We cannot say what caused the bark to separate and break off. There may be several reasons such as physical damage, root issues, poor drainage, etc. You did not mention what the canopy looks like and how you care for the tree. 

We recommend that you consult with a certified arborist for an onsite visit regarding the health of the tree. An arborist can assess the situation and recommend practices to support the tree's health if needed. 

Many mature oak trees are in decline in various locations in Maryland. Some contributing factors are described in the following articles:

https://extension.umd.edu/resource/why-oak-trees-are-declining-or-dying

https://extension.umd.edu/resource/browning-and-defoliation-white-oaks

You can find an arborist using this directory: https://www.treesaregood.org/findanarborist/arboristsearch.

Marian

This is hard to say. It is probably fine but hazards can be hard to identify without seeing the situation on site. An ice or snow load might be risky. Legal implications of liability become fuzzy when a tree falls on a neighboring yard. It looks like many tree roots are outside your yard and is hard to know what stresses they experienced even if your tree care was fine. The bark is separating low on the trunk and the tree top is heavy looking.  You may not want to delay having the tree assessed.

Marian

On Tuesday, November 23, 2021, 1:27 PM, Ask Extension <<personal data hidden>> wrote:

You are welcome.

Marian

Thank you for the update Eleni. When arborists recommend treatments for ailing trees (which can be costly in some cases), it's prudent to seek a second opinion. Similarly, make sure the assessments are made by certified arborists rather than just any tree-care professional; arborists have more thorough training and keep track of current science regarding tree care. For instance, scientific evidence has not concretely supported the use of mycorrhizae (as an additive) due to lack of any measured benefits for plants. (Healthy soils have plenty of mycorrhizae already present, and adding more doesn't necessarily benefit the plants, nor do we always know what species of microbes are needed; plus, if the soil isn't healthy enough to support naturally-occurring mycorrhizae, it probably won't support added mycorrhizae either.)

The kelp might be providing a few plant growth-stimulating hormones as well as a mild dose of fertilizer, but this would only temporarily boost a tree's appearance or growth if the core cause of its demise is tissue damage to the trunk or roots or chronic environmental stress. Therefore, we are skeptical this treatment would provide any substantial results. Usually, detaching bark is not a good sign and suggests damage to vital tissues in the trunk, though we are of course only going by photographs and the arborist should be able to do a thorough assessment in person.

If you are instead comfortable and satisfied with their assessment and treatment proposal, just keep an eye on the tree's canopy for signs of decline going forward. Bear in mind that decline in the canopy might manifest years after conditions initially caused damage or stress to other parts of the tree, so the lag time can make diagnosis challenging. Changes occurring gradually can be difficult for us to notice, especially on a tree this size, so one suggestion is to take pictures of the tree during early or midsummer (when the canopy is fully leafed-out) around the same time each year so you can compare how dense and full the canopy is; it will be easier this way to see areas thinning-out or a pattern of branch loss, should it occur in the future. If you notice decline in this way, then you can consult an arborist again to see if a condition that may not have been serious or urgent now has progressed into a hazard.

Miri

You're welcome.