Understanding Erosion, Deposition, and Burial 1.7

Opener What do you think happens to sediment after it is formed? Describe your ideas in your journals.

Last time…. The candies in our model in the last session represented large pieces of the same type of rock. If the candies represent rock, what did the small bits of candy represent? We learned in the last session that smaller pieces of rock that have been weathered from a larger rock are called sediment. Sand can be an example of sediment. Sand, dust, pebbles

Sediment Small pieces of rock and other earth materials, such as mud or sand The candies in our model in the last session represented large pieces of the same type of rock. If the candies represent rock, what did the small bits of candy represent?” [Sand, dust, pebbles.] Say, “We learned in the last session that smaller pieces of rock that have been weathered from a larger rock are called sediment. Sand can be an example of sediment.”

Erosion The movement of sediment from place to place, often caused by wind or flowing water Sediment can get carried by rivers and streams to ocean or lake bottoms, just like we read about in the Geologist at Work article. Sediment can get carried by gravity, wind, water, or ice in a process called erosion. The wind or water drops the sediment as it travels over very long distances, and the sediment can pile up in one place over time

Sediment is often dropped, or deposited, in layers at the bottoms of lakes and the ocean. Over time, as more and more layers of sediment are deposited in a location, the sediment on the bottom gets buried and pressed together by the weight of the layers above it. We’re going to model this by “burying” our sediment under some books overnight.

Model and predictions In your journals, make predictions about what you think the sediment will be like after it has been buried under the books overnight.

Remember this mystery? You went through a few steps to get to the point where you were able to make an argument that explained everything that happened here, given the available evidence. First, you only had a little bit of evidence. You started by making observations and inferences about what this first set of footprints meant. Once you had more evidence, you were able to make more observations, more inferences, and put together a more complete story. Even if we didn’t all agree, you still were able to make an argument about what you think happened here.

Remind students that geologists work in the same ways—they make observations, gather evidence, and try to come up with the strongest explanations for how rock formations or landforms came to be on Earth’s surface. Say, “During this investigation, you will be collecting information and evidence that will help you explain what happend to form these rocks at the bottom of the Grand Canyon.

Writing an argument about Black Sand Beach. You will use the same skills as scientists—making observations and inferences, collecting evidence, and explaining— in order to make an argument about a unique and beautiful beach and how it came to look the way it does.

Explain that the sand they just observed came from this beach in Hawaii. Say, “This beach is made of sand—which is a type of sediment—that was broken down from larger rocks.” Explain that this is a black sand beach, but ask if students observed any other color sediment. [There is also a little light-color sediment.] Say, “Black sand beaches are rare. How did this beach get to look this way?”

Explain that this is actually a fictitious map, created for this activity, so students can think about how and why the sand on this beach got there and why it looks the way it does. Explain that geologists use maps like this one to help them explain rock formations and features on Earth and how they came to be. Point out the river and the areas in which different rocks can be found.

Questions 1. Why does the sediment (sand) on Black Sand Beach look the way it does? 2. How did this sediment (sand) get to this beach? Explain that students will use the map, which is also in their notebooks, along with a set of Evidence Cards, which you will distribute, to answer these two questions. Read the questions aloud and keep them projected during the activity. Remind students that some sediment was a lighter color. Explain that students will think about how both the black sand and the lighter-color sediment formed and got to the beach.

Explain that different ideas can come from the same set of evidence Explain that different ideas can come from the same set of evidence. Say, “You may have come up with very different ideas about how this beach came to look the way it does. That is fine—geologists often disagree about explanations like this as well, even when they have the same set of evidence.” 

Share your thinking Where did the black sediment come from? How did it get there? Where did the lighter-color sand come from? How did it get there?  [Black sand is igneous rock and could come from a mountain/volcano. It traveled by erosion to the beach.] [Light sand could come from the sedimentary rock near the river. It likely was carried by the river to the beach.]

Have students turn to pages 26–28, Homework: Completing an Argument About Black Sand Beach, in their notebooks. Let them know that they will get a chance to write a scientific argument explaining their thinking about how the sediment on Black Sand Beach came to this spot and why it looks the way it does.  Refer to graphic organizer. Say, “Your argument will have a claim that answers this question. In order to support your claim, you will need these four pieces of evidence.” Briefly review each evidence bubble. Explain that format of homework assignment looks different than the projected graphic organizer. Let students know that in order to have more room to write, they will write their evidence for Parts 1–3 in boxes instead of bubbles. Explain that each box has a sentence starter to help students to begin writing. Describe evidence sources. Also explain that photos of the the Evidence Cards are provided on the first page of their homework assignment (page 26), Students can use the cards, the Black Sand photograph (on page 24), and the map (on page 25) while they think through their arguments at home.  Review Part 4. If you decided to have students complete Part 4, review the directions with them. If you decided to have your students forego Part 4, point out that they do not need to complete that part.