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BASIC GEOLOGY – THE BIG IDEAS BY LOOKING AT A ROCK, YOU CAN TELL HOW AND WHERE IT WAS FORMED AND SOMETHING ABOUT WHAT HAS HAPPENED TO IT SINCE Bedrock is the crust of the Earth. Sometimes we see bedrock in a mountain or rock outcropping, but bedrock is there even when we cannot see it, under forests, houses, schoolyards, roads and oceans. The rock type is determined by observing its composition and texture (size of crystals or grains). The minerals in a rock, their proportions, crystal size or grain size, and whether minerals or grains are arranged randomly or in layers are all taken into account when interpreting how the rock was formed. Rocks are always changing. Rocks may seem solid and unchanging, but surface rocks are constantly being weathered into smaller pieces, eventually into soil. Bits of weathered rock are moved – eroded by wind, and water. New rocks are constantly being made from hot liquid rock deep under the crust of the Earth and at its surface. New rock is also being made from sediments that are now collecting in oceans and lakes. 15,000 year ago water in the form of a glacier covered this area. As the glacier flowed like a slow-moving river from Canada across New England, it moved pebbles; rock and big boulders in front of it like a bulldozer. It also carried rocks and sand on its surface is like a conveyor belt. The glacier, with sand and rocks embedded in its base, scraped like sandpaper across bedrock it could not move, smoothing and polishing the rock. Rocks and sand from the melted glacier are scattered all over the New England landscape.

Igneous Rocks (from Latin meaning fire) 1. Scientist tell us that rocks formed from interlocking crystals are Igneous Rocks. Deep under the Earth, heat and pressure can be so high that rocks melt. This molten rock is called magna. As magma cools, crystals form which grow together to become solid igneous rock. If the magma cools in pockets deep under the Earth (perhaps several miles), the overlying rocks act as insulation and magma cools slowly. This give chemicals in the magma time to organize into crystals that are large enough for us to see. The longer the magma takes to cool, the bigger the crystals. The deeper the magma is, the longer it takes to cool. Therefore, the deeper the magma was when it solidified, the larger the crystals. Rocks cooled miles under the Earth's surface have medium to large, easily visible crystals. 2. When magma reaches the surface through cracks in the Earth, a volcano forms. The magma cools so quickly that very small or even no crystals are formed. Volcanic rocks usually have very small crystals only visible with a magnifier. Magma cooled at the surface is called lava.

The faster crystals are formed, the smaller they are. The closer to the surface when forming, the smaller the crystals.

3. The kind of igneous rock depends on a. the minerals present b. how long it took to crystallize Adapted from Bev Morrison's work with Fran Ludwig, 2006 Do not try to identify worn or weathered rocks – you cannot. Look for a fresh surface. Make a fresh surface by wrapping a rock in an old cloth and breaking it with a rock hammer. Be sure to use goggles for all who are watching.

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4. Examples of igneous rock are:

Granite - minerals are quartz, mica and feldspar with small amounts of dark minerals. Feldspar may be either pink or white. Granite is a light colored rock. Crystals are visible.

Diorite – minerals are white feldspar and black iron rich minerals. Diorite looks like salt and pepper all mixed together. Diorite is medium colored rock. Crystals are visible. Gabbro – minerals are

mostly black iron rich minerals. Gabbro is a dark rock with visible crystals. Crystals are visible but sometimes hard to distinguish as the minerals are all about the same color.

All of these rocks crystallized deep underground (evidence = medium to large crystals). The magma pocket from which they formed might have been the "roots" of an ancient volcano. 5. Sometimes liquid magma flows into a crack in a solid rock and hardens. When this happens we see a vein of one rock in the middle of a different rock! Often this happens with quartz leaving a white stripe or vein in the middle of a darker rock. Some people call

pebbles with a stripe found on the beach "lucky stones".

6. Sedimentary Rocks Rocks at the surface of our planet are weathered and eroded by rain, ice, wind, plants and other environmental factors. Slowly but continually, rocks are broken down into small pieces and eventually into tiny particles – pebbles, grains of sand and silt or mud. Most of these rock particles are carried by water and deposited as sediments in lake or ocean bottoms. The particles are worn down and rounded by water. The faster the stream, the larger the pebbles it carries. As the stream slows, larger pebbles will drop to the bottom. Finally, far from shore, the fine mud and silt particles will settle out of the slowly moving current. Materials will be continually added on top, especially during spring floods, until many layers are formed. The layers are always laid down flat and parallel. The youngest layers will be on top. These sedimentary layers may be thousands of feet think. In ocean trenches near the edge of continents they may even be seven or eight miles thick!

Color Crystal

Size Light Medium Dark

Formed near

surface… Lava flows

Small (Almost too tiny to see)

Rhyolite Andesite Basalt

Formed deep under

surface… forming

large crystals

Large Granite Diorite Gabbro

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Pressure from other layers, water, and gravity plus heat and pressure from inside the Earth compress the lower layers into sedimentary rocks. Therefore, sedimentary rocks are always layered, parallel to the surface and to each other, made from particles of other rocks – usually worn down to a single mineral, and with a grainy not smooth texture. The grains or particles in most sedimentary rock are the same size, but sometimes water worn pebbles or rocks were mixed in with the sand or clay forming a conglomerate rock looking somewhat like concrete, i.e. a sandstone or shale with rounded larger rocks imbedded in it. Sometimes other things fall into a layer, are buried by other layers, and become part of the rock. Plants, animals, footprints, shells, all may leave their mark in sedimentary rocks and become fossils.

Sedimentary rocks found in Wellesley, for example, are sandstone containing mostly quartz grains, shale containing feldspar and mica grains and conglomerate – a sandstone or shale containing rounded water worn pebbles. Clay deposits are a layer not yet compressed enough to form shale. Other common sedimentary rocks include limestone – made from marine shells, chalk, a fine form of limestone, and coal, plants laid down in layers in swamps. Since sedimentary rocks are always laid down in flat layers, a rock with layers tilted or twisted must have been moved after it became a rock.

Igneous and Sedimentary Rocks Granite as seen through a hand lens.

• Feldspar

(pink or white) has two sides

(cleavages) at nearly right angles. • Mica (shiny, clear or black) has one

cleavage and forms very thin plates. • Quartz has no cleavage. It is the last mineral to

cool and fills the spaces leftover, forming irregular shapes.

• A sedimentary conglomerate has rounded grains cemented together by smaller grains of sand, silt, or clay. An igneous rock has interlocking mineral grains.

A sandstone or shale has rounded grains all the same size cemented together.

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7. Metamorphic Rocks Rocks that have been changed by intense heat and pressure, but have not melted, are called metamorphic rocks. Examples of metamorphic rocks are gneiss (pronounced "nice") and schist. Gneiss is usually banded with black and white or black and pink stripes. Some gneiss was originally granite. Schist has thinner layers than gneiss and with high proportion of mica, has a shiny appearance. Schist may have originated as sedimentary rock. The rocks along Route 2 in Acton are mostly metamorphic with some veins of granite. The Stories Rocks Tell By looking at a rock you may be able to tell how and where it was formed and something about what has happened to it since. What is the story each rock tells us?

• Rocks with visible crystals such as diorite, granite, and gabbro were all formed from molten magna many miles under the surface.

• Surface rocks are exposed to wind and rain,

temperature extremes and plant roots. Iron stains on rocks are evidence that the minerals are reacting with air and water to form rust. Sometimes earthquakes occur and cracks form, making it easier for water and air to get into a rock. This weathering breaks up a rock into smaller pieces and makes it soft.

• Erosion carries away the bits of rock. Wind,

l water, and ice move weathered rock. 30,000 years ago a glacier carried rocks and sand from further north to Massachusetts. It polished and smoothed bedrock in its path.

If this weathered rock is deposited, possibly in layers, and over a long period of time, the grains are cemented together by minerals in water, sedimentary rock is formed. In Boston, such sedimentary rock

formed in an ancient ocean millions of years ago. Fossils of trilobites from this ocean bottom were found when excavating the foundation of the Prudential Center. Sedimentary rock is not as common in Massachusetts as igneous and metamorphic rocks are. As igneous, sedimentary and metamorphic rocks are buried or thrust deep under the earth by movement of continental plates, they melt and the rock cycle starts all over again.

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How the Hills Were Made (and also mountains and valleys and the bays and the sea and even Cape Cod) by Helga Burre If you're headed to Cape Cod this summer, or if you're thinking of buying some land there, or, worse yet, if you happen to live there, you might keep in mind that the Cape is a temporary phenomenon. It is the sea's great sandbox, a loosely piled arm of sand and gravel, a great place to play, but not a good long-term real estate investment. On the other hand, compared to bedrock areas in the center of the United Sates, you might say the same thing about all of New England. The whole Northeast is a relatively young region geologically. Over a billion years ago, sediments deposited in the ocean on the eastern edge of the continent were pushed up into a tremendous mountain chain by the approach of a drifting continent. After these continents separated, a new ocean was formed, and erosion wore down the mountain chain to create the present-day Berkshires. About 500 million years ago, the new ocean started to close, and in the process, an island chain was formed, not unlike the Japanese island chain. This chain eventually collided with North America (435 million years ago), crumpled the ocean floor sediments, and pushed them up into folds (like pushing a flat tablecloth with your hands), thereby creating the Taconic Mountains. Some of the folds actually broke loose, and one was pushed up and over the Berkshires to form Mount Greylock. As the ocean continued to close, two other slivers of land – one carrying the present site of Boston – were crushed into North America, creating another mountain range in central New England, which has now totally eroded away. The collision compressed sediments into hard rock that was able to withstand erosion and eventually became the summits of

Mount Monadnock, Mount Washington and the Presidential Range. Magma melted deep in the Earth and welled up to form the Mount Cardigan Dome in New Hampshire and Maine's Cadillac Mountain and other Down East hills and islands. In this period, a visit to Africa would have been a relatively short land journey – just a stroll through Rhode Island; in fact, 250 million years ago, a land journey to any of the continents would have been possible because the continents had all joined into one super continent called Pangaea. Pangaea soon began to separate (245 to 150 million years ago), putting tremendous stress on the Earth. The stretching of the crust of the Earth caused earthquakes and faults, which caused the Connecticut Valley, the Newark Basin in New Jersey and the Bay of Fundy. These basins filled with reddish brown sediments. Wandering dinosaurs crossed the basins periodically – leaving their tracks, which can still be seen in some areas. The separation also caused upwelling of dark basalt magmas, which oozed up through cracks in the Earth to flood the surface of the Earth. The exposed edges of these (now tilted upward) sheets form the Palisades on the Hudson River, Mount Tom and the Holyoke Range. To the north, reaction to stress resulted in thick basaltic flows of lava, and, when the outpouring of lava left a hollow underground, the Earth collapsed into a caldera and the Ossippee Mountains in New Hampshire began to form. Upwelling of lighter colored granitic magmas also added to the White Mountains and formed the face of the Old Man of the Mountains. Lexington Gabbro formed over 400,000,000 years ago. http://www.platetectonics.com/

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Then, 150 million years ago, the Atlantic Ocean started to open. Erosion moved sediments onto the new continental shelf and deposited the clays that now form Gay Head on Martha's Vineyard. Erosion continued to lower mountains and fill in valleys to form a flat peneplain, and then, 65 million year ago, the land arched up, and erosion increased, leaving the hardest rocks exposed as our present-day New England mountains. One million years ago, a series of ice ages scoured the new land, carving cirques in mountains such as Tuckerman's Ravine, gouging out lowlands to form

lakes, and moving loose rocks, sand, and gravel south to form the place that today is known as Cape Cod. Helen Burre is Massachusetts Audubon's regional director for programs in the Education Department. Reprinted with permission from the Massachusetts Audubon Society.