Students will be introduced to the environments in which different sedimentary rocks form and relate these environments to the history of the rock. They will present histories from sequences of rock layers.
Show students how sedimentary rocks tell a story about past environments.
Objectives: Students will be able to:
Define extrusive and intrusive igneous rock.
Understand how rate of cooling affects crystal size.
Learn how to classify igneous rocks based on texture.
10 minute setup
30 minute activity and discussion
Many samples of sandstone, shale, limestone and conglomerate
Materials to determine the type of sedimentary rock (see original activity)
Sedimentary rocks can be read like a book. Each layer of sedimentary rock is like a chapter in the history of the earth. Remember that sedimentary rocks are always deposited from oldest to youngest, and unless some incredible earth movements (faults and folds) have deformed the sediments, the oldest layer is always going to be on the bottom and the youngest layer is always going to be on the top.
Depositional Environments of Sandstone
As mentioned in the original activity, sand that commonly forms sandstones is found in two very important places: beaches and deserts. So, when geologists see a sandstone, how do they tell whether it was deposited in a beach or desert environment? There are two ways: One way is to look at the layering in the rock to see whether it mimics the pattern of sand dunes alone (desert) or whether there are signs of water deposition, such as preserved ripple marks, flat layers representing the swash zone of a beach (the place where the waves come onto the beach and flatten the sand out) or evidence of waves from large storms moving the sand underwater. Another way to distinguish between desert and beach deposits is to look at the rock above and below the layer you are looking at.
Depositional Environments for Shale Shale is a rock that comes from relatively deep, calm water. Because shale is made of very small particles (fine sediments), it must be deposited in water that is calm enough to no longer suspend such fine particles. Think of water that can deposit shale as water in a bathtub. When you're really dirty and you take a bath, you scrub all the dirt off and don't realize how dirty you were until after you get out. When you are in the tub, you create enough current in the water to suspend the dirt. Once you get out, however, the current in the water slows and eventually stops, allowing all of the dirt to settle down to the bottom. Shale forms in the same way. Any place the water is calm enough for fine sediments to settle out, you might find a shale.
Depositional Environments for Limestone One place where limestone deposits are found is in very deep ocean water. Way out in the ocean, where the water is calm and deep, live multitudes of tiny marine animals. These animals use the calcium carbonate in sea water to make their shells. When these animals die, they fall to the sea floor. Over millions of years, the shells accumulate, and are eventually buried, cemented and turned into limestone. It is not common to find large fossils in deep-water limestone. Limestones can also form when the concentration of calcite increases to the point where the molecules start to fall out of the water. Think of chocolate milk. When you put too much chocolate into the milk some falls out. Limestones can also form in this way.
Another environment that creates carbonate deposits is a coral reef. Reefs are composed of almost 100% calcium carbonate. If you see a limestone with fossils that resemble corals or any other reef creature, you can imagine that the environment that deposited that limestone was probably associated with a reef. Reefs commonly reside in shallow marine water and are found only in tropical environments.
Generally, in a typical ocean environment, you'll find sand on and near the beach. Out into the water a ways you'll find mud that could someday become a shale. If you keep moving farther out to sea, you'll find the area where the carbonate marine animals live and die. The type of sedimentary rocks in an area can be used to determine the depth of the water at the time the sediments were deposited.
Depositional Environment for Conglomerate The last deposit we will consider is conglomerate. Conglomerate rocks can sometimes have spaces between the rocks and other times be filled in with a matrix cement, like concrete. A common place to find conglomerate is near a river. Rivers, because they move at different velocities, are able to transport and deposit rocks of all different sizes. If the river flowed at a constant velocity it would have about the same sized rock throughout. In places along the river the where the rate of flow slows, there you will find rocks surrounded by a matrix mud. Often, when geologists come across a conglomerate, they can assume that there was once a river in that area.
How Do These Environments Fit Together? In order to complete this activity, you'll need to think about depositional environments in the following manner: Say you come across a layer of conglomerate, and above that layer of conglomerate you find a sandstone, and above that sandstone you find a layer of shale. What would this tell you about past environments? Well, the conglomerate tells you that there was once a river in that place. The sandstone tells you that there was once a beach or a desert in that place. The shale tells you that there was once deep, calm water in that place. So, which scenario makes the most sense? A: First there were rivers, and then those rivers dried up. The climate change was so drastic that as soon as those rivers dried up, a huge desert covered the region. Then, a few million years later, there was suddenly really deep water to deposit the shale. Scenario B: First there were rivers in that place that emptied into a big lake or ocean. The level of the lake or ocean was rising, so eventually the place were the rivers were became a beach. As the level of the water continued to rise, the area was covered by deepening water and the shale was deposited. It seems as though scenario B makes the most sense.
Divide students up into groups of four.
Give each group four rocks that they can use to interpret past environments.Tell them which rock is the bottom layer, the next to bottom layer, the next to top layer, and the top layer.
Tell them to inspect each rock, determine what kind of sedimentary rock it is (see original activity), and determine what kind of environment that rock may have come from.
Then tell them to re-create the scenario that took place to get those rocks into the sequence that they were found.
There are many, many options for scenarios. Perhaps sea level was deep, went up (deeper), then down (shallower), then up(deeper again)? You might find a shale on bottom, then a limestone, then a shale and then a limestone. Perhaps sea level was very deep and went down? You might find a limestone on bottom, then a shale, then a sandstone and conglomerate on top! Remember that sedimentary rocks are deposited oldest on bottom to youngest on top.
Have each group present their findings to the class or have each group do many different scenarios.
In order to make this activity work, assume that all limestone is deep-water limestone rather than reef limestone.
This activity can be done by providing students with a list of possible scenarios or by allowing them to come up with scenarios on their own.
How can geologists use sedimentary rocks to learn about ancient environments? How do rocks tell a story of past environments? What do sedimentary rocks tell geologists about climate change? What do you think geologists would learn about ancient environments if they found fossils in sedimentary rocks?