!channel!for!the! river.)! ! At+the+view+to+Echo+Bridge:+ This!Aqueduct!used!to!carry!water!to!Boston.!!It!is!called!Echo!Bridge.!!It!is!the!second!largest! single!stone!arch!in!America.! Notice!the!cell!tower!antenna!–!that!is!about!500!feet!high!–!about!as!high!as!a!mediumHsized!