Can I go to Morocco from Spain and back to Spain on US passport

Your friend is misinformed. I've left the Schengen zone while on vacation in Europe numerous times and returned a few days or weeks later before going back to the US with my US passport. Never a question or a raised eyebrow. I assume one can visit Morocco without a visa with a US passport but am not sure - I would just google it.

Your friend probably is confusing a Schengen visa (which US visitors don't need to visit for 90 days at a time) with a visit using a US passport.

You friend is completely wrong. Is she going by her own experience?

As others note, this is nonsense. I have personally entered Schengen and then traveled to non-Schengen countries and re-entered Schengen on more than one recent occasion. The only limit is 90 days in 180 which is increasingly carefully enforced.

Thank you all. I felt very strongly that this was the case. My friend said she spoke with US and Spanish Consulates and Embassies. I can see one giving her misinformation but all four? She must've worded her questions differently than she stated to me. Now I have to make a decision! Monkeys in Gibraltar or camels in Morocco. Thanks all.
Lynda

Lynda, I can't imagine making a day-trip to Tangier from Seville. It's 125 miles from Seville to Tarifa, the port from which the ferry probably departs. Then there's time on the ferry, the fastest of which seem to take about an hour. Of course there's time spent waiting to board the ferry, etc. So I figure you're going to be traveling a minimum of 8 hours back and forth in order to see...a port town. Morocco is great; Tangier is far from its most interesting city. I wouldn't want to give up a full day of my vacation time for a few hours in Tangier.

Why not take the one-hour trip from Seville to Cordoba instead? Or a side-trip from Lisbon?

The limit is exactly what you were quoted by the embassies: "90 days in 180 day period", which means exactly what it says, citizens of non-EU countries who do not need a visa can enter and exit the Schengen Area as many times as they like, so long as they do not at any time exceed 90 days in any 180 day period.
Entry and exit days count as full days. Your passport will be stamped for every entry and exit, so the immigration officer may need to look at all the stamps and do some arithmetic to check you have not exceeded 90 days.

"but if I leave at any time in those 90 days, I can not get back into Schengen countries for 90 days." - that is total hogwash.

This limit does not apply to EU/Schengen area citizens, who can travel to and live in other EU/Schengen countries without limit.
If you are a citizen of a country which needs a visa to visit the Schengen Area, it depends on whether you have a "single entry" or a "multiple entry" visa.

Go to Gibraltar to see the monkeys, then take the hydrofoil to Morocco. In the late 1970s when Spain and the UK we’re at odds over something, the border between Spain and Gibraltar was closed. If you wanted to visit it you had to go to Morocco first.

I have done both of the day trips you are thinking about, so I'll share my thoughts on both with you.

Tangier was very interesting and I don't regret going one bit, however there is some uncertainty surrounding this trip because the ferries from Tarifa and Tangier are frequently cancelled or delayed due to high winds and rough waters, it's good that we had a little flexibility because our trip was cancelled the first day which left us scrambling to reschedule and rearrange. This was actually how we ended up in Gibraltar. Our ferry was delayed and very late arriving in Tangier so our time was cut very short, but it was still cool to see the little bit that we did. It was nice to have someone else take care of all of the arrangements and transportation, especially in a place that is so foreign to me. The tour consisted of a bus tour around the city in a mini bus, a carpet demonstration, a brief walking tour of part of the medina and lunch. It was market day the day we went so many of the women from the country side were there wearing their traditional dress and selling their fruits and vegetables. The lunch was decent. The camels were a few minute ride around a parking lot for a few Euros. This trip was good for a very brief glance at a different culture/city. What I saw/experienced interested me enough that I plan to go back. My dad who was with me saw enough and is happy he got to go to Africa.

Gibraltar worked better as a day trip in my opinion. There's not really any uncertainty surrounding the trip and you can plan things out a little better time wise. Our tour consisted of a guided ride up the mountain with a local guide (the cable cars were closed because of the wind), entrance to the caverns, time with the monkeys, a quick look at some of the military tunnels, and a driving tour around the city and rock. Then we had free time for lunch and a little shopping. We chose to walk back across the boarder to meet our transportation back to Seville so that we could walk across the runway for the airport. The monkeys are interesting and quite the site! The views from the top are breathtaking and the history of Gibraltar is interesting.

Both days were very long days. The company we used, picked us up and dropped us off at our hotel which was nice. Hope this is helpful to you. If you have any other questions, I'd be happy to try and answer them for you!

Thanks Angie,
Going to try to fit both in.
Lynda

.... "90 days in 180 day period" - no mention consecutive or if multiple entries are allowed that add up to 90 days. ....

That is correct as the others have been saying. Your friend is wrong or simply misunderstood -- and that is common. Let me rephrase it slightly and hopefully make it clearer. 90 days in any PRIOR 180 day period. It is a look back situation. Take today and count back 180 days - if you have been in the zone for any combination of 90 days in those prior 180 days -- your done. Tomorrow you do the same thing. And the week after that and so on. You can go and come as often as you like but each going and coming day counts. When you leave the zone be absolutely certain you have exit stamps that clearly show when you left. Do not treat this causally as it can have consequences.

Tangier as a day trip works better if you're starting at the ferry port of Tarifa. Starting from Sevilla adds a 3-hour bus ride one way (that's by a public bus, the first of which leaves Sevilla at 9:00; travel time may be faster or earlier by dedicated tour bus).

Continuing to Gibraltar also works better from Tarifa, about one more hour by bus. So even if you don't want to sleep in Tangier, you should at least sleep in Tarifa on one or both ends of the Moroccan "day trip."

I say go for it, and have fun, Lynda!