In our video “Coring Sea Floor Cake,” we use the example of putting a tube into a layered cake to discover its contents, to help explain why the JR is currently drilling on the Louisville Seamount Trail. The video is a good introduction to the concept, but you can take it one step further and involve the students even more by having them core their own layered cake.

One way the scientists interpret cores is by identifying the order and type of rock in the different layers and using this to reconstruct the story of the seamount. Students can do a simplified version of this with a layered cake (or other layered item similar to a cake) and a clear glass or plastic tube.

The cake should have three different layer colors to represent the three different rock types the scientists are finding most often in the seamounts. You will want to arrange these three colors into at least five layers in the cake to offer some complexity. The teacher can create the layered cake, or small groups of students can each create their own layered cake that represents a seamount and then another group can core the cake and compare their interpretation to the group-that-made-the-cake’s seamount history. Cake should be frosted (the frosting can represent sediment) so the layers are not visible from the outside. As a large group, or in small groups, students stick the clear tube in the cake, pull out their core of cake, mark the top layer and bottom layer with a marker, and then interpret the layers they see.

The three types of rock layers found:

• Sandstone: formed from the sand of the beaches when the seamount was still an island.
• Breccia: formed when the lava flows into shallow water, resulting in an explosive eruption.
• Basalt: formed either when the lava cools above water when the seamount was an island, or when lava flows very deep in the ocean where the high pressure prevents it from being explosive.

Assumptions to share with students as they interpret the cores they bring up:

• The cake contains 5+ layers, but a real seamount would contain hundreds or thousands of layers.
• In general, on a seamount the lower layer will always be older than the layer on top of it.
• Seamounts can rise above the surface then start sinking. Even while sinking, new eruptions can occur. These new lava flows form layers on top of the older layers that were once above the surface of the water.
• A basalt layer over a layer of sandstone or breccia most likely resulted from a lava flow that occurred above the water.
• A bottom layer of basalt may be the deepwater lava flows, but you would need to keep drilling deeper to make sure, because there may be breccia or sandstone underneath it. (They cannot drill any deeper on the cake, but the assumption would be that this is just the top portion of the seamount).

Here are some examples of what the cakes layers might look like. I wrote sample interpretation of each cake that can be found below the diagram.

Cake 1: The bottom layer of basalt is the oldest layer of rock. It may be part of the deepwater basalts but also may be a lava flow from the island. We do not know for sure because we do not know what is underneath it. The breccia layer above it formed in a shallow water environment, either after the deepwater basalt was near the surface, or when a lava flow on top of the island had sunk into the ocean, and an eruption occurred on top of it. The middle layer of basalt most likely occurred on land when the breccia accumulated until it was above the water. The top two layers of breccia are the youngest layer of rock and most likely occurred in shallow water after the seamount sank some and the middle lava flow was under water.

Cake 2: The bottom layer of basalt is the oldest layer of rock. It may be part of the deepwater basalts but also may be a lava flow from the island. We do not know for sure because we do not know what is underneath it. The breccia layer above it formed in a shallow water environment, either after the deepwater basalt was near the surface, or when a lava flow on top of the island had sunk into the ocean, and an eruption occurred on top of it. The breccia must have reached above the surface, because the middle layer of sandstone most likely occurred when the seamount was an island and the eroded volcanic rock sediment formed a beach. Since the basalt layer is over the sandstone it must have been from a lava flow from the island when it was above water that occurred above the beach. The top layer of breccia is the youngest layer of rock and most likely occurred in shallow water after the seamount sank some and the island lava flow beneath it was under water.

Cake 3: The bottom layer of sandstone is the oldest layer of rock and most likely occurred when the seamount was an island and the eroded volcanic rock sediment formed a beach. The breccia layer above it formed in a shallow water environment after the island had sunk and the sandstone was underwater. The breccia must have reached above the surface again, because the next layer of sandstone most likely occurred when the seamount was an island and the eroded volcanic rock sediment formed a beach. The top layer of basalt is the youngest layer of rock and most likely occurred on the island above the water and the beach

Cake 4: The breccia layers on the bottom are the oldest layers of rock and formed when the seamount was near the surface in a shallow water environment. The breccia must have reached above the surface, because the top layer of sandstone (the youngest rock layer) most likely occurred when the seamount was an island and the eroded volcanic rock sediment formed a beach.

For some background on why seamounts rise and fall above water, read these two blogs:
The Rise of a Volcanic Island
The Decline and Fall of a Volcanic Island