Geology is the study of rocks, how they form, and the processes that shape Earth. Many people go to National Parks like the Grand Canyon, Hawaii Volcanoes, and Yosemite to study geology. These are great places to learn about Earth, but did you know there is a national park where you can learn a lot about geology right in the middle of Washington, D.C.? The National Mall may seem like a strange place to study rocks, but when you dig deeper you’ll be surprised how much you learn here.
Geologists classify rocks into three types: sedimentary, igneous, and metamorphic. Build a GeoFlag to learn how to identify each type of rock, then see if you can find all three types of rock during your tour of the National Mall.
From hills and valleys to mountains and beaches, geology has left its impact on the Washington, D.C. area by shaping the diverse terrain found within it. Soil and vegetation may cover much of the rock at Earth’s surface here, but a rich history can be discovered below. The geologic history of this area is a combination of unique and complex events that include plate tectonics, erosion and deposition, cataclysmic impact, and sea level fluctuation. Once we understand the impact of geology on the area, we can see how the scenery, development, and history all come together.
We can also look at the geology of the building materials used in all the monuments and memorials. The stone not only provides the building blocks of each structure, but strengthens the themes and ideas of the monuments and memorials as well. Rock types for each memorial were chosen for their unique color, texture, and strength – as well as the mood they create. What kind of rock is the National Mall built out of? Explore the rest of the tour to answer this question.
Sand dunes, whether at the beach or in the desert, are made of loose sand grains. You could pick up these grains in your hands, or bury your toes in them. Wind and water easily change the shape of the dunes. If these sand dunes turn to rock, what clues could tell us if they started as beach dunes or dessert dunes? With enough time, pressure, and cement, loose sand grains can lithify, or become rock. Rock made from sand is called sandstone. Sandstone is a sedimentary rock, often cemented with silica, iron, or calcium.
The mucky bottom of a shallow sea is made from sediments like sand and mud. Whatever lives in this shallow sea has the potential to get buried in the sediment below and become a future fossil! What makes limestone a unique sedimentary rock is all the calcium carbonate, or lime, in it. Calcium carbonate found in the bodies and shells of shallow sea creatures provides the cement for limestone. Remember the dunes from the last page? They have been flooded by a sea! Calcium rich water trickles down from the layers above to provide cement for the sandstone.
A dinosaur walking across the muddy plain left her footprints. If the footprints are preserved in rock, they will be called trace fossils. (So will ripple marks left by the stream and mud cracks left when the puddles evaporate.) Another type of sedimentary rock is shale, or mudstone. Shale forms in areas where mud has been lithified, or turned to rock. In shale, you can find trace fossils such as footprints or burrows. How are trace fossils different from fossils such as bones or shells? What happened to all the water? Sea level fluctuates (goes up and down) all the time. Now we are in a drier climate, but evidence of the sea can still be found. You can see fossils of shallow sea organisms in this limestone?
When molten magma comes up from the earth’s mantle into the rock layers above, it is called an igneous intrusion. Igneous means “fire formed.” When magma cools, the minerals in it crystallize to form different kinds of igneous rocks. If the magma cools slowly, large mineral crystals form. If the magma cools quickly, the mineral crystals are small.
Magma that cools on the surface of the earth forms extrusive igneous rocks. Basalt, andesite, and rhyolite are some examples. Usually extrusive rocks cool very quickly so their crystals are small. Obsidian, also called volcanic glass, cools so quickly that no crystals form.
Magma that cools inside the earth forms intrusive igneous rocks. Granite and gabbro are two examples of intrusive igneous rocks. Frequently, granite has large mineral crystals that can be identified by the naked eye. Because of its strong interlocking minerals, granite is a good building stone.
Increased heat and pressure can cause metamorphism. The minerals in the old rock, the parent rock, recrystalize to become something new. These new rocks are called metamorphic rocks. The molten magma in this igneous intrusion is hot enough to melt and recrystalize the sedimentary rocks around it. What happens to the shale, limestone, and sandstone when they are metamorphosed? Slate is the metamorphic rock formed from shale. Marble is the metamorphic rock formed from limestone. Quartzite is the metamorphic rock formed from sandstone.
Do you remember the three rock types?
From hills and valleys to mountains and beaches, geology has left its impact on the Washington, D.C. area by shaping the diverse terrain found within it. Soil and vegetation cover much of the bedrock, but that cannot hide the story below. Millions of years ago, the area that is now Washington, D.C. was a very different place. Geology has changed the landscape and waterways, and influenced the development of the area. Can you think of ways geology affects the area where you live?
The geologic history of the Washington D.C. area is a combination of events that include plate tectonics, erosion and deposition, cataclysmic impact, and sea level fluctuation. Luckily, the word “Impact” makes these events simple to remember. What does I.M.P.A.C.T. stand for? Imagine the Movement of Plates that created both the Appalachian Mountains and the Atlantic Ocean basin along with the Crater from the Chesapeake Bay meteorite that helped shape the Washington, D.C. area over Time. Let’s investigate each of these concepts with the letters below.
The first three letters of I.M.P.A.C.T. focus on plate tectonics: Imagine the Movement of Plates. Earth is not one solid rock. It has a solid core, surrounded by liquid molten magma. On the outside is a thin solid crust. The crust is broken into pieces, or plates, that move around on the molten magma, like broken ice on a lake. Earth’s landmass was once concentrated in a single continent called Pangaea. Eventually the plates broke apart and they have been moving into their present day locations ever since. Because of the movements of plates, Washington D.C. has a great location on the Atlantic Coast of North America.
The letter A in I.M.P.A.C.T. represents evidence of two important tectonic events within this area - The creation of the Appalachian Mountains and the Atlantic Ocean basin. About 450 million years ago, the North American plate and the African plate collided. Earth’s crust reacted by folding and building the Appalachian Mountain Range, originally 20,000 feet high. These ancient Appalachians might have looked like the Himalayas, found today between the colliding plates of India and Asia.
Mountain building lasted for a few hundred million years, while the plates were still pushing against each other. Then, the plates reversed directions, suddenly spreading apart, and released the compression that had pushed the mountains skyward. As the plates drifted apart, huge rift valleys were created between them. The largest of these valleys filled with water and the Atlantic Ocean was formed.
Without uplift, the folded, crumpled layers of ancient sedimentary rock gradually wore away due to erosion. Exposed ridges of more resistant limestone and sandstone were left behind while softer shale layers were eroded into deep valleys. Streams carried sediment eroded from the Appalachians to the ocean. There, layers of sediment were deposited, building a flat coastal plain. Washington D.C. developed on the coastal plain because ships could navigate through the rivers to bring people and goods to the city.
It has taken 450 million years to get through the first four letters of I.M.P.A.C.T., but only a second for the C – the Crater from the Chesapeake Bay meteorite. 35 million years ago, a meteorite a mile in diameter crashed into Earth, creating the mouth of the Chesapeake Bay. Before the meteorite there were several separate rivers cutting across the coastal clain, emptying into the Atlantic Ocean. The meteorite’s impact created a crater 50 miles wide and thousands of feet deep.
Many of the ancient rivers took drastic bends to converge in the crater before continuing together out to the ocean. As rivers cut across the coastal plain at steeper inclines, deep canyons were carved into the soft sediments. These canyons became increasingly deeper as sea levels dropped during the last Ice Age. When the glaciers melted, sea level rose 600 feet, flooding the canyons. As seawater from the Atlantic Ocean mixed with the freshwater of five major rivers, Chesapeake Bay, the largest estuary in the United States was created.
The Chesapeake Bay meteorite did not only affect water on Earth’s surface. Major underground aquifers were destroyed by the impact crater, leaving behind layers that now hold super-saline water. Records of salty water in area wells go back as far as the Civil War. Fresh water was needed for the Union troops at Fort Monroe. Maps of these “salty wells” show a peculiar ring pattern that has only recently been explained by geologists as the impact crater. Drill samples, rock fragments, and seismic mapping of the bay floor confirm the existence of the impact crater.
Finally, we come to the T in I.M.P.A.C.T. Time. In Geologic Time, events such as mountain building, or the creation of an ocean can take millions of years. We know Earth is always changing, even if the changes are happening so slowly that they are difficult to perceive. A geologist’s job is to uncover the stories about Earth’s past by looking at clues inside rock, such as minerals and fossils, as well the area around a rock. Through their studies, geologists discover stories billions of years in the making. Combining these stories with our knowledge of the present helps us understand Earth’s history.
Stop 1: The Geology of the Washington D.C. Area
Stop 2: The History of Washington D.C.
Stop 3: Finding D.C.’s Foundation
Stop 4: A Watery Past
Stop 5: GeoStory of the Lincoln Memorial
Stop 6: Remembering War
Stop 7: Stories in Stone at the Franklin Delano Roosevelt Memorial
Stop 8: Thomas Jefferson Memorial - A Place of Controversy
Stop 9: Washington Monument - The Nation’s Most Unique Rock Collection
Stop 10: Who Cares for the National Mall
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