Part II of the activity leverages the novel geologic map of the southeastern face of El Capitan (Putnam et al., 2014, 2015) as an advanced, real-world relative dating exercise. El Capitan is one of the most famous landmarks in Yosemite; it is an imposing, nearly kilometer-high vertical-walled monolith of exposed intrusive rock steeped in the history of rock climbing. With the help of rock climbers and gigapixel photographs, geologists have mapped intrusive units with eight different chemical compositions projected onto a vertical plane with great detail and precision (10 cm accuracy for most contacts). While some of the units may be time-transgressive (e.g., multiple generations of pegmatite dikes) or coeval (overlapping geochronologic ages and gradational contacts), the relationships are consistent enough that students can screenshot a particular region of the map and interpret the relationships to sort the eight units from oldest to youngest (Fig. 4). A solution file is available through the SERC website with verified instructor credentials.

With author permission, a version of the geologic map (Putnam et al., 2014) was edited to remove details such as the geologic summary, correlation of mapped units, and geochronology so that students could focus on the mapped relationships. An El Capitan stop on the VFT provides the necessary background that students need to understand the basics of this geologic puzzle exercise. The concepts of cross-cutting relationships, gradational contacts, included fragments, and magma mixing are introduced, using relatable examples where possible (e.g., a cracked phone screen for cross-cutting relationships), and students are provided with an embedded link to download the modified map. Because students could search for the geologic map and companion journal article, this exercise works best as an in-session group exercise with lenient grading. Emphasis should be placed more on the locations the students decide on, to make screenshots to highlight the least ambiguous relationships, and their explanations provided.

Many cross-cutting relationship exercises given to students may involve sorting the timing of layered units, unconformities, folds, faults, and dikes. This El Capitan exercise is made more challenging by focusing on eight intrusive units with limited spatial layering and real-world relationships of varying ambiguity.

In Part III of the activity, students are tasked with producing a geomorphic map of the Quaternary (mostly Holocene) surficial deposits in Yosemite Valley using Google Earth Pro (desktop version; Fig. 5). Features to be mapped include deposits of talus (i.e., rockfall), debris fans, rock avalanches, and glacial moraines, as well as river terrace risers. The VFT highlights examples of each of these deposit types with additional background concerning processes and linked hazards (rockfall, flooding, and debris flows), including historic events. All the instructions and data links needed for this mapping exercise are contained within the Google Earth KMZ file provided to students. An example of each deposit type or feature to be mapped is provided to students in the folder structure that they will use to submit their final map (as a KMZ file). Students create their geomorphic map by interpreting web-served high-resolution hillshades (grayscale NW-illuminated representation of surface) derived from unfiltered (i.e., includes trees and buildings) and last return (i.e., bare Earth) airborne lidar data collected in 2006 (e.g., 10.5069/G9GQ6VP3, Yosemite, CA, 2021) and the internal web-served Google Earth satellite imagery built into the Google Earth engine. The different Quaternary features (debris fans, rock avalanches, etc.) have very Distinct textural and slope styles readily distinguishable in the bare-Earth lidar hillshade; once students gain an eye for it, they can efficiently map the 75 km2 Yosemite Valley region.

This mapping exercise is most appropriate for an advanced undergraduate course (e.g., geomorphology, applied geology, and capstone field geology). The maps are scored based on correctness, completeness, and neatness; the quality of the work is expected to be comparable to that produced by an entry-level professional geoscientist. An instructor-produced map is available through the SERC website with verified instructor credentials. An entire class's KMZ maps can be added to the instructor's Google Earth, allowing them to efficiently and objectively sort the maps based on quality, check for cheating, and identify obvious gaps in quality that signify score boundaries. From our experience in 2020 and 2021, these maps are among the best products students have produced in UCR's remote summer field alternative courses. In the process of mapping, most students (82 %; 18 out of 22) recognize that nearly all of the valley walls have talus or debris flow deposits at their base; many (64 %; 14 out of 22) recognize the variable density of river terrace risers along the length of the valley.

Part IV of the activity builds on the detailed geomorphic mapping the students did in Part III, which, in turn, builds on the knowledge gained in Part I's VFT. In a KMZ file students are provided with, there are the following two real-world hazard extents that Yosemite National Park uses for planning and preparedness: the known extent of the January 1997 flood in the valley, approximating a 100-year flood, and a rockfall hazard extent that considers talus slopes and isolated boulders but not rock avalanches (Stock et al., 2014; Fig. 5). Students immediately recognize that some places in Yosemite Valley are expected to be susceptible to both floods and rockfalls, and that most of the valley floor is susceptible to one of the hazards. Looking at the hazard extents overlain on the Google Earth satellite imagery, students can readily see what existing park infrastructure is within or near the hazard extents.

Students are tasked with using both their geomorphic mapping and the hazard extents to (1) provide recommendations for existing Yosemite Valley infrastructure that could be relocated to a less hazardous location and (2) identify locations that would be suitable for additional development with varying levels of risk (e.g., a hotel, visitor center, storage facility, or parking lot). Students turn in a KMZ file indicating their recommendations and a technical report written as if they were consulting for the park (an oral presentation would be an effective format too). It is emphasized that the recommendations students provide must be well-reasoned and geologically sound. Students should consider the nature of the respective hazards (e.g., rockfalls occur instantly and without warning and can be fatal, whereas floods in the valley are predictable several days out and are rarely fatal) and the facility use (parking lots in flood zones can perhaps be evacuated with limited damage, a seldom-visited storage yard is better near a rockfall hazard zone than a campsite, etc.) when providing their recommendations. Students become particularly invested seeing how the numerous facilities are placed throughout the valley. By the end of the exercise, they gain an appreciation for the limited real estate available for further development in the valley, that natural hazards are an active part of Yosemite Valley, and that there are considerable challenges associated with making decisions that affect the safety of over 4 million people a year.

The Projects feature of Google Earth Web is a robust and adaptable format for semi-immersive virtual field trips that can be created with relative ease and presented intuitively. Instructors and students alike can learn the basics of project creation in about 15 min, making it an efficient format for instructors and also suitable for students to create their own VFT as part of a course's final project. The VFT can be made readily available on web-connected tablets (via Google Earth) or computers (via web browser such as Google Chrome) using a simple web link. The abundant fair-use imagery available through the Google Earth Engine (topography-draped satellite images, 360∘ street-level images, and user-submitted 360∘ photospheres) often means VFT creators do not have to start from scratch. Using a 360∘ camera or the Google Street View application, the VFT creator can also upload their own photospheres. The ability to not only geotag a field trip stop as a point on a map but also to curate a precise starting view for that stop (e.g., magnification level, oblique 3D vantage, or particular view orientation in a photosphere) allows the creator to draw attention to the vantage most directly relevant to the learning goals for that stop. Text, images, and videos can be added to a sidebar supporting the stop's view; annotated photos of a similar or identical view can be particularly illustrative. However, each stop's view is not fixed, allowing the viewer to explore off-script, which can improve their situational awareness and provide opportunities for independent learning. Advanced creators can customize the HTML for a given HTML stop, providing an opportunity to add custom icons, narration, and built-in quizzes. Though less self-contained, hyperlinks offer an opportunity to send students to external web addresses for background information (e.g., Wikipedia), quizzes or surveys (e.g., Google Forms), and even web-hosted 3D models of outcrops or hand samples (e.g., Sketchfab or V3Geo).

As a result of the COVID-19 pandemic, Google Earth Web VFTs were created for several locations that instructors would otherwise be taking students to for physical field trips, such as Greece (Evelpidou et al., 2021b) and Colorado, USA (Mahan et al., 2021). A notable pre-pandemic general, public-focused implementation of Google Earth Web Earth projects as VFTs is Streetcar 2 Subduction, hosted by the American Geophysical Union (Rowe et al., 2020; https://www.agu.org/streetcar2subduction, last access: 25 January 2022). On their dedicated web page, eight separate Earth projects are linked that cover geological field trips in the San Francisco Bay area, building on the classic field trip guidebook A Streetcar to Subduction and Other Plate Tectonic Trips by Public Transport in San Francisco by Wahrhaftig (1984). While these trips are fully functional as VFTs, they include information on transportation and safety logistics and are designed as self-guided walking tours. Because there is often cellular data reception in these urban-adjacent field trip areas, a self-guided participant can use their GPS position (as a blue dot) to navigate from one stop to the next within the VFT frame and access the text, annotated images, and videos provided at each stop along the way. There is a vast collection of existing field trip guidebooks that could be adapted into immersive VFTs to reach a broader audience. Despite a global pandemic, United States national parks still hosted 237 million visitors in 2020; the top 10 visited parks all prominently feature geology (National Park Service News Release, 2021). One could imagine the value of linking immersive VFTs on park websites to help visitors plan their physical trips and phone-scannable QR codes displayed outside visitor centers that would allow self-guided trips.

Currently, there are several limitations of the Google Earth Web project format that are worth discussion. There is no functionality to add georeferenced layers to a Google Earth Web project, such as a geologic map that could be turned on or off over a landscape or a folder of data points that are distinct from field trip stops. Analysis is limited to measuring distances and areas. Google Earth Web also currently does not have the ability to switch between different imagery dates, a stellar feature on Google Earth Pro (desktop) that better highlights landscape changes (e.g., before and after a wildfire). If there was a more straightforward way to cache all imagery and media related to a project, then the VFT could be taken to remote field locations and actually be used as more of an interactive field trip guide. At the moment there does not seem to be a way to directly embed 3D models into Google Earth Web project sidebars; we attempted to embed web-hosted Sketchfab, SketchUp, and 3D PDF models but were blocked by script or cookie permissions. The ability to add an embedded 3D scan of a rock sample or a detailed outcrop model would likely add to a VFT viewer's experience (these can still be hyperlinked, though). Adapting the Google Earth Web project format to Google Earth VR would certainly boost the immersiveness of a VFT but at the cost of being a less accessible format for many due to the specialized equipment currently needed for virtual reality (VR; e.g., Hagge, 2021). For public outreach efforts and student engagement, it would also be beneficial if statistics could be accessed about the number of viewers and how long they viewed the VFT. Google Earth has been around for over 20 years, and software development continues to actively be supported, but, as with any format, longevity is not guaranteed.

ArcGIS Online allows multiple layers of vector data to be viewed on a single customizable map interface (though currently custom raster layers are not supported) and provides the ability to analyze and filter the data (e.g., West and Horswell, 2018); however, this format does not really support a presented or guided structure, and so additional resources are needed to support a VFT. ArcGIS StoryMaps, best characterized as a map-centric dynamic web page, is another adaptable format suitable for geographically oriented tours or narratives (for VFT examples, see Evelpidou et al., 2021a; Senger et al., 2021). StoryMaps offers a more structured and less immersive VFT option than Google Earth Web projects. Other more specialized formats exist on pay-to-create software platforms. StoryMaps GPS (e.g., California State University Fullerton's Yosemite Fire and Ice tour at https://www.travelstorys.com/tours/154/Yosemite%20National%20Park, last access: 25 January 2022; Gutierrez and Guinto, 2021) offers a similar presented format to Google Earth Web, with an overview map and field stops that can be selected from a table of contents, but lacks the key ability to associate a discrete view with each stop.

Arizona State University hosts many publicly accessible VFTs built in the Smart Sparrow software platform (Mead et al., 2019; https://vft.asu.edu/, last access: 25 January 2022). These trips are largely photosphere-centric, with built-in links to video and image pop-ups and links to additional photosphere stops; the result is an immersive experience with high production value (there are even ambient bird sounds) in a completely self-contained format. Several educators have been experimenting with using video game platforms (e.g., Minecraft, https://itch.io, last access: 25 January 2022) to allow exploration-based field simulations in both scanned real-world sites and fictional environments (Needle et al., 2021; Rader et al., 2021); these formats typically require more commitment on the part of the educators to design and implement but can be engaging, allow interaction between students, and mimic the freedom of mapping a region for the first time. VR experiences offer the most immersive VFT possibilities by allowing the viewer to have the virtual environment surround them (e.g., Peterson et al., 2020; Hagge, 2021; Métois et al., 2021); unfortunately, the specialist VR goggles, software, and computer are likely unavailable to most students at home, and students will almost certainly need to take turns in a classroom setting. Arguably one of the more flexible formats for presenting VFTs remains a custom HTML web page; this format allows embedding of maps (e.g., ArcGIS Online and Google My Maps) and 3D outcrop or rock sample models (e.g., Sketchfab), as well as external web links or links to download supporting materials (e.g., Bond and Cawood, 2021).

Undoubtedly, a major advantage of VFTs is their immediate accessibility to locations around the globe and beyond. Additionally, VFTs through Google Earth provides opportunities for students to participate in cognitively engaging and interactive learning experiences, which have been found to improve student outcomes in STEM (science, technology, engineering, and mathematics) courses (Freeman et al., 2014; National Research Council, 2011). In a recent meta-analysis synthesizing the findings from 225 studies, Freeman and colleagues (Freeman et al., 2014) found that interactive and active learning experiences, such as those provided by VFTs, are better for students' STEM learning than pedagogical experiences that rely heavily on didactic instruction (e.g., lecture-based instruction).

VFTs using Google Earth allow students to engage in guided-discovery learning (Mayer, 2004). Virtual learning environments based solely on discovery learning allow students to independently explore and solve problems with little to no guidance (Lee and Anderson, 2013; Mayer, 2004). However, one drawback to this type of learning environment is that it can be a source of extraneous cognitive load, especially when it comes to users with limited knowledge about geology and conducting fieldwork. Cognitive load refers to the load that performing a particular task imposes on the cognitive system (e.g., Paas and Van Merrienboer, 1994; Sweller, 2011). The amount of information one's cognitive system can process at a given moment is limited. Thus, the presentation of too much information, some of which is unnecessary information when it comes to solving the task, can result in artificially increasing the cognitive resources needed to process the relevant content. This is referred to as extraneous cognitive load by Sweller (2010), and it results in decreasing the efficiency and efficacy of the learner's cognitive system (see the review of cognitive load theory by Paas et al., 2010). With the aim of reducing the learner's cognitive load, the embedded instructional prompts that the Google Earth VFT format affords enables the instructor to promote a guided discovery of the environment by allowing for the integration of direct instruction into discovery learning (e.g., Lee and Anderson, 2013; Mayer, 2004). This direct instruction provides students with the scaffolding (i.e., additional learning supports that can eventually be decreased with increasing ability) necessary to navigate and learn from such a perceptually and information-rich environment (e.g., Lee and Dalgarno, 2011).

Google Earth-based VFTs are also an effective means to scaffold students' spatial skills. Spatial skills are a set of cognitive skills that enable us to manipulate, organize, reason about, and make sense of spatial relationships in real and imagined spaces (e.g., Atit et al., 2020; Newcombe and Shipley, 2015; Uttal et al., 2013). Field geology heavily relies on the use of spatial skills, as the goal is to use present-day spatial properties to infer the geologic history of a region (e.g., Atit et al., 2020; Shipley and Tikoff, 2016). In particular, identifying the relevant spatial properties in the field requires the geologist to focus their attention on the critical spatial information (e.g., the orientation of a bedding plane). Focusing attention on the important spatial information involves actively ignoring many other aspects of the scene (e.g., the size of the minerals, the fault slightly offsetting the layers, and the talus pile at the base of the outcrop as these geologic features are not pertinent to the problem at hand). The spatial skills used to identify relevant information for further cognitive processing is called disembedding in the geosciences (Manduca and Kastens, 2012; Reynolds, 2012) and selective attention in psychology (Moran and Desimone, 1985). Novices find disembedding to be difficult (Coyan et al., 2010; Shipley and Tikoff, 2016). Google Earth allows the user to remove the extraneous irrelevant information from the scene, thereby bolstering geologically relevant disembedding tasks for novice users.

Through our geology of Yosemite Valley activity, we provide an example of how a VFT can be designed to be approachable to a broad general public audience and, at the same time, serve as background information for an advanced undergraduate student mapping project. Hyperlinking technical words to external resources is an invaluable way of unobtrusively broadening the target audience. Distilling VFT stops to the most critical learning goals (especially with annotated images and videos) and encouraging interaction with the immersive view at the stop likely increases learning and engagement. Designing exercises that require students to utilize a combination of data they create (e.g., mapping) and real-world data (e.g., hazard data and stream gauge data) to justify decisions trains them to develop professional skills and increases their investment in the task. The progression from VFT to mapping to professional-style consultant report produced the highest quality work of any of the eight activities and exercises covered in UCR's 2020 and 2021 virtual summer field geology course offerings. Student feedback (n=10) collected through an anonymous web survey in the 2021 offering indicates that the geology of Yosemite Valley activity garnered the most positive response. In total, 50 % of the class said the activity was their favorite component of the course, and 60 % said they learned the most from it; no student thought it was their least favorite or that they learned the least from it (unfortunately, no anonymous feedback was collected from students in 2020).